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Thread: 77 280z running lean

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    Default 77 280z running lean

    Hi I'm new to the site so i hope I am posting this in the right spot. I have a 1977 Datsun 280z that pops back thru the intake when you give it gas!!! I replaced all the injectors and gave it fresh tune up, but still runs like crap the fuel presure is 20-39 psi. I jumped the thermotime switch and it stopped popping thru the intake but makes it run to rich- black smoke DOES Anyone have any ideas?

    Thanks Steven

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    Merchant of Cool Z train's Avatar
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    Vacuum leak is the first thought.
    Faster than anyone in Oceanside

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    I looked every where for a vac leak and i found the air bypass was leaking so i caped it off. so i know its not a vac leak that what I thought too.

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    Another Oregonian. Woohoo. How come you're not watching football?

    The thermotime switch only comes in to when play you're Starting. Maybe, instead, you jumped the temperature sensor that tells the ECU how warm the engine coolant is? If so, you told the ECU that your engine was very hot. Or you think you jumped it but actually just disconnected it. In that case, you told the ECU that your engine was very cold. That would give lots of rich black smoke.

    Did you just get the car? The symptoms look like those of a bad AFM. I had a bad one and mine idled fine, went well at 3/4 to full throttle, but did what you describe at part-throttle. There is a test procedure for the AFM and most of the other EFI parts. You can spend a lot of money replacing parts, when all you might need is some contact cleaner.

    By the way, your first post was pretty well formatted except for the large, bold font. Second one looked like cell phone.

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    Yeah I was wondering about the AFM. I took the cover off of the AFM and it looks like new. I moved the weight by hand to make it run richer which made the car idled better but it would still pop back if i gave it gas.
    When I jumped the thermotime I also jumped the temp sensor too. When I jumped the temp it didnt really effect it much. I switched the jump around just the temp or just the thermo and jumped both at the same time. It ran the best by jumping the themo I think it ran halfway decent other then poor idled and black smoke. I know which wire is which and i know for sure its the thermo is the one im jumping.

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    A little info on the car. 1977 280z non AC, 4speed that has been sitting for 10yrs every couple years i would start it up and drive it around the block. The reason it was parked is cause of the pop back and it didnt pass deq. now i live out of deq so now its time to fix the driveabiltly issuses. I drained the fuel tank and put fresh gas in it so i know its not bad gas.

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    Quote Originally Posted by 77Oregon280z View Post
    A little info on the car. 1977 280z non AC, 4speed that has been sitting for 10yrs every couple years i would start it up and drive it around the block. The reason it was parked is cause of the pop back and it didnt pass deq. now i live out of deq so now its time to fix the driveabiltly issuses. I drained the fuel tank and put fresh gas in it so i know its not bad gas.
    You should download the Factory Service Manual (see the web site below) if you don't have one, get a multimeter and test things. The AFM test is pretty simple (it's in the Engine Fuel section). Some here have had good luck just cleaning the AFM contact area. I bought a remanufactured AFM but only after lots of testing to confirm it was bad (the resistance readings were off and it popped and bucked when I drove it). Spend a while testing components and you'll know what to think about when it still doesn't work right.

    Deoxit is a good contact cleaner but the others work well too. The electrical connections tend to corrode over time.

    http://www.xenons30.com/reference.html

    The 76 manual is broken in to chapters, the 77 wasn't last I looked.

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    I tryed downloadin it and it said its a bad archive

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    Quote Originally Posted by 77Oregon280z
    I tryed downloadin it and it said its a bad archive
    Try this link.
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    What is your vacuum gauge reading at 800 rpm idle ?

    Jumping the temp sensor is leaning it out, actually you should use a 330 ohm resistor to jump the temp sensor, since it's technically never shorted out with 0 resistance.


    I've been battling a very similar issue on my 78. I'll be watching what you come up with here. I've got quite a lengthy thread going on another forum about it..

    http://www.zcar.com/70-83_tech_discu..._893825.0.html

    Here is what I've done so far..
    complete test of entire FI system per FSM and FI guide, including wiring harness, AFM module, injectors flow tested and serviced..
    replaced all vacuum lines that were suspect. Pinched off, plugged, prodded, etc.. any possible source of an external vacuum leak
    replaced EGR valve
    replaced intake/exhaust manifold gasket
    replaced both intake boots
    complete tune up, wires, plugs, rotor, cap, etc..
    Checked timing, verified timing mark with TDC
    Compression check, all cylinders around 150psi or so.
    All new FI connectors (there are 10, including 6 injectors)
    used 3 full cans of carb cleaner giving the whole intake a bath trying to find a leak

    It acts like a vacuum leak, but I can't find one. The thing is, when you add more fuel it smooths it out, but I could just be masking a vacuum leak. On the other hand, incorrect air/fuel ratio will look like a vacuum leak as well.


    I fooled my AFM into supplying more gas as a test by adjusting the gear up 12 notches, which helped, but only down low. When I drive it after fooling the AFM the top end was completely down on power.

    Vacuum on mine is only around 14hg. I can't get it higher than that no matter what I do, other than giving it more fuel.

    I've seen a few threads around on the net describing similar situations with 77-78 cars, but no one ever seems to fix the issue, or at least post back to the thread stating what the fix was (more likely).
    Last edited by cozye; 10-22-2010 at 05:54 AM.

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    Add in more initial lead and see how it responds. Motor should pull a MINIIMUM of 17 inches of vacuum at a HOT 850rpm idle. 18 is better. 19 is ideal. Watch/listen for detonation when adding in timing.

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    anyone in the hillsboro area wanna come help me find out whats wrong with the car?

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