HELP! Engine Issue, can't diagnose!

[93formulalt1]

Hey guys. . .

I have a pretty serious engine issue I've been trying to track down for about 6 months now, and it's just gotten to the point that I can't figure it out, and I'm dumping entirely too much money into parts, with no results.

Here are the symptoms:
Spark-Knock under load
Backfire/Afterfire at low RPM's under high load
Slight stumble at idle

What I've replaced in the last 6 months to attempt to fix it:
Plugs & Wires
Header Gaskets
Optispark
Coil
ICM
Fuel Pump
Fuel Filter

Fuel pressure is good, no more exhaust leaks, and the optispark is good (Has shown same symptoms on 3 optisparks now.) I'm planning to replace Plugs and Wires again next week, but I have decided to walk away from the car for a couple of hours to prevent my frustration from becoming anger. EGR/AIR were deleted, but not tuned out yet, though I was told that wouldn't cause my issues. Car is drivable, but gets horrible gas mileage, is embarrassing as hell, and is down on power. Not as bad as it was before fuel pump swap, but still not 100%. I'm hearing a bunch of generic gearheads (knowledgeable about cars, know nothing about LT1's specifically) tell me it's a valvetrain issue, or timing issue, or anything other than what it is. I know this is not an internal engine issue because 1) the problem didn't happen "all of a sudden", it started small, as a hesitation, and became worse and worse, until it reached the point it's at currently, and 2) Everything I've done so far has helped to lessen the problem for a while, or reduce the severity of symptoms, which would remain largely constant if it were an internal engine issue.

The car is a '94 Formula LT1/T56, BBK Shorty headers (so pulling #6 & #8 spark plugs is impossible, IMO), Magnaflow Cat-Back, stock Y pipe and cat, cheap ebay CAI, and Accel Coil. Car is otherwise stock, except for aesthetic modifications.

I'm looking for either help diagnosing the car in person (I'll come to you, anyone who is close to Belleville and has time, between now and tomorrow afternoon), over the phone, by forum posts/PM's, or otherwise. I'm just trying to rule out any parts I can, so I don't needlessly spend more money. . .

All help and suggestions are appreciated!

[Darren]

So did you replace plugs 6&8?

[1BADAIR]

will it make it to higher rpm? exhaust sound funny? convertor rattle at all?
change those two plugs then check the cat

[s.reef]

Sounds like plugs to me

[93formulalt1]

will it make it to higher rpm? exhaust sound funny? convertor rattle at all?
change those two plugs then check the cat

No converter rattle, exhaust sounds fine, and it will run all the way through the RPM range just fine. Spark knock is mostly at lower RPM under load, loudest when taking off from a stop and attempting to accelerate in higher gear (I.E. trying to go from 70-75mph in 6th gear).

#6 and #8 plugs are literally impossible to get to. These headers are the crappiest design I've ever seen, and I'm probably going to pull my long tubes from the '93 tomorrow and take them with me, so I can install them next week. Looking at true duals also, just for ease of swapping. If I do that (replace headers with PS LT's), I'll swap all the plugs with NGK's, and set gaps to .050-.055. Going to replace the wires again, the last time I replaced the wires, I noticed a vast improvement in the way the car ran, but it eventually started running poorly, and continued degrading until it got to where it is now.


Also worth noting: Car is still "driveable", but all of my tools are on this side of the state, I live 181 miles from here, and the Formula is my only transportation to get back home. . . If something needs to be done to fix this, it has to be done here, or I'm stuck limping this piece of **** home tomorrow, and bringing it back next time I have a weekend off.

Would be really cool if someone had a garage, knew LT1's well enough, and was willing to help me troubleshoot this thing for a couple hours. I'm desperate, I'll pay whoever figures this thing out for their trouble. I'm not even looking to have the work done, I just want someone who knows LT1's to spend some time with it, tell me definitively what is wrong with it, and I will make the repairs myself. I've thrown enough money at the car, I'm not spending another dollar on it until I know for sure that I'm buying the parts to resolve this issue.

[meissen]

I'm having flash backs to the problems we had with dad's '95 Formula. You know how we fixed it? He sold it.

Have you checked the wires lately? When we were troubleshooting with my dad's car we swapped wires and it seemed to fix the issue. A few weeks later and it was running like crap again. Aaron (Steel Priest) bought the car off dad (knowing full well he was buying a project that wasn't running right) and has since been trying to fix the issue. He said that he fixed the backfiring issue when he was replacing the wires -- one of the wires on the passenger side was arcing to the header... and that was a brand new plug wire we had installed. I don't know if we just pulled on the wire while I was routing it to the optispark or if it moved on its own from the car rattling and/or vibrations, but the wire found its way to the header and shorted out. I don't know if that's going to be the 100% fix for my dad's car, but Aaron says it's been running great since then.

Might be something to look into.

On a positive note - from the sounds of your thread on LTxTech, it definitely sounds like it needed the fuel pump regardless of the fact that it didn't fix the issue. That's what annoyed the piss out of my dad and me.... Every time we did something it seemed to help lessen the problem, but it wouldn't fix it entirely.

[93formulalt1]

I'm having flash backs to the problems we had with dad's '95 Formula. You know how we fixed it? He sold it.

Have you checked the wires lately? When we were troubleshooting with my dad's car we swapped wires and it seemed to fix the issue. A few weeks later and it was running like crap again. Aaron (Steel Priest) bought the car off dad (knowing full well he was buying a project that wasn't running right) and has since been trying to fix the issue. He said that he fixed the backfiring issue when he was replacing the wires -- one of the wires on the passenger side was arcing to the header... and that was a brand new plug wire we had installed. I don't know if we just pulled on the wire while I was routing it to the optispark or if it moved on its own from the car rattling and/or vibrations, but the wire found its way to the header and shorted out. I don't know if that's going to be the 100% fix for my dad's car, but Aaron says it's been running great since then.

Might be something to look into.

On a positive note - from the sounds of your thread on LTxTech, it definitely sounds like it needed the fuel pump regardless of the fact that it didn't fix the issue. That's what annoyed the piss out of my dad and me.... Every time we did something it seemed to help lessen the problem, but it wouldn't fix it entirely.

I'd bet if you asked your Dad if he misses that car, he'd say he does. . . I'm not giving up on it, because if I sell it, and someone else spends $15 on a part that fixes it instantly, I'm going to be pissed. I'm too far into the rabbit hole to turn back, I need to keep going until I figure this out.

You're right about the fuel pump, the filter was shot too. I'm going to check the grounds, and it's dark out, so I'll check for arcing plug wires while I'm out there. I'm almost thinking it could be a coil issue at this point, I have replaced the coil/ICM with the ones off my '93, but IDK if either of them are in the best shape.

New plug wires are inevitable, I'm sure. Going to use the Taylor OTVC wires, as I need to do all I can to avoid the issues caused by that driver's side abortion of an exhaust header. . . If I can tie the plug wires up out of the way, I'll be alright.


I have considered getting rid of the car, but I'd take such a hit on it at this point, it wouldn't be worth selling, at least not if I wanted to get into anything decent.

[meissen]

Nope, I agree with you 100% - my dad has been depressed about selling his car since it was out of his driveway.

[JoeliusZ28]

plugs and plug wires. We werent blessed with a robust ignition system, so its important set it up as close to factory spec as possible. Anything that adds resistance such as extra plug gap or a larger wire is going to be detrimental. That means AC Delco plug wires, no exceptions. Theyre $120 because they work. This is exactly what I told Aaron when he came to me for advice on Larry's old car, and the car hasn't had a single backfire in 3 weeks. Problem is 100% solved. Do it once, do it right, and have a smile on your face!

Also the best tool for plugs on these cars is a plug socket with an open ended wrench (instead of the socket wrench).

[JoeliusZ28]

Going to use the Taylor OTVC wires

No no no no no....

This is how great my car ran with those garbage wires. Engine bay was a firework show in a dark garage.

[nascarnate326]

I see absolutely no reason that Delco wires must be used. Tons of cars out there using cheap wires that have no issues, mine included. $30 at autozone for duralast wires, route them correctly and no issues.


Only thing I can think of is it being the MAF, or IAC.

I had issues at one time and it turned out to be the MAF. I got a MAF from a Corvette at a junkyard and it fixed my problem. Its on top, and easy to fix...its worth a shot and you can borrow one from another car to test with.

[hoogiesngrinderz]

Make a plug tool! This is the only way I can do plugs on my IROC

[JoeliusZ28]

I see absolutely no reason that Delco wires must be used. Tons of cars out there using cheap wires that have no issues, mine included.

I see absolutely no reason Taylors and MSDs both made my car run like absolute shit either, but hey thats what happened. AC Delco's ALWAYS work. If they don't solve your problem, you can at least confidently check it off the list. Spark plug wires are responsible for issues on these cars as much or more often than the optispark is.

[93formulalt1]

I see absolutely no reason Taylors and MSDs both made my car run like absolute shit either, but hey thats what happened. AC Delco's ALWAYS work. If they don't solve your problem, you can at least confidently check it off the list. Spark plug wires are responsible for issues on these cars as much or more often than the optispark is.

I just bought a set of Autolite wires about 6 months ago, and the coil wire went out at some point. . . I'll return them and pay the difference for a set of ACDelco's. I'm just tired of this car giving me fits. Actually, I was going to ask, with all the confusion about who makes what and who owns who anymore, is there any difference between AC Delco and Delphi parts? Found a set of Delphi plug wires, and they are about $35 cheaper than the Delco's. . . If they are essentially the same thing, I'll get the Delphi's. If not, I'll order the Delco's. Just thought they were both GM suppliers, so the quality may be about the same.


I just changed the MAP sensor, with a spare I had laying around in the garage. The car idled lower (around 650-700RPMs, as opposed to 750-800 from before the swap), and seemed to run slightly better, but not enough to really make a difference. Just seemed to want to die out more frequently. . . The old one was pretty much always reading 10iHg, the new one was more. . . "Dynamic", and ranged from 9iHg-30iHg while driving, sticking around 10-11 at idle.

I'll swap plug wires tomorrow if I can get a set in my hands, otherwise, they will have to wait until I get back home and have a day off. Given the fact that I had to replace the coil wire already, I'm willing to believe that a new set of wires would be an improvement.

Just so you all know the extent of my involvement, I've blown through close to $600 on parts alone in the last 2 weeks on this car. If I sell it now, I'll have to sell for less than I paid for it, ignoring the new parts I bought, as well as my labor. I'd be taking a MAJOR loss on the car. There's no way I could sell it at this point. It's become my goal in life to get this thing running 100% and take it to the strip before the end of the season. Once I accomplish that, I can start spending the repair money on mods, and I'll be that much happier.

[JoeliusZ28]

I cant confirm or deny delphi quality, but iirc delphis have a slightly different color to them, so i doubt they are a "rebranded" AC Delco wire.