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  1. #1
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    Default Backfire/Misfire, Part Deux...

    So, some of you know of my recent Optispark fiasco. . . After the last Opti went out, I pulled it, got a refund, and installed a known working Opti from my parts pile. Sure, it's not the ideal situation, but I couldn't really afford to blow $500 on a new Delphi part, so this worked.

    Anyway, when the last opti died, it left the car dead, and I didn't have the motivation to do any more work to it, so it sat for about 8 months. Now it runs, but has a pretty bad random misfire, and backfires on acceleration. I'm not sure what is causing it, but it's not just under load, so I'm leaning toward either a fouled plug (likely, since when the opti went out, I was driving down the road, car is a manual trans and was in gear, so it "dieseled" until I got it out of gear and off the road) or a bad plug wire. Plugs and wires were replaced last summer, car ran great before the optispark failure. No codes, O2's were replaced when I did plugs and wires, timing chain was replaced at the same time, as was the starter. Compression check on all 8 cylinders were within 5psi +/- of 192psi. Haven't done a leakdown test yet, although I'm not ruling out anything, since I've had 4 opti failures on this car, and an alternator that was bad out of the box from the same auto parts store.

    The car is a '94 Formula LT1/T56 with 164k miles, only mods are K&N CAI, Magnaflow Cat-Back, and BBK Stainless "Shorty" Headers. Looking for different possibilities to check out, I'm leaning toward ignition, since the car is running rich, and the misfire is random, but consistent (doesn't always happen on one particular cylinder, but it is consistent in that it doesn't resolve itself at a certain load or RPM level). A friend of mine suggested a clogged cat could be to blame, but given that the car ran fine before all of this, I'm hesitant to believe that the cat could have failed, when a fouled plug could just as easily explain the symptoms.

    Anyways, if anyone has any suggestions, I'm all ears, and I'll likely be doing some wrenching on the car this week to attempt fixing the issues. Thanks in advance!

    1994 Pontiac Formula-PPM, LT1/T56, BBK Stainless Shorty Headers, Magnaflow cat-back, 17" C6 5-spokes.
    Coming Soon: Full UMI tubular suspension, UMI STB, Eibach ProKit coils, KYB AGX adjustable shocks, and a tune.


  2. #2
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    With the opti swaps, have you changed the extension harness between main harness and opti? Have you looked at icm and coil? Clogged cat is possible, but it usually smells like sulfur melting.
    2021 Charger Scat 392
    1994 Firebird Formula 381 stroker - Carrying the torch! - 9.90 @ 134.3mph on a 200 shot

    The wrench/driver for LTConvert's 94 Z28- Ellwein 383/LE Trickflows/ D1SC / CPT Ultra Pro Race 4L60E
    10.78 @ 125mph. Shooting for 9.99 for Frank Cahall!

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  4. #3
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    Power house has ac delco opti for under 400

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  6. #4
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    I second what tubby said about the ICM and coil. My car went nuts when the ICM went out(for obvious reasons)


    383, Eagle crank, scat rods, Mahle pistons, 238/248 .565 cam. 3k stall, AFR 210 heads, Pacesetter LT, 58 MM TB and a few other goodies.

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  8. #5
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    How were you able to determine that the misfire is not always the same cylinder?

    Misfires cause it to run rich, and can contribute to rapid cat damage.
    SOLD - GONE TO A (VERY) GOOD HOME ! - 94 Formula A3+1: 381ci forged stroker - Callies Stealth, Oliver 5.85 billet rods, BME nitrous pistons / CNC LT4 heads / CC solid roller / TH400+GearVendors OD / 4.11 Strange 12-bolt / 300-shot N2O / Spohn Suspension / roll bar / MoTeC M48 Pro engine management system /a few other odds 'n ends.

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  10. #6
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    Quote Originally Posted by TubbyFormy View Post
    With the opti swaps, have you changed the extension harness between main harness and opti? Have you looked at icm and coil? Clogged cat is possible, but it usually smells like sulfur melting.
    No sulfur smell, the exhaust smells like fuel, and is strong enough to irritate your eyes if you stand too close while the car's running. The opti harness has been replaced every time I replaced the opti, that was my first thought when I first started having issues with the car backfiring.

    Quote Originally Posted by spikester7878 View Post
    I second what tubby said about the ICM and coil. My car went nuts when the ICM went out(for obvious reasons)
    Coil was replaced last Summer. I have a spare, known-working coil in my parts pile at home, but the ICM is something I don't have a spare of. I'm fairly certain I can take the ICM to one of the parts stores to have it tested. I can do that this morning if you all think that may be the problem, it only takes a couple minutes to remove, and I'm fairly certain it's not terribly expensive to replace. . . Just want to verify before I start throwing money at the car again, I put probably $3k in parts into this car while I've owned it, and the whole idea of throwing money at the problem in hopes that it will be resolved simply isn't a possibility right now. . .

    Quote Originally Posted by Injuneer View Post
    How were you able to determine that the misfire is not always the same cylinder?

    Misfires cause it to run rich, and can contribute to rapid cat damage.
    I haven't been able to determine that it's not always the same cylinder. The plan for today was to pull the plugs after work to see if it's always one cylinder. I really wish I wouldn't have forgotten my scan tool and infrared thermometer, as they would have been EXTREMELY useful in diagnosing this issue, but they are 60 miles away, and I won't be back out that way again until the end of the week.


    I'm going to go pull the ICM/Coil, and I'll stop at Autozone on my way to the office to have them tested. If the ICM is faulty, I'll go ahead and replace it.


    Another symptom worth mentioning: The car doesn't backfire when cold, it only does so after the car's been running for a few minutes. Not after it goes into closed-loop, before it reaches operating temperature, but it presents as a misfire on cold start, and deteriorates into a full-blown backfire after the car has been running for 3-4 minutes.
    Last edited by 93formulalt1; 04-21-2014 at 08:49 AM.

    1994 Pontiac Formula-PPM, LT1/T56, BBK Stainless Shorty Headers, Magnaflow cat-back, 17" C6 5-spokes.
    Coming Soon: Full UMI tubular suspension, UMI STB, Eibach ProKit coils, KYB AGX adjustable shocks, and a tune.


  11. #7
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    Quote Originally Posted by 95albinobird View Post
    Power house has ac delco opti for under 400
    Is it in stock there? This is the non-vented opti, BTW, which usually costs a bit more than the 95+ Vented Optispark II. If we determine the optispark is the problem, I'll be looking for a Delco, and assuming it's in stock, I will purchase it from Power House.

    1994 Pontiac Formula-PPM, LT1/T56, BBK Stainless Shorty Headers, Magnaflow cat-back, 17" C6 5-spokes.
    Coming Soon: Full UMI tubular suspension, UMI STB, Eibach ProKit coils, KYB AGX adjustable shocks, and a tune.


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  13. #8
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    Default Backfire/Misfire, Part Deux...

    Quote Originally Posted by 93formulalt1 View Post
    Is it in stock there? This is the non-vented opti, BTW, which usually costs a bit more than the 95+ Vented Optispark II. If we determine the optispark is the problem, I'll be looking for a Delco, and assuming it's in stock, I will purchase it from Power House.
    They ship directly from delco to you when u go thru gail i believe. @AmericanPowerhouse
    2021 Charger Scat 392
    1994 Firebird Formula 381 stroker - Carrying the torch! - 9.90 @ 134.3mph on a 200 shot

    The wrench/driver for LTConvert's 94 Z28- Ellwein 383/LE Trickflows/ D1SC / CPT Ultra Pro Race 4L60E
    10.78 @ 125mph. Shooting for 9.99 for Frank Cahall!

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  15. #9
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    Quote Originally Posted by TubbyFormy View Post
    They ship directly from delco to you when u go thru gail i believe. @AmericanPowerhouse
    This^

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  17. #10
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    Another update, I took the ICM to be tested, and it tested fine. . . Bought a set of AC Delco Double Platinum spark plugs, since I went with NGK Iridium plugs last time, and I've been told by a number of auto enthusiast friends that they are junk plugs. . . I'll pull the old plugs this evening and find out what's going on. I replaced the wires last year also, but had to go with the cheaper wires, since I was down to one vehicle, and couldn't wait for Delco wires to arrive. I may replace the wires if the plugs are in rough shape or point to a single cylinder being worse than the others. I have to pick up a couple of things from my Mom's place (where all of my tools are at the moment), so I'll grab my scan tool and IR Thermometer this week, although I have a feeling fouled plugs may be the problem.

    1994 Pontiac Formula-PPM, LT1/T56, BBK Stainless Shorty Headers, Magnaflow cat-back, 17" C6 5-spokes.
    Coming Soon: Full UMI tubular suspension, UMI STB, Eibach ProKit coils, KYB AGX adjustable shocks, and a tune.


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