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Thread: 94 Corvette LT1 Troubles

  1. #16
    Fuel Injected!
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    30lb ford style red tops

  2. #17
    LT1 specialist steveo's Avatar
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    the opti isn't just for spark, it's for engine position reference, which means injector firing is also based on that sketchy sensor. a badly messed up opti sensor will show up in a datalog as wild rpm flucutations.

  3. #18
    Fuel Injected!
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    Try turning the key on with the engine off, and wiggle the optispark 4-pin harness on the passenger side. If you hear the injectors clicking, this is indicative of a bad harness. They do fail, and you can buy replacements cheap on Ebay. There's a reason most optispark vendors now include that harness.

  4. #19
    Fuel Injected! beasleyrb's Avatar
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    I just spent 6 months finding a problem very similar to yours. I don't know if your car has the MAF Burn Off Relay and the MAF Burn OFF control Relay but my 88 Corvette does. I had a bad MAF Burn OFF Control Relay. Bad contacts on the relay resulted in low voltage. I didn't find this. A troubleshooter with Oscilloscopes, tools and over 25 years of experience did. It took him two days to find it. I checked very wire in the loom, every sensor to ground, every voltage at the ECM and replaced about 90% of the parts on the engine that have to do with fuel supply and things that control the injectors, etc. You name it, I checked it per the service manual or replaced it out of frustration. I finally gave up and had this young fellow come trouble shoot it. He has all the equipment and the experience. No one wants to be a part changer but it could be the most obscure thing you can imagine. I doubt I ever would have found this because I thought I had Good MAF reading on my scan tool. Again where the experience comes in. I feel your pain my friend. I hope you find the problem and report back so it may help others.

  5. #20
    Fuel Injected!
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    The LT1 does not use a MAF relay of any kind, unfortunately.
    1990 Corvette (Manual)
    1994 Corvette (Automatic)
    1995 Corvette (Manual)

  6. #21
    Fuel Injected!
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    I finally got some time to play with this again today, I actually do have a few codes here now. One in the CCM that is H41 Loss of ECM serial data. and the other in the ABS module H72 Serial data link fault. I know the one in the ABS has been there intermittently for a while, sometimes my traction control light comes on, other times it works fine so I am not sure if that issue is related to my current problem. As to the CCM H41 code, Am I correct that the CCM has nothing to do with fuel and spark after it tells the ECM its OK to start? That leaves the ECM in charge of running things correct? Could that leave me with the ECM itself being bad? I have flashed it about 100 times over the years, and I can still read and flash it now. Its an expensive WAG part. Not sure how I should proceed at this point. Help appreciated. Thanks.

  7. #22
    Fuel Injected!
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    Both of those codes are historical codes set by connecting to the PCM and flashing. When you shut up the bus and interact with the PCM directly it causes a loss of serial data to other modules. This is expected. Those codes can be reset from the dash and will not come back until the next time you connect to the car and shut up the bus.

    But yes, the CCM has nothing to do with controlling the engine. All it can do is disable the fuel pump relay (though not permanently; the fuel pump is also tied into the oil pressure switch) and not send the VATS activation signal to the PCM (which will kill the injectors if the PCM is set to operate with VATS).
    1990 Corvette (Manual)
    1994 Corvette (Automatic)
    1995 Corvette (Manual)

  8. #23
    Fuel Injected! spfautsch's Avatar
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    Are cranking times fairly long or does it pop right off when you start it?

    Sounds a bit like a failing hi-res signal - particularly when you mention it started manifesting symptoms when hot and they disappeared after cool-down. How old is your opti?

  9. #24
    Fuel Injected! beasleyrb's Avatar
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    This isn't going to help much but I feel your pain. I had a problem evade me for 5 months. It turned out to be a relay in a circuit I never considered could be a problem. Caused the ecm to get a bad signal. Not no signal but off enough (low voltage) to screw up what the ecm thought it should be doing. I guess I'm saying consider EVERYTHING in the fuel and ignition system. Something changed and your engine performance went south. It's probably a sensor or 5 volt signal back to the ecm. Like I said, this wasn't going to help much.

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