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Thread: Frustrating Intermittent Misfire

  1. #1

    Default Frustrating Intermittent Misfire

    I've been battling a very intermittent "misfire" all season in my Holley HP EFI RAMJET 350 setup. I have a "TPI" type setup, I.E. GM HEI Small Cap, remote coil, with computer controlled ICM, Holley 36 lb injectors, 43 PSI fuel pressure, etc. While intermittent, I.E. it can go days without happening and then sometimes in a single drive happen 4-5 times. It seems to be a part throttle acceleration, so typically say when pulling out from a light/stop sign and then going through the gears at mid throttle level. It usually one good "kick" then everything is fine. I managed to capture a datalog today of a typical one. Conditions were:
    TPS 58%
    MAP 83 kPa
    TPS RoC and MAP RoC 0 (This occurred you could say halfway in my part throttle 3rd gear pull.)
    CTS 189°
    RPM 3817
    MAT 97°F
    Advance 32.3°
    Fuel 40 psi and steady as a rock.
    AFR left 12.6
    Knock level bouncing around but all low I.E. 19%
    Closed Loop/CL Comp -1%.
    Fuel Flow 97.6 lb/hr Base Fuel 96.4.

    Everything looks great and all of a sudden this kick. I haven't had it happen to the best of my knowledge at idle or WOT. Seems like it always occurs in the steady acceleration in 2nd/3rd if it is going to occur and usually into the pull so AE has already played out, etc. I get a big kick, tach swings down, you can see AFR then goes lean, CL Comp goes to max (15%), but all these actions seem to be occurring in response, not the instigator.

    I'm thinking it was ignition. I've already methodically changed distributor, coil, plugs, wires, it's just so strange. I don't think this is fully the case, but it does like seem to either happen in a drive (I.E. A couple of times, say every 3rd or 4th time I go through the gears), or basically not at all and it can be good for several days and then happen again. This seems like a very steady operation area and out of blue this hiccup happens.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  2. #2
    Join Date
    Dec 2009
    Location
    Connecticut
    Posts
    28,767

    Default

    I always recommend using a CD ignition box with distributor type ignition systems. Here's why: LINK.

    Defective ignition modules - common problem with MSD 8366 distributors.
    Replace the ignition module with a genuine GM ignition module.
    (This same rationale applies to Ford TFI distributors too.)
    ‒ Ignition module for MSD 8366 Pro-Billet Distributors:
    GM 19179578
    ACDelco D1943A
    http://www.rockauto.com/dbphp/x,cata...CO_D1943A.html
    http://www.msextra.com/doc/ms1extra/...EI_modules.jpg
    ‒ I'd also verify the GM HEI module polarity is wired correctly:
    https://forums.holley.com/showthread...ity#post120395
    http://www.megamanual.com/ms2/GM_7pinHEI.htm
    ‒ Apply heat sink (thermal conductive) paste on the bottom of the module before installing it.
    Often times, dielectric grease is wrongly used and will lead to an overheated (failed) module.
    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, Holley Dominator MPFI & DIS, 36-1 crank trigger, 1x cam sync, 200A 3G alternator, Optima Red battery, R134a A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans, Frostbite 3-core 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, BMR upper & lower torque box reinforcements, LenTech Strip Terminator wide-ratio AOD, 3000 RPM converter, B&M Hammer shifter, Stifflers trans crossmember & driveshaft safety loop, 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.

  3. #3

    Default

    Thanks, I've thought about adding that and also while I'm at it a Dual Sync Holley distributor.

    I guess in full disclosure the reason this is extra frustrating for me is I have over 10 third Gen Camaro and Trans Ams and while a couple have LS conversions, the rest either have factory TPI or a version of factory TPI with a modified factory controller (flash added). I don't have problems like this with any of those '80s era ECMs. Sure I get an occasional ICM that fails, but generally it fails dead.

    Holley is a great in that it beats burning PROMs, I love the Pro Dash, and all the flexibility. I have to say though so far the '80s era ECM is beating it though for driveability with the standard setup.

    I'm doing another LSx upgrade this winter and was thinking of standardizing all my projects around Holley HP. And I still want to, but I'm getting a little nervous about it, as my other swap with OEM E38s have worked great. Like I said, I really like all the Holley extras, I just want to be confident in the basics though. Over the next week or so, I'll get the CD box in along with the Dual Sync while I'm at it. Maybe Holley HP core just isn't robust with the old ICM type interface.

  4. #4

    Default

    If RPM is lost Closed Loop will go to 0%. It doesn't here, so we can likely eliminate crank and cam signals. This sounds ignition related to me. Try doing a wiggle test (not spark plug wires). Take a look at your plug wires for carbon tracing.

  5. #5

    Default

    Do you have any vacuum accessories on the engine such as PCV, brake booster, etc? I had an engine with a PCV valve with some sort of weird intermittent failure that acted like that. 90% of the time the engine ran fine, every once in a while it would open up and the extra vacuum would change the MAP setting enough to upset the tune.

    The problem with self-tuning systems, is that if they get fooled by a bad sensor they'll take you somewhere where you don't want to go. I also had a MAP sensor once that was intermittent. That thing really messed with my head.

  6. #6

    Default

    Quote Originally Posted by allan5oh View Post
    If RPM is lost Closed Loop will go to 0%. It doesn't here, so we can likely eliminate crank and cam signals. This sounds ignition related to me. Try doing a wiggle test (not spark plug wires). Take a look at your plug wires for carbon tracing.
    Yes, seems like ignition to me as well the intermittent nature is what's made it extremely challenging. Since it's TPI style the wiring is quite simple. Basically a couple plugs 4 + 2 to the ICM and then 2 plugs on the coil 2+2. I'll give them some additional wiggle and also double check the ground to the block. I was worried Holley was maybe losing sync with the RPM signal from the ICM, but doesn't appear that's the issue as you say.

  7. #7

    Default

    Quote Originally Posted by AndyF View Post
    Do you have any vacuum accessories on the engine such as PCV, brake booster, etc? I had an engine with a PCV valve with some sort of weird intermittent failure that acted like that. 90% of the time the engine ran fine, every once in a while it would open up and the extra vacuum would change the MAP setting enough to upset the tune.

    The problem with self-tuning systems, is that if they get fooled by a bad sensor they'll take you somewhere where you don't want to go. I also had a MAP sensor once that was intermittent. That thing really messed with my head.
    Yes, thanks for the thought. I do have brake booster, vacuum regulated FPR & PCV. At this point it didn't seem to me that this was likely the cause for a couple of reasons. MAP looks steady, fuel pressure which is referenced to vacuum, is steady. Both those gave me the thought that it wasn't anything to do with these systems.

  8. #8

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    This seems real simple to me, unless I'm missing something? Tach is hooked to ignition module. Tach goes to zero yet Holley does show RPM. Ergo, it's a bad ignition module.

  9. #9

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    Well here's my thinking on it. I did already completely change the distributor which also has a new ICM (not saying that both couldn't be bad, but usually when I've had bad ICM it generally goes dead pretty fast, I have been living with this for months on both distributors with their own ICMS).

    Also in this setup the connection to the ICM which is ECM controlled, I.E. goes to Holley. I'd think if that was the problem, besides seeing the tach move, which is feedback from the coil you would also see the Holley ECM go to zero. It doesn't it, it seems to follow the RPM blip.

    I have plenty of ICMs, I just didn't go there again with another new one based on the above thoughts.

  10. #10

    Default

    This is an 8-pin HEI module, right? The signal that goes to the ECU from the ICM is not the same as what goes to the coil. It's much earlier in the circuit and is just a cleaned up signal from the pickup. The ECU sends a signal back to the ICM when it wants it to fire the coil. The ICM then grounds the coil to create the spark and the tach is hooked to this wire as well. In your case I can't see any other answer than that the power stage in the ICM that fires the coil is bad. Nothing else explains the ECU reading RPM, but tach goes to zero.

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