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Another Weird Problem...

This is a discussion on Another Weird Problem... within the General Help forums, part of the LSx Technical Help Section category; I seemed to have fixed my last problem (see This post) by replacing the ICM. For reference I have a ...

  1. #1
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    Another Weird Problem...

    I seemed to have fixed my last problem (see This post) by replacing the ICM. For reference I have a '93 LT1 Formula. I don't know if any of these problems are related, but I've got three of them.

    1. The tach reports a different RPM then my scantool does. Ex. Tach says 2000 rpms and scan tool says 1825. At idle, tach says 800, scan tool says 625. It didn't use to do this before. It started after I replaced the ICM, but since the other ICM is dead, I can't double check it.

    2. I get no codes about a bad TPS sensor, but at WOT, the scan tool reports the throttle angle is only 60%. I haven't visually checked it yet, but I will soon.

    3. This is subjective, but the car sounds different now (with new ICM). It might be related to issue #1. The car seems to be running richer then before and perhaps it's running at lower rpm even though the tach reads the same. Does anyone know if the dash tach rpm signal comes from a different source than the ECM tach signal?


    As a side note, when trying to troubleshoot my dead ICM, I've tried starting the car many times with no spark, which means I probably had plenty of unburnt fuel in my cylinders. I may have fouled my spark plugs temporarily - is this the reason I'm running rich now? Is it going to go away?

    Another thought, I do have some pretty old and cracked vacuum lines, but my pressure reads 22psi at idle at 24 at 2000rpms. Is that good? I can't find a solid answer to that....

    Thanks for any directions you can provide - the service manual doesn't cover these issues so I'm kinda lost on what could be wrong.

  2. #2
    Your friendly policeman FinZ28's Avatar
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    1) If I recall correctly, the tach reading isn't always exact, so being off by 100-200rpm isn't unusual. I'd definitely go by the reading the scan tool gives you. Hopefully somebody else will chime in with some better info.
    2001 Chevy Camaro Z28 A4
    SLP cold air intake/lid, SLP smooth bellow, Pacesetter LT's, ORY, Magnaflow exhaust, BMR STB, adjustable Panhard rod, SFC's, LCA's, 1LE sway bars, cross-drilled rotors

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    The service manual suggests that +-100 rpm difference is acceptable and it's currently at 175 rpm difference, but it didn't use to do that. In fact, it used to be nearly exact. If I go by the scan tool, then my idle is 625 rpms, which is pretty low. Ignoring the scan tool for the moment, the car does in fact feel rougher at idle then before. Do you think a new ICM could be causing any of my problems - low rpms, rougher running, running rich, more drone coming from the exhaust? Or is my problem a bunch of other misc problems?

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    What do you think of this theory: The Duralast ICM that I am using is not a GM reference design, it uses it's own proprietary chip. I have confirmed that the tach gets it's signal from the coil and the coil is controlled by the ICM. So, since I am getting a higher rpm from my tach than from the computer (and it didn't use to), this would imply that my ICM is firing the coil more then it is supposed to. Which might explain running rich and the extra drone I'm hearing.

    What do you think?

  5. #5
    Your friendly policeman FinZ28's Avatar
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    Sounds like something you need to look at, especially if these problems started out after the swap.

  6. #6
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    I just wanted to update you, I do believe my problems have been solved.

    I did end up replacing the ICM twice. The first time with a Duralast part, which worked, but it seemed to work too fast as my tach was reading 175rpms higher than the signal from the optispark. I than bought an AC/Delco ICM and it was much more accurate.

    Number 2 from my OP: I thought my TPS sensor was bad, but it checked out fine. I believe my problem here is a throttle cable with too much slack. I'll be checking into this more. At the moment, I can pull my throttle cable almost a full inch of slack from the throttle body before my pedal moves.....

    Last, number 3 from my OP: I discovered I had a bad spark plug. Judging from the amount of carbon build up on it, it must have been bad for at least a few weeks. Once I replaced it, my sound came back. It must have still been firing, just weakly, cause the car never ran bad....just sounded different.

    Thanks for everyone's help!

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