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Thread: Frustrating Intermittent Misfire

  1. #11


    You're correct, 80s style 8-pin ICM mounted on the base of the small cap GM distributor. I appreciate your thoughts, would be great if it's that simple and I'll definitely give it a try if I don't get anything out of Allan5oh comment on the wiggle test. I had looked at the wiring already, but wondering also based on your comment as well, could the 2-pin ICM output to the 2-pin connector on the coil have some intermittent and therefore be giving me still good ECM RPM, but cause the actually tach bounce and also the misfire. This wiring was new with the RAMJET, so it isn't old but who knows maybe bad crimp or something.

  2. #12


    Well I did try and do some movement of the wiring that was on the ICM and coil while the Camaro was idling in the driveway. It didn't seem to see anything obvious, but I decided to take the 2-pin to 2-pin wiring harness off between the ICM and the coil to have a better look using an Ohm meter since that's an easy one to remove and check.

    This wire harness from the 2-pin to 2-pin came as part of the RAMJET package. I was very surprised when I Ohm checked the red wire that instead of showing the expected low Ohm "short", i.e. good wire, it read about 7 Ohms. I took the protective tubing off and was surprised to see that instead of just a wire running from one connector to another there was a splice in it. When I opened the splice up there is a resistor in series between the two connectors.

    All the TPI jumper coils and circuits I've ever dealt with going from ICM to coil always have just two straight wires through. Since I have a lot of spare parts from all my 3rd Gens I put an old ICM to coil jumper wire on. I have run about 60 miles tonight without hiccup, but since this is so intermittent I'm not declaring an finding yet at this point

    Clearly the resistor in this GM RAMJET jumper is intentional. I did not use the supplied RAMJET coil as the mounting base was not a good fit for me and instead have been using a typical remote coil from a 80s Camaro. When I Ohm checked the primary they both seemed to be basically the same. Maybe something is different about the ICM in the RAMJET distributor, I haven't looked up what ICM that is. I did though have the bucking with that distributor also though before I put the DUI unit in I'm now running with their ICM.
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  3. #13


    Well so far after 150 miles with the traditional TPI jumper, everything is working well, but too early to tell. After thinking about it and try to find a real diagram of the 8-pin ICM which I haven't really found, considering the resistor is in the '+' wire, GM must have added it. I believe to act as a current limiter in case the ICM module were to fail instead of blowing the fuse is what I think it's there for. I also think that I can't be having an issue on that wire as if I did, I believe it supplies power to the electronics that sends the reference pulses back to the Holley so I think I'd see that reference go to zero during the issue. Of course if the other wire was intermittent it definitely would cause an issue similar to what I'm seeing as the coil primary wouldn't get its pulse, the tach wouldn't either, but the reference pulses would continue back to the HP. So I believe this could potentially be the cause of the issue, but we will see.

    I'm avoiding replacing anything else, i.e. the ICM again until I see if the issue comes back as I'd like to know what the issue is. Also knock on wood the issue has always been intermittent vs engine dies. This has also been one of the reasons I haven't been convinced it's likely the ICM as in my past experience with ICM issues either I have had a truly dead no start until replaced or occasionally car dies and after it cools down it has come back to life for a little while.

  4. #14


    So today my crazy misfire came back (I was out of town over the weekend, so didn't drive since Friday). I was about to try replacing the ECM to ICM wiring, but decided to replace the ICM again since it's easy to do as one last try before more radical efforts like the wiring. I was a little disappointed with the DUI module when I took it off as they appear to use just a silicone grease vs heat sink paste. I cleaned that up, applied a new Delphi module with heat sink paste and fired everything back up. Everything is good after a quick drive, but of course that doesn't really mean anything. I'd be delighted if this was it as I had ruled it out, since I had replaced the entire distributor, but we will see.

    Also the RAMJET jumper wiring with the resistor in it still seems very strange to me and not sure of its exact purpose. I left the straight wiring original TPI type jumper in when I put the Delphi ICM module in.

  5. #15


    Well I got the misfire back even now with the new ICM module, so at this point I'm convinced either I have a very strange intermittent wiring issue, or there's something going on with the Holley HP output either due to hardware issue or some weird software interaction/configuration. The strange thing is while I'm not completely convinced of this yet, it does appear more and more either the issue is there in a specific key on drive or not. I.E. I've been able to drive around a lot with no issue then fill up at the gas station and all of a sudden it's there. last night I replaced the ICM and went for a long drive and no issues. This morning driving to work it was there. If I had time I was half thinking about pulling over and shutting off and turning back on. Even when it's there it's very intermittent, i.e. it might happen once every 5 to 10 miles and it happens just once at a moment in time, never a series of them.

  6. #16

    Default Strange System Log crank position captured during misfire event.

    As I've posted in this thread I've been chasing a strange "misfire" that is very intermittent on my RAMJET 350 that utilizes GM HEI ICM remote coil, i.e. TPI type ignition.

    I've been chasing everything and changed everything in the ignition with no effect on this random misfire feeling type event. it happens very infrequently to the point that after i have changed something like the ICM, the distributor, plugs, wiring to the ICM, coil, plug wires, etc. It seems like the issue is gone and then all of a sudden within a day I get this lurch/misfire that happens and immediately goes away. It happens enough that you would normally probably be thinking, oh no is this about to die and leave me stranded, but the strange thing is it just happens and then goes away. Sometimes in a 25 mile drive it'll happen a couple of times, sometimes it won't happen for one or two days.

    I finally captured a System Log and the crank position calculation by the Holley, looks strange to me during this event. Look at just after 47.9. I'll try and get the datalog posted up. The computer I use for datalogging/tuning isn't connected to the internet. All the crank signal pulses are there (didn't plot these so that the plot was readable).

    How can it be right that the crank position is counting fine and then all of a sudden starts counting up and decides to stop & start over? If this happens does the ignition or fueling event momentarily stop and seemingly cause a momentary misfire until the Holley resets itself.

    Anyways just thinking out loud and posting this as it certainly looks strange to me. Also seems more than a coincidence that the only time it happens in the datalog is when I have one of these issues. Now the problem is I don't think I can do anything about it myself!
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  7. #17


    Attached here is my .sl System Log file of the above event and my current tune file. I had to .zip them it seems to get the forum to allow them to be posted. As I noted, above look in the 47.9 section and you'll see what I'm talking about.
    Attached Files Attached Files

  8. #18


    I used to run an 8-pin HEI in my C3 'Vette but removed it because it got too hot. In my case, I'd be driving down the road and all of a sudden the engine would just shut off. Coast to the shoulder, wait 5 minutes and it'd fire back up and drive fine for a bit again. Tried different brand ICMs and all did it.

    Maybe yours is getting hot also? Not quite hot enough to shutdown, but hot enough to glitch. I have the Dual Sync now and no heat issues.

  9. #19


    I appreciate your thoughts Zwede. I've tried three different new ICMs, a GM original with the RAMJET, DUI, and replacement Delphi (as well as swapping distributors. I have a lot of experience with ICM and know they can go bad. I really don't think this is the case (I have many of these 3rd gen F bodies and never have had an issue like this, but of course there is always a first)

    I believe that the fact i captured the crank position resetting itself in the Holley software is an indication something is wrong with their logic, I.E. the ICM just sends it pulses, but I'm not yet familiar enough with the System Logs to know exactly what they're recording.

    Also there have been some comments in other threads of people and Holley techs not recommending any longer using GM HEI small cap/Ford TFI due to some unexplained troubles. Could it be there is a firmware issue?

    The fact I have replaced everything (including the wiring) at least once and the issue remains exactly the same leads me to more and more believe it's in the Holley. Due to the fact I have so many of these cars I'm already contemplating pulling the trigger and giving up on the Holley and going back to an upgrade version of the '80s style ECM. I'd be willing to make a very strong bet the issue would go away at this point.

    I'd like not to do that as the Holley has a lot of nice features, but I'm getting to the point I'm stuck.

    I agree with you point of maybe if i just put in a Dual Sync the issue resolves itself, but then I also need to add a CDI box, etc. Not opposed to that, but it just seems that this issue should be able to be resolved.

    I'm thinking of backing up to a much older firmware and see if the issue is resolved that way. I like working on these types of projects, but I have to say I'm getting tired of this.

    Here's a screen snip that better points it out. You can see that the Crank pulses from the ICM remain nice & steady, and all of a sudden in the circled red area the Holley Crank position calculation decides to reset itself. Also then end itself up at the top before "normal" as the other position saw tooths. I'm obviously not an expert on their EFI software or these System Logs, but this doesn't seem right and I suspect is leading to my issue.
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  10. #20
    Join Date
    Dec 2009


    (Alan91Z28, PLEASE don't type your posts like you're texting. It takes me a long time to edit it. At least use Spell Check before posting.)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

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