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Thread: back with new problem on 90 camaro 3.1 v-6

  1. #1
    Fuel Injected!
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    back with new problem on 90 camaro 3.1 v-6

    Have a fuel related problem with my 1990 camaro 3.1 v-6 auto trans car. Drove car to library last Monday. Came out and engine would start & run for about 4 seconds then die. Would repeat this behavior on repeated start attempts.

    First thought was security system failure. But when key turned on security light cycles on then off like it should. Used scanner and security checks out with it also.

    Ignition system is fine. Good spark. Pulled fuel rail, attached spare distributor which I hand turned with ignition on. All fuel injectors spray with a good spray pattern when dist is turned. Fuel pump also runs for key on prime and when dist is turned. Fuel pressure at key on prime is 43 psi.

    Can run engine if I spray gas into throttle body. With scanner hooked up in data mode, all sensor readings are within normal limits except injector pulse width. Normal pulse wdith at idle usually runs about 1.7 to 1.9 . With engine not wanting to run unless gas sprayed into tb, pulse width is running at 1.1 to 1.2 .

    Here's the crazy thing. Last Thursday, I came out started car and it ran normally, including inj pulse widths running in the normal 1.7 to 1.9 range at idle. Had not done anything to engine to account for this change. Engine ran and car drove fine.

    Went out the next morning, started engine and it was back to dying after 4 seconds run time.

    I've installed new spark plugs, swapped in known good iac, 2 other ecm, with no change. Checked power supply wires and ground wires and all is good. Engine still starts and runs for 4 seconds and dies.

    I need some ideas on where to take my investigation next.
    Last edited by cocobolo95; 06-16-2015 at 03:25 AM.

  2. #2
    Fuel Injected!
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    What is the fuel pressure doing? Is the pump priming with key switched to on and the car is running on that initial prime? With many GM vehicles, the fuel pump runs off the oil pressure switch. When the key is first turned on, it primes the pump while the motor has zero oil pressure, then relies on the oil pressure switch to continue to run the fuel pump while there is oil pressure. My guess about the pulse width being so low might be that the pulse width is calculated from RPM and since it isn't running the RPM is lower than idle. Good luck! Let us know how it turns out!

  3. #3
    Fuel Injected!
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    Fuel pressure is 43 psi @ key on prime. And when idling it is 36 psi, just as it should be. There's no problem with fuel pump running when engine is running. It's just that pulse width of injectors are too low to keep engine running without spraying gas into tb. I have the carpet rolled back from gas tank area, trap door for pump area is removed. I can actually hear the pump running with engine running.

  4. #4
    Fuel Injected!
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    Have a question. Are the injector drivers in this ecm part of a quad driver circuit? And if yes, what other components are on this quad driver?

    The injectors of 1990 camaro 3.1 are batch fire with 2 circuits, 1 for each side of engine.

  5. #5
    Fuel Injected!
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    Is it possible to get a datalog of the car starting up and running, then a datalog of the car trying to start? Are the injectors batch fire on that engine? Maybe putting a scope probe on the ECM side of the injectors to make sure they have full voltage swing? Does the car have VATS?

  6. #6
    Fuel Injected!
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    My scan tool reads sensor data but does not save it. I have read data when car is running normally. Have read data when assistant sprayed gas into tb. Big difference is pulse width of injectors. When normal pulse width is 1.7- 1.9 @ idle. When you open the throttle pulse width increases.

    When car starts, runs, dies, pulse width is 1.1- 1.2 @ idle and stays there when throttle is opened. Injectors are batch fire.

    Car has vats. It cycles normally when key turned to on. And scan tool reads vats as being good.

    I asked in post # 4, if injector drivers were part of a quad driver circuit in ecm. And what other drivers share the same circuit.

    Have scoped the injectors and wave form doesn't change when car is in start, run die period.

    As stated in original post, last thursday, came out car started & ran normally. Drove car, off and on, all that day. Data on scan tool was normal. Went out the next morning and car did start, run, die again.

    Car starts easily. One chug of starter and car runs. This is both when normal and problem occurs.

  7. #7
    LT1 specialist steveo's Avatar
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    if it was a quad driver issue; you wouldn't have a predictable failure mode like 'starts for 4 seconds then dies'.

    just a theory, but it's on prime cycle (for initial startup) in a primitive fueling mode based just on temperature/tps/rpm, then dies when it switches to speed density or maf or whatever. does it even have a maf? i don't know much about the 3.1...

  8. #8
    Fuel Injected!
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    Engine has no maf, runs speed density, and uses batch fire injector strategy.

  9. #9
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    with the MPFI 60V6 code, unless you disable a certain flag, idle and coastdown fueling is actually done more like a hybrid alpha-n calculation. TPS and IAC are absolutely critical to proper operation in this mode. I would take a look at those two items to make sure they're operating properly.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  10. #10
    Fuel Injected!
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    Tps is functioning correctly, I will take a look at readings new iac gives. But when I have an assistant spraying gas in tb to keep running, if I press throttle down, fuel injectors still don't increase pulse width. I've had engine up to 3500 rpm in park by increasing the sprayed gas amount.

    As stated in earlier post, last thursday car ran fine all day. Injector pulse widths were back to normal # idle and increased as throttle was opened. Next day car again started, ran, died.

  11. #11
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    you have an intermittently bad sensor. opening the throttle should have increased pulsewidth through more than one function(higher MAP/higher TPS, both of which should have been read as a large increase in airflow and require more injector on-time to keep the desired air/fuel ratio.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  12. #12
    Fuel Injected!
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    When my assistant sprays gas in tb, and I open throttle, tps gives smooth change in open %, map reading change with change in throttle position also. What doesn't change is injector pulse width. And 3 different ecm's show same symptoms.

    Checked iac counts, around 30 at idle, increasing a little as throttle opens then drops down in counts when throttle opens wider. Goes back up in counts when throttle closes.
    Last edited by cocobolo95; 06-16-2015 at 06:56 PM.

  13. #13
    Fuel Injected!
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    Decided to check my injectors again. Last time all 6 had great spray patterns, didn't leak when closed. This time I had 2 that dribbled when closed, 1 that wouldn't close and sprayed all the time, and 1 that wouldn't spray at all, wtf. Here goes another $300 for parts that I'm 95% convinced won't solve the original problem.

    Going to try cleaning my 25 year old oem injectors. Have an ultrasonic cleaner that I will use. After cleaned or new injectors installed and car still won't stay running, will start tearing the wiring out looking for opens & shorts.

    Any member have a set of new 15lb injectors for sale at a reasonable price?
    Last edited by cocobolo95; 06-19-2015 at 02:46 PM.

  14. #14
    Fuel Injected!
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    I had similar problem with a subaru years back, would run for 5 sec then die ,instant restart ,5 sec die...
    the short story ; after 5 sec voltage source (for what i don't know) changed from batt to alternator, which was toast.
    Just something to check off the list.
    Nice test with the spare dizzy , I'll have to remember that one.
    "I tried mediation... I tried medication... Then i realized what i really needed was more Horsepower!"
    - = 1984 Corvette CFI - 700R4 - 7427 $0D - 18psi - ex262H-14 = -

  15. #15
    Fuel Injected!
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    System voltage shows normal reading at gauge and scanner. I thought about alt putting such a heavy load on engine and making it die. So I unplugged alt, car still died. Removed drive belt also to see if power steering was killing engine. Won't run more than 4 seconds with accessory drive belt off.

    Good suggestion, however. Keep those ideas flowing.

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