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Thread: ecu float

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    Registered User rcb280z's Avatar
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    Default ecu float

    Okay, before I go into detail of my new F.I. issue I'm posting a question:

    Anyone with the so called "ecu float" issue, can you describe symptom(s) or reason(s) you believe that is what's happening.

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    Boat Anchor Repairman Captain Obvious's Avatar
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    The ECU output stage uses open collector Darlington transistor drivers to pull the low side of the injectors toward ground in order to open them. That open collector style of driver is sometimes referred to as "floating" when it's OFF.

    Ummm... I'm assuming that has absolutely nothing to do with what you're talking about?

    If that's the case, then I've never heard of it. Give me just a little hint (PM if you don't want to influence the results from others) and I'll see if it rings any other bells?

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    I think he might be talking about FastWoman's theory of "lean mixture drift". Aging electronic components causing a shorter injector open time. Typically fixed by adding resistance to the coolant temperature sensor circuit.

    If that's the case, then the signs would be slight hesitation and surging during driving. The engine doesn't get enough gas for the amount of air it's pulling in.
    1976 280Z, with some minor modifications

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    Registered User siteunseen's Avatar
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    Yea it's drifted out of spec is what I read. Mine ran terrible between idle and 2500 RPM, it wouldn't even spin the tires from a dead stop. I first thought it was the TPS though. I did what Fastwoman and Zed Head suggested, also on Atlantic Z's tips for fuel. I'm so glad I did too, made mine run like it should. Cheap and easy to do, the way I like it!

    http://atlanticz.ca/zclub/techtips/t...pot/index.html
    Last edited by siteunseen; 02-25-2013 at 12:24 PM.
    1972 240Z #918 New Sight Orange
    1977 280Z #305 Light Blue Metallic
    1972 240Z #110 Persimmons Red

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    Yes I am speaking of FastWomens lean mixture drift. My 77 is running similar to siteunseen's did but I was under the impression that it only happens with the 78 model. I haven't had a chance to trouble shoot anything yet. Has anyone experienced the same thing with a bad altitude switch which is a CA only sensor, supposedly. Damn, everything had been replaced when I was putting it back together a year ago. CTS, TTS, and harness to those, injectors (new) and connectors, TPS, AAR, CSV, Iv'e had all these new parts for quit some time and when I was installing my header I went ahead and replaced these items. Oh and a refurbished AFM. I swapped my old one in and it ran like crap also so I put the rebuilt one back on with the new setting and it runs good. So Far mileage hasn't gotten any worse either, plugs look good. I could keep on and on but I don't want to do that yet and waste peoples time here. It seems to have had the symptoms of "lean mixture drift". And I wanted to know what the symptoms are. Thanks

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    Registered User black gold man's Avatar
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    pop out the intake .

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    Quote Originally Posted by rcb280z View Post
    Yes I am speaking of FastWomens lean mixture drift.
    "Oh, that's very different.... Never mind."

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    Pure BS. Solid state components don't drift. Vacuum tubes drift as do oil filled paper capacitors. Neither of which exist in an S30 ECU. The ethanol in today's gas and or fuel delivery issues are far more like the culprit causing your off idle/low rpm stumble.

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    That's why I posted the question. I'm no electrician but I didn't think it was possible. Interesting Powerglide...that you would say fuel type related. Never gave it any thought. Why would it have been all of a sudden. My curiosity is up now about fuel additives is SoCal. Didn't think it got cold enough for it. As far as fuel delivery, no problem there. Observed no change in fuel pressure at all. I can see my inline gauge under the hood scope from inside the car.

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    Registered User siteunseen's Avatar
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    I get my fuel for my 4 vehicles from the same Gas Station. The '77 280 had a problem that was fixed by putting the potentiometer in line with the coolant sensor. I've actually done this. Maybe "drift" wasn't the right choice for a word so I'll just say it was broken and now it's not.
    1972 240Z #918 New Sight Orange
    1977 280Z #305 Light Blue Metallic
    1972 240Z #110 Persimmons Red

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    Quote Originally Posted by Powerglide View Post
    Pure BS. Solid state components don't drift.
    Haha! You must be in Marketing or Sales.

    Seriously though... There's no way I would guarantee the performance of the ECU would be dead-nuts identical today as it was the day it left the plant almost forty years ago. No way.

    Things are better than vacuum tubes and oil filled paper caps, but we haven't figured out how to get all the long term drift out of everything yet.

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    I still say putting a potentiometer inline is not "a fix" rather a mask to cover the problem. No different than making adjustments to the AFM IMHO. The problem is still there.

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    Z geek at large FastWoman's Avatar
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    Quote Originally Posted by rcb280z View Post
    Okay, before I go into detail of my new F.I. issue I'm posting a question:

    Anyone with the so called "ecu float" issue, can you describe symptom(s) or reason(s) you believe that is what's happening.
    Just speaking for myself, I bought the car running lean, and I fixed/replaced/verified/reconditioned EVERY part of the EFI, bringing EVERYTHING up to spec. And still I was running lean. And yes, I even tried real, non-ethanol gas, which makes a small difference, but doesn't really solve the problem. All that was left was the ECU. I bought another '78 ECU off of ebay, and the engine ran pretty much the same.

    Then I thought about all the old electronics I've fixed/adjusted/restored in my lifetime, including tube-type, component transistor, and early linear IC. And I thought about the hostile conditions (lots of heat!) under which the ECU had to operate for decades. I've seen lots of drift in lots of components manifested in wonky calibration, as viewed on an oscilloscope. Components do drift, and semiconductors do break down. Even a linear IC, with its numerous feedback circuits, will ultimately break down. More significantly, add to that that capacitors lose their capacitance, and therefore charge faster than they should, hence faster voltage ramps and shorter trigger times. It made sense to me that the base pulse might be shortened, and so I took measures to lengthen it.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

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    Registered User rcb280z's Avatar
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    Default update, electrical or injection, not sure which.

    I guess its once again time to break out the diagnostic equipment and start trouble shooting the fuel injection AGAIN! Would be nice to go more than a year without problems.
    So the reason I asked about the lean drift is because my Z has been running really good, great mileage and all. Was getting around 17 or 18 mpg in city and around 25 on the freeway. Anyone that lives in Southern CA knows that is good mileage. On a trip out one day recently I noticed surging on acceleration, and a rough idle. So when I got back to the house I popped the hood, its a 77 280, and looked at my fuel pressure gauge and fuel pressure is good. Checked timing, checked for vacuum leaks (pulling 18in), everything seemed fine until I pulled the plugs. They are usually a nice tan color and this time they look almost white! I'm assuming lean. So I started adjusting the bypass screw on the AFM and no change! I turned it all the way in and no change. I shut the car down and set the set screw back to where it was, I removed the cover from the AFM and loosened the screw holding the large "geared" wheel. Started the car and began inching the wheel counter clockwise and it was nearly a half turn (180 degrees) before I started noticing a change. Then it was running good. Plugs were back to normal, all seemed good until today. Now I think its running really rich! I did some running around town today and I can smell its rich now. Plus I burned through 1/4 tank of gas with a total of 35 miles. Not good. This is why I was asking about the lean drift issue. But, I don't see where anyone has had the same problem with it drifting out and then back. Which is what it seems like with mine. The only parts that haven't been replaced would be the ecu, EFI harness, dropping resisters, things you wouldn't normally have to replace. Here is what I replaced just over a year ago, the TTS, CTS, TPS, AAR, and their harness's and or connectors, injectors and injector connectors. She has been running really good until now. I have had injection issues with this car a few times now and they weren't easy ones to fix. I really haven't had the time to diagnose it this time not to mention I'm tired of having to diagnose injection problems. This was a quick run down of whats going on now. has anyone else experienced this before?

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    Z geek at large FastWoman's Avatar
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    It doesn't sound like drift to me. Drift never goes back. It just keeps drifting in the same direction. It also happens gradually, not suddenly.

    It sounds to me like an intermittent sensor problem -- perhaps a shorted air temp sensor, shorted coolant temp sensor, or some weird problem with your AFM. I suppose you could also have an intermittent vacuum leak, although it seems rather unlikely. Perhaps it relates to mode settings on your HVAC system? (One of your pneumatic control tubes could have broken or come loose.)
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

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    Registered User rcb280z's Avatar
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    Forgot about the HVAC system. Better check that first. However, I don't ever use it here on the beach. Never need any of it. I think I'll get everything ready (Diagnostic equip and tools) and the next time it "drifts" for lack of a better description I will check everything and log results. I should go ahead and run through everything tonight so I have something to compare to later....to see if there are differences.

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