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Thread: When cold runs good- Warm Runs Bad?

  1. #1
    EZ
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    Question When cold runs good- Warm Runs Bad?

    I have 75 Z FI. I runs very good when it's cold but the engine stalls and hard to restart when hot. I know that its running rich because I blow black clouds when I dig my spurs into it, but the more I lean out the AFM the hotter the engine runs and thus the worse the problem becomes aftr warm up. I have a new distributor, wires, cap, rotor, intake/exhst manifold gasket, no appearent vacume leaks, new PVC, PVC hose good. Runs great but the time I get work (14 Miles) I'm fighting to keep it running and heaven forbid it cuts off, because it will be hard to restart. I have to disconnect all of the injector terminals, run out the excess gas and then I can usually get it running again. I've only had the car for a few months and have never driven it more than 15-20 miles but I'm pretty sure that it would over heat if I had to stop in traffic for more than 20 minutes or so. Also I'm getting horrible gas mileage (about 9-10 MPG). I'm having a rough time figuring this one out.

    Other:
    Firing order is OK.
    Injectors all seem to be firing.
    Tried changing the Coil.
    New Fuel Filter.
    Newly sealed tank.
    Fuel pump is hot-wired on all the time.
    Shallow Ed's 76-280z

  2. #2
    Old Z Guy LanceM's Avatar
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    I'm not a FI guy but I would suspect the fuel pressure regulator.
    Lance

    73 240Z, tripple webbers, 5 speed, 4 wheel disk, Einbach springs
    98 BMW 540i/6

  3. #3
    Former frequent poster sblake01's Avatar
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    Check the thermotime switch/cold start valve circuit. If the thermotime switch is bad it can keep sending the signal to the cold start valve even after the cars starts which will cause it to run very rich. Also, of the cold start valve leaks it will cause richness.
    2004 Ford Ranger EDGE Supercab
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  4. #4
    EZ
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    HMMMMM haven't swapped that out yet! Could it be that simple? I think I have an extra I'll try and jam that in tomorrow & see what happens.
    Shallow Ed's 76-280z

  5. #5
    EZ
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    I know that my thermotime switch is bad cause when I connect it I get erratic idle and cutoff. right now it's disonnected. How does that register with the computer? Is it sending rich signals to the injectors?
    Shallow Ed's 76-280z

  6. #6
    Former frequent poster sblake01's Avatar
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    No, all the thermotime switch does is send a signal to the cold start valve when the engine temperature is cool enough to need it. Kind of like a choke. It gets a signal from coolant temperature and the starter. If its cold enough it will send the signal to the cold start valve to help the car start. If the thermotime switch is bad it will either never send the signal or send it all the time depending on whether its shorted open or closed. On the other hand if the cold start valve is bad it can leak also causing the car to run rich. With the thermotime switch off the car (or with the engine cool, check it's two terminals for continuity. It should have continuity when cold and none when hot. If it has continuity when hot then it's bad. The cold start valve can be tested by unsrewing it and lifting it out of the manifold. Place the end of it in a small container like a spray can cap and with everything else still connected (a good thermotime switch and the fuel and electrical connection to the cold start valve) start the engine. When cold if the cold start valve is good you should see a spray of fuel come out of it and the stop. When hot nothing should come out of it.
    2004 Ford Ranger EDGE Supercab
    (@Moonpup: This one really is an EDGE!)
    2005 Pontaic GTO
    2010 Mercedes Benz C300 AMG Sportline (Wife's car)
    2014 Kia Rio LX (Wife's daily driver)
    Certified HVAC/MVAC Technician

  7. #7
    EZ
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    Sorry I was confusing the cold start valve with the air regulator. I have the cold start valve disconnected as well as not to add more fuel to the fire. I live in Florida so cold starting is not a big problem. I do not see any noticable difference in starting or running when the cold start is connected/disconnected. Do Zs vary the duration of injector spray? And if so what governs that duration?
    Shallow Ed's 76-280z

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