strange issue with 78 280z running - I'm at the end of my rope!

[gheiser70]

Hi, I've got an frustrating issue with my 78 280z. I bought it last fall from the original owner ( a very nice lady). It had been parked in her barn since the early 1990's. She said she was afraid it would leave her stranded. Any way, she had a lot of work done to it before it was parked, new AFM, fuel pump, injectors ect. I dropped the fuel tank, cleaned every connector, put new plugs, wires, tune -up parts, cleaned the 35 pin connector and reflowed the solder connections in the ECU, fuel filter and pressure gauge. I'm getting 40 PSI with the FPR disconnected and 32 or so with vacuum to it. I don't believe I have any vacuum leaks. I was pulling between 17-18 in. Vacuum. All FI components spec out per FI bible. Valve adjust also done. Timing set at 10 BTDC.

After all that I was still fouling plugs with what seemed to be a rich condition. I then found that my master cylinder was leaking into the brake booster (it was full of brake fluid). I thought I might have been burning some brake fluid causing my "rich" condition. So, I replaced the booster and M/C, then I found a new issue. When running down the road, the car would cut out under hard acceleration, like some one flipped a switch, when I back off the gas it kicks back in and bucks like a bronco. After reading information here and the Atlantic z site I installed a 5K pot in line with the water temp sensor. It helped some but I still need to adjust for the best condition.

Now though, my idle has become unstable and I'm only pulling about 15 in vacuum. The real brain teaser is that when I pull off the vacuum line for the air con. control to hook up my vacuum gauge, the idle increases and smooths out. So the car idles better with a vacuum leak - what gives???

Any input would be welcomed. Sorry the post is long, I just wanted to give some background.

[Jeff G 78]

The resistor in the water temp sensor circuit will make the mixture richer, so that should have made the situation worse, not better. A vacuum leak will lean it back out making it run better. As for the original problem, I'm not sure what to try next. What kind of plugs are you using?

[Zed Head]

I'm getting 40 PSI with the FPR disconnected and 32 or so with vacuum to it.

All FI components spec out per FI bible.

You said everything checked out but you're reporting fuel pressure that's 10% too high. The proper number is in the Fuel Injection Handbook and the FSM. High fuel pressure will cause things to be rich. With the potentiometer, you have rich on rich.

The cutting out sounds like a problem with the TPS. Did you wash the engine recently? They get wet and do what you're describing.

[siteunseen]

I'd clean the AAR. Maybe get a new PCV valve? Sounds like it needs more air, maybe a varmint got in the air box?

[gheiser70]

Thanks for the replies. I'm using NGK plugs, but I probably need a new set. No water has gotten into the TPS and it checks out. I did replace the PVC valve. I don't get any black smoke from the tail pipe. My in line FP gauge is just after the filter and was cheap from jegs so it may be off a little bit. The reason I wanted to try the pot was for the cutting out problem. Many posters (from my searches) seem to think this is caused by a lean condition. I have good hot spark, but if my plugs were slightly fouled from before I changed the brake booster, I suppose they could be failing to fire under a hard load when accelerating. The Pot did seem to reduce the cutting out under load, but the idle went to crap. It was a 5 dollar attempt and no harm was done, as I just wired in in the engine compartment, without cutting the harness. I'll get a new set of plugs and try from there. It has a new air filter, but I'll take a look for varmints. I had the AAR off and I seemed to work well. The fast idle works when the car is started and settles down as it warms up.

[Zed Head]

Good luck. There are so many components that work together, that "close enough" just doesn't work very well. You'll end up adjusting something else instead of the actual problem. The best method is to make sure that everything is exactly as it should be.

If you report numbers in your posts, instead of looks good and checks out, you'll probably realize that the numbers aren't quite right. An ohm here, a psi there, a small vacuum leak...the tiny things add up.

If you find yourself deciding to adjust the AFM spring, make sure you mark your starting point. So you can go back.

On the ignition module - often they don't fail completely. They show signs of failure, then come back to life when they cool down a little bit. So, spark testing won't tell the whole story. Watch the tachometer needle next time it cuts out and starts bucking. The tach sees what the module is doing.

[FastWoman]

Your high fuel pressure reading could be an inaccurate fuel pressure gauge. You might try confirming with another gauge. You might be able to borrow one from an auto parts store. (You leave a deposit with them that gets refunded when you return the gauge.)

It's entirely possible your engine was running lean, and then you added enough resistance to make it run rich. Or perhaps it was running rich, and you made it run richer. Any of that is possible. You really have to adjust the potentiometer to maximize the vacuum and performance -- and to give the plugs a nice mocha color.

The cutting out issue does sound like the TPS. Perhaps the previous owner/mechanic rewired it incorrectly, or perhaps it's broken. It sounds to me like it's sending a throttle-closed signal to the ECU when you step on the pedal. If the engine is turning above 3k or so, the ECU will then cut fuel. You should check this switch very carefully to make certain it's doing what it should. You should check it both visually and with a meter at the big ECU connector.

Another thought: When you reflowed the solder on the ECU PCBs, is it at all possible that you introduced a short between traces leading to the TPS? It might be worth a look.

[Blue]

Data:
Fouling plugs means you are running rich
Vacuum leak experiment adds unmetered air. The improved performance with the extra air makes sense due to the rich condition... you are compensating for it.
Fuel pressure seems in the ball park and FPR is working

Questions:
Are all plugs the same? This shows that spark and injectors are working relatively the same.


Most probable problems causing richness:

1. Water Temp sensor and wiring to ECU. Measure resistance at sensor and also back at the ECU. Check to ensure it is nominal for the water temp. (do this on a cold engine).
2. Incorrect injectors. Check their published flow rate against part number.
3. AFM spring too loose. Did anyone adjust it before (look for signs of entry). If the spring unwound by a previous butterfingers mechanic then it may still be too loose.
4. ECU failure.

[Blue]

BTW great report and work/checks that you have done to get the car up and running!


IMPORTANT. Be sure to clean the plugs with solvent and a wire brush once they are fouled or further attempts to resolve will be hindered by the fouled plugs. No need to buy new ones as once the air fuel gets close to nominal, the engine will clean up the plugs. Just ensure they are not carboned-up before trying something new.

I like to put fouled plugs in a bowl then hit with carb cleaner spray and wire brush.

[Blue]

280Z stock injectors are 188cc units
Injector list: http://www.atlanticz.ca/zclub/techti.../injectors.pdf

Water Temp Sensor Resistances vs. Temperature

C	F	Kohms
-30	-22	20.0-33.0
-10	14	7.6-10.8
10	50	3.25-4.15
20	68	2.25-2.75
50	122	0.74-0.94
80	176	0.29-0.36

[gheiser70]

Thanks again. I'll recheck the TPS, because it does seem that the fuel (or spark??) is completely cut when the car acts up. I have considered the ignition problem as well (module failure or coil) when the car has run for a while. At this point the problem is not related to how long the car runs.


Anyway I have my niece's baptism today, but I'll try to get to the garage this evening.

I'll remove the pot as well and try again with clean plugs. I'll also check injector P/N. I don't think the AFM was open It was replaced < 1000 miles before the car was parked. The AFM P/N is A31 625 000 - Is that correct for 1978? It looks like there is clear sealant around the black cover.

[Blue]

What are pn's on the injectors?

[Wwildman]

When I got my car it was sitting for 14 years and the AFM had clear silicone around the cover . I opened it and found that a butter fingers mechanic unwound the spring. I reset the spring and replaced the coolant temp sensor, all injector connectors with injectors,and new FPr and pump. Car stalled and would not start. I had no spark or injector pulse. I put a new Ignition transistor under the dash and a new pick up coil in the distributor. Also I cleaned all the grounds and added two more grounds

[Zed Head]

I have considered the ignition problem as well (module failure or coil) when the car has run for a while. At this point the problem is not related to how long the car runs.

It's not how long the engine has run, but how much current is running through the module that heats it up. High RPM passes more current through the sensitive parts as it generates more sparks. High RPM, heat, weird behavior, turn off engine, cool down, back to normal.

[hr369]

bucking during decleration is probably tps adjustment.
did you take the cover off and inspect the contacts?


http://www.atlanticz.ca/zclub/techtips/tps/index.html

[gheiser70]

Blue, The best I can tell the injector PN is A46-00 but its awful hard to see.

Zed Head -Tach needle remains steady during the "episodes".

So, here is my update. Replaced plugs with new, removed pot and reconnected temp sensor as stock. Temp sensor OHM = 2730 at 70 degrees this AM in the garage, the car sat all night. Rechecked vacuum = 18in at idle. Rechecked Fuel pressure. Looks like 28 PSI at idle. Checked TPS - function was OK. Idle running much better and smoother with pot removed. Went up and down the road and still cut out between 3 - 4K RPM when accelerating hard. I was also able to peek at the under hood FP gauge and I didn't see any pressure drop during the episodes.

So, back to the garage and I did the old screw driver to ear check on the injectors. Found a problem - injector #5 - No click at all. Swapped injector wires from cylinder 5 and 6 and injector 5 still fails to click. Injector 6 acts normal no matter which wire (5 or 6) is attached. So I believe the problem lies with the injector, not the wiring. I figure running on only 5 cylinders is not good for hard acceleration!!!

So what do you guys recommend - one new injector or a new set of 6? Digging through the PO's paperwork the injectors were replaced in 1998 and about 3,000 miles ago. Any recommendations on sources? From what I see on the internet, prices vary quite a bit. Anyway, new a injector or injectors are in order before any more tuning takes place.

On second thought, I'll probably buy 6. If one went south, who knows when the next one will fail.

Thanks for the advice so far. Greg

[Blue]

Thanks for the update:

Temp sensor checks out at the sensor. I assume the wiring from the sensor to the ECU is OK?

Are the injectors green? The full part number should be A46-000001 for stockers.

Getting a full set of injectors would give piece of mind however you can easily pick up a few used ones and flow all and clean some to get a relatively matched set.

[Zed Head]

If I had a stock system and was looking to buy new injectors, I would get Standard brand, either the FJ3 or FJ707T. (Rockauto has good deals. Data is sparse but they might be better for heat soak than others.

Before you buy though, since you're going to take things apart, you might leave the injectors connected to the rail when you remove them (they'll come out as an assembly), reconnect or leave connected the fuel hose and electrical and squirt in to some containers for a short time ( use small coke bottles). You'll get an idea of how unbalanced the injectors are. Do it outside though, with a breeze and no open flames.

You should notice if #5 is totally dead. The engine will run a little rough and the spark plug would be dry or oily, but different from the others.

[gheiser70]

Thanks,

Blue the temp sensor was checked at the ECU (35 pin). The injectors are green.

Zed, My son and I had the fuel rail off this winter to replace the plastic holders and O rings on the injectors as I think 5 were broken (the holders). We cranked the engine and all the injectors were firing then. (we didn't measure flow, just confirmed they were all spraying). Spark plug looks much whiter than others after running yesterday.

I believe the injector failed during the time I replaced the leaking brake booster and M/C. This is what increased my frustration. I assumed fixing the leaky booster would make a big improvement in how the car ran. When the new issue with cutting out cropped up I went a little bonkers and got away from basic troubleshooting. I'll update again after the injectors are replaced.

[Blue]

Good stuff. You know what you are doing...keep at it and you should have it in good running order soon.

[FastWoman]

A single MIA injector isn't going to cause your engine to cut out completely above 3kRPM. It will give your exhaust a chugging sound, and you will lose 1/6 of your power throughout the power band. But no, it won't cause your engine to drop out.

A non-working injector also will not affect the fuel/air ratio balance. The cylinders that are running will be getting the same mix as they would if the #5 injector were working.

Your injectors could just be gummed up with varnish. If there's any flow at all through your #5, there's hope for cleaning it up. (A stuck injector might free up once it gets hot. Does it show a normal resistance across its contacts?) I once cleaned up a gummy intake by running some Chemtool B12 through it. It took a bit of persistence, but it finally paid off.

Have you taken the cover off of the TPS to see if anything is amiss? Have you confirmed the correct signals at the big ECU connector when you move the accelerator pedal?

[madkaw]

As Zed said-it's the amount of current that might make an ignition issue, not so much the heat. The coil might be caving under high load situations and thus cutting out the ignition. I agree with Sarah that one injector is not your issue.
Keep up the good work, most guys with only 5 posts are still wondering what the PCV does-

[gheiser70]

Fastwoman - I did have the TSP apart and all looked well. My son and I check for continuity at idle and at full throttle at the 35 pin connector. I don't remember the exact pins, but the continuity checked OK at both positions.

Thanks for the encouragement. I have quite a bit of experience with air cooled VW's and old Mopars, but alas, they were all carbureted. Those are fairly forgiving systems, fuel, spark and compression and they generally run well. I'm learning a lot about 40 year old electronics though. The trickiest part of electronics I've dealt with on the old cars was the old "ballast resistor" in the early Chrysler electronic ignitions, and for that you just kept a spare one in the glove box if it crapped out!

I do have a spare 12 volt coil, when I get the injectors sorted, I'll swap it out and report back.

[Zed Head]

I've never really understood why water in the TPS would cause the problem that it does but I think I have it figured out now, thanks to this thread. I attached the relevant passage from the FSM. Water in the TPS apparently makes the ECU think the throttle is closed. Details in attachment.

One way to get a better idea of if it's the TPS is to watch the tachometer and note the RPM, when the cut out happens, and when it bucks. The tachometers aren't known to be super accurate so you may not see exactly 2800 and 3200 RPM, but if it happens consistently around those numbers, that would be a good indicator.

If that's not it, then the old fuel pressure gauge poking out of the hood plan would let you know that you're maintaining fuel pressure. If you have proper pressure but it still cuts out, then electrical has more potential.

[sscanf]

Hmmm... I have some observations to make here based on the work I have done on my 1976Z.

1. When I bought the car, most of the EFI parts had been removed (it was a jigsaw puzzle). While it was apart, I replaced the injectors with FJ707T (Standard Brand).
2. After putting it back together it ran rough and rich. Had periodic cut outs, and the tach was jumpy.
3. Replaced already upgraded ZX distributor with a rebuilt from Rock Auto which includes the ignition module.
4. This fixed the tach and the cut out problem. Ran pretty good but still had issues.
5. Replaced the AFM with a rebuilt from MSA and fixed vacuum leak BDCC (or is it BCDD). This made it run great except when cold.
6. Replaced the Aux Air thing with a ball valve from Home Depot. When its cold, I pop the hood and open it half way before I start and let it run for a few minutes that way, then I close it and go.
7. I have been running it daily for a while now (maybe a month). Getting 19MPG which I am disappointed in but I do lots of short trips. Work is only 8 miles from home. It does still seem a little rich to me based on exhaust smell.
8. I suspected a problem with the TPS so .... I dis-connected it (yes, I will put a meter on it soon). It runs just fine without it. I want to see what the gas mileage is with it disconnected. My theory is that if it was stuck on rich, then it should now be stuck on normal. Its main purpose, as I understand it, is to richen the mixture a bit when cold and when full throttle (and shut off fuel when coasting). So, if you suspect the TPS, you can disconnect it but, based on my experience, I kind of doubt it would cause some of these problems unless there was something else going on as well.
9. I have a home depot fuel pressure gauge after the fuel filter and its right on specs.

Dan

[Zed Head]

Your theory looks right. When disconnected, no idle enrichment and no full throttle enrichment. Mileage would only get better if you spent a lot of time at full throttle, I assume.

When the TPS gets wet, the idle circuit shorts, apparently, then when RPM go above 3200, the ECU cuts fuel because it thinks you've just shut throttle. Obviously, the algorithm doesn't consider that 3200 RPM could be reached with the throttle closed.

[FastWoman]

Here's a thought: If the ECU has so high an input impedance on the TPS circuit that a bit of water between the contacts can cause trip the logic, then it might also be sensitive to a buildup of electrolytes over the years, perhaps from exposure to road salt. (I note that you're in west Ohio.) Considering this, I would recommend you remove the TPS, scrub it down with gasoline. Let dry. Then wash with detergent and water. Finally, soak the switch overnight in distilled water (because electrolytes might be sandwiched between parts), do a final rinse in distilled water, and let dry. Re-lube. Reinstall.

[Zed Head]

Fastwoman - I did have the TSP apart and all looked well. My son and I check for continuity at idle and at full throttle at the 35 pin connector. I don't remember the exact pins, but the continuity checked OK at both positions.

Expanding on FastWoman's point - you're not reporting "no continuity" or open circuit at 4 degrees + off idle. You should see the idle circuit go open when the throttle blade is moved (looks like Pins 2 and 18). You might need a good meter to be sure that there is not some slight conductivity that would "trip the logic" as she suggests.

In the same vein, I assume that the ECU TPS idle circuit monitoring itself could be bad. But I've never seen anyone report that.

Still, the observations at 2800 and 3200 RPM should tell if any of this is even worth following up on.

[gheiser70]

Weekend update:

So new injectors from rock auto were installed with new O-rings. Slight miss at idle is now gone. Screw driver to ear test confirms all injectors "clicking" now.

TPS saga: Zed - I reconfirmed that pin 2 at 35 pin shows continuity at idle and open when accelerator is depressed all the way to full throttle. Also, pin 3 is open at idle and all the way until the last couple of degrees till full throttle is reached then it shows continuity. I cleaned the contacts with 1500 grit sandpaper and spray contact cleaner and repeated the above tests. They are the same at the TSP and 35 pin connector.

The fuel cut seems to be consistent at about 3K rpm, only when at full throttle - ie. it will rev past 3K and accelerate well unless the TPS sends the full throttle signal. As soon as I release the throttle, normal running resumes.

As a further test, I disconnected the TPS wiring from the TPS, and there is no cut out in this mode of operation - ie. I can run at WOT with no cut out of the engine. I think these tests isolate the issue to the ECU. It seems to be sending the fuel cut signal under WOT operation.

I was wondering if anyone has a schematic for the ECU. My son and I were trying to trace the circuitry for the TPS, but it seems nearly impossible without a diagram.

There is also a test in the FSM (section 8.3) for throttle valve insulation. The reading between all three pins on the TSP and body ground should be open. My TPS passes this test as well.

Has any one purchased a rebuilt ECU from MSA? And if so, how was it? If I can't find the schematic for the ECU that may be my next step.

[Zed Head]

TPS saga: Zed - I reconfirmed that pin 2 at 35 pin shows continuity at idle and open when accelerator is depressed all the way to full throttle.

The fuel cut seems to be consistent at about 3K rpm, only when at full throttle - ie. it will rev past 3K and accelerate well unless the TPS sends the full throttle signal. As soon as I release the throttle, normal running resumes.

As a further test, I disconnected the TPS wiring from the TPS, and there is no cut out in this mode of operation - ie. I can run at WOT with no cut out of the engine.

.

I guess I'll keep harping on the TPS. This is not right - open when accelerator is depressed all the way to full throttle

I don't know if you can see the picture I attached in my last post, or if you haven't seen that page in the FSM. Pin 2 should show continuity at idle then OPEN when the throttle is just off-idle - 4 degrees is barely cracked open. Basically, when you crack the throttle, Pin 2 shows NO MORE IDLE. What you're writing is that the ECU sees IDLE all the way up to full throttle. That's what I've been proposing. Plus the fact that you don't see the cut with the TPS unplugged supports that it's the IDLE circuit continuity (Pin 2) causing the fuel cut. There appears to be something wrong with the TPS signal to the ECU, not the ECU itself.

[Blue]

Seems like the TVS wiring in the harness is reversed? The WOT pin on the TVS seems to be sending the Idle signal to the ECU by the behaviour?

[gheiser70]

Zed,

I guess I'm not saying it correctly. Pin 2 shows continuity at idle. Then, when the throttle is depressed, it is open (no contiguity) all the way through the rest of the stroke of the throttle, including wide open. At all positions other than WOT pin 3 shows No continuity. At WOT pin 3 shows continuity. This is correct per the FSM.

Blue, I also thought the wires might have been reversed, but they are correct at the 35 pin connector, so I don't think that is the case.

[Zed Head]

I guess I read it the way I thought showed that the TPS would be the culprit. Sorry about that, looking back I did read it wrong. Bummer then, problem not solved.

It's odd though, that disconnecting the TPS removes the problem. That doesn't fit with the ECU being bad. Going back to what FastWoman suggested, when you say open, do you mean infinite resistance? Do you have a high quality meter that shows infiinty, or are you using a test probe with a beep? It might be that there's enough conductivity to trip the ECU's idle circuit, bit not enough to show on your probe.

[gheiser70]

Zed,

I'm using a middle of the road multimeter to do my checks. It's no Fluke, but no harbor freight either. It shows infinite resistance. Dan's post above (# 25) shows a similar result with the TPS disconnected. I surmise that the disconnecting the TPS removes any signal from entering the ECU's fuel cut circuit. I'd love to get a schematic for the ECU, so I could see how the signal is processed.

[Zed Head]

Send a message to superlen, he's been deep in to the ECU functions. He's building a standalone engine management system to replace the Z EFI.

And the ECU is really easy to take apart. Maybe you'll find a shorted or corroded circuit. I've seen some crusty ECU internals, they live in a spot that gets moist if there's a windshield leak.

[Blue]

If the RPMS are over ~2800 and the idle contact touches the center contact then the ECU cuts fuel (this is normal behavior).

From the description in post #29 paragraph 3 seems to indicate the fuel cuts off over 2800 rpm when the WOT contact connects to the center contact thus my supposition of reversed wiring. The fact that it does not happen with the TVS disconnected further points to the source.

Comrades... a reference with chocolate hugs:
http://www.atlanticz.ca/zclub/techtips/tps/index.html

[Blue]

If you want to dig deeper there are heaps of ECU info here: http://www.classiczcars.com/forums/f...od-system.html

[Captain Obvious]

Are you sure you aren't drowning your engine when the ECU gets the WOT signal? You keep thinking that fuel is being cut because the engine misfires... Maybe you're dumping so much fuel at WOT that it just can't light it?

[FastWoman]

OK, fuel isn't cut at WOT above 3k with the TPS unplugged. It IS cut at WOT above 3k with the TPS connected. Therefore we can assume there is a connection (perhaps a very high impedance one) between pins 2 and 18 when the TPS is connected to the ECU and the throttle is wide open. Somehow, some way, THIS IS YOUR PROBLEM. We know the TPS mediates this connection, but we cannot assume we have isolated the source of this connection to either the TPS, the ECU, or the wiring inbetween.

We have these possibilities that I can see:

(1) There is a high impedance salt bridge between 2 and 18 in the TPS. (See suggestions above for washing electrolytes from the TPS.) What resistance range did you use on your multimeter? If you used something like a 200 Ohm range, you're not likely to see the sort of salt bridge I'm talking about. You should use the highest possible resistance range on your meter.

(2) Your middle contact (18) floats off idle and below WOT (which is really just a "healthy" throttle, not necessarily to the floor). Perhaps the contact is hovering close enough to the 2 contact to bump it when the engine is vibrating hard. If the middle contact floats pretty evenly between 2 and 3, I'd say there's very little likelihood of the contact bumping. I've never heard of it.

(3) I'm tentatively ruling out a wiring problem that shorts pin 3 to pin 2 or that reverses pins 2 and 3, based on your observation that 2 and 18 have continuity at idle and at no other throttle position. However, I'm ASSUMING you mean that 2 has continuity WITH 18 and not with something else. I suggest, again, that you reconfirm 2 (idle contact) runs to 2, 3 (WOT contact) runs to 3, and 18 (center contact) runs to 18, and that none of these is shorted to ground.

(4) You re-flowed the solder on the ECU boards. I suggested a while ago, but I don't think you've investigated the possibility, that you might have introduced a solder bridge between pins 2 and 3 on the ECU board. That would be easy to do. If you have done this, then at either idle or WOT, there would be continuity between pins 2, 3, and 18. Otherwise there would only be continuity between pins 2 and 3. Of course unplugging the TPS would eliminate any continuity between 2/3 and 18, so the ECU wouldn't enter into a fuel cut state above 3k. I suggest you test for continuity between pins 2 and 3 of the ECU (not the 35 pin connector). If continuity exists, then there's your problem.

(5) I suppose it's possible there's a solder bridge upstream from the 35 pin connector and its board contacts. Without a schematic of the ECU, it would be difficult to identify possible trouble spots. The adjacent contacts 2 and 3 sound as likely a trouble spot as I can imagine. BTW, I don't think anybody has a schematic. The ECU uses proprietary chips in oil cans, so even if you can map out all the traces and small components, god only knows what's inside the oil cans.

[gheiser70]

Captain O - I don't get a misfire at WOT, the engine stops running like a switch was flipped. Tach remains stable however.

Fast - I'll try to clean the TPS as you suggested. I've taken the ECU apart and checked for solder bridging per your recommendation. Checked with the Multi meter where the pins hit the board and the pins themselves. No continuity between 2&3.

I was checking at 2000K ohm range (for the TPS switch checks), still showed open.

The inside of the TPS looks visually OK. I agree with you that the chance of contact jumping is small. I'll try to get some measurements but the idle contacts are well apart in WOT mode.

I'll also double check the wiring.

Greg

[FastWoman]

I suppose I should add...

(6) There's a possibility that the "logic" state for the idle circuit has become unresponsive and is either permanently high or permanently low. That is, your ECU might be bad. I've never heard of one failing in this way, though.

[Captain Obvious]

Captain O - I don't get a misfire at WOT, the engine stops running like a switch was flipped.

I guess what I'm saying is that if you're dumping so much fuel that the mixture won't light and you're misfiring on all the cylinders 100% of the time, then wouldn't that feel like a switch was flipped?

You could wire a small 12V bulb across one of your injectors and hang it out of the engine bay taped to the cowl or something. Drive with it attached and see what the bulb brightness does as you drive.

If fuel is being cut, the bulb will go dark. If fuel is being poured in there at great volume, the bulb will glow bright.

[Blue]

After more thought I think the WOT fuel enrichment circuit in the ECU has a bad component that kills the pulse shaping when activated.

[FastWoman]

Er... Please ignore my #6 (last post). Obviously the involvement of a connected TPS falsifies that possibility.

Thinking about Captain's theory, assuming the ECU is delivering WOT enrichment correctly, there is nothing different >3kRPM (vs. <3kRPM) that would suddenly deliver excessive fuel and choke out the engine. However, if you look at it from a standpoint of engine load, perhaps 3k at WOT is where the engine begins to suck in enough air to move the AFM vane to a dead spot in the potentiometer, thereby mucking up fuel delivery. Maybe the 3k mark is just a cruel coincidence.

There's one thing I'd like to know: Does this happen only under load? Can you recreate the issue out of gear in your driveway? Can you lay the pedal to the floor and have the engine bounce around the 3k mark as though it's rev limited? If you can do this, then it would be interesting to plug a lightbulb in to one of the injector plugs and observe its behavior when this happens.

[gheiser70]

Fastwoman, I think your salt bridge insight may have been the answer. Yesterday I scrubbed the TPS with a thoothbrush and good old soap and water, plus some white vinegar for good measure. The rinsed everything in distilled water. I also did the same for the TPS connector in the engine bay. I then dried the components with compressed air. Drive way testing shows no cutout when revving to WOT. I haven't had a chance to road test, but I may today. As for the other question, the cut out does (did) take place whether or not the engine is under load, ie, the engine would cut in the drive way at WOT as well as on the road.

Now I just need to get out on the road, get her good and warm and see what happens.

[gheiser70]

So - More good news.

20 min road test with multiple WOT runs (I may have exceeded the speed limit -) NO cutting out was observed, just smooth acceleration. So, it appears the salt bridge theory was the answer.

Thanks to Fastwoman, and all others on the board that helped me through this troubleshooting adventure.

I'll keep you guys updated if anything changes, but right now I'm going to enjoy the sweet sound of the inline six humming along!

Thanks again to all.

Greg

[rossiz]

wow... you guys amaze me.
salt bridge??!? who knew!

[FastWoman]

I'm glad we ferreted out the problem! Damn that Ohio road salt! It's still taking its toll on my Saturn.

It's odd they'd make this circuit soooo sensitive. The conductivity of the short had to be > 2 MOhm! Maybe they wanted to pass as little current as possible to minimize corrosion in the contacts -- to make them last longer.

Anyway, congrats on getting your Z back on the road! Enjoy the ride!

[superlen]

Greg,

It sounds like you're back up and going. (For the record, after reading this post & BEFORE you did your last round of cleaning, I was thinking the same thing Blue said...the WOT pulse shaping circuitry in your ECU was kaput. I do think now it was just bad external connections.)

If you still think it might be the ECU, give me a yell. I have several old stock units. I can send you a loaner one to compare against yours to see if it makes any difference. You can send back yours if you decide the loaner is better. I don't care if I get back a goofy-running ECU.

Len

[Attila]

I had an issue with my 280z and took years to resolve. It ended up being a temperature sensor. The car would run rich with no performance and would run only between 2,000 and 3,000 rpm. This would happen intermittingly until the sensor completely failed. This was a problem with a lot of the 78Z's. I hope this sheds some light to your problem.

[mjr45]

I am once again amazed by the enormous amount of knowledge by folks in this club!