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Discussion Starter · #1 ·
My '94 Caprice has headers (also cam, ported Z28 heads, etc) and has been misfiring at idle, cruise and load. I found a few threads that suggested that it could be a plug wire shorting out, so I'm going to replace the whole set. Is there any set of plug wires that would be better (more heat resistant, for example) for use with headers?

(The car has already gotten a new Opti, new coil, and new PCM, all with no improvement on the misfire. Mechanic thinks I broke something internally but I want to address any remaining external fixes before he starts doing leak down tests or adjusting the rocker arms.)
 

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Discussion Starter · #3 ·
what brand of header and what plug # is "burnt"?

Magnacore, MSD are 2 brands of recommended wires

if your header is "burning" plug boots you need to run a heat sleeve...
My car is at my mechanic's so I can't check to see what brand of headers are on it. I don't know that any of the plug wires have actually been burned, but I can hear a ticking noise coming from the back passenger's side of the motor. It may well be an exhaust leak, too.

Basically I am taking the shotgun approach to trying to get the misfire fixed.

Do I need longer/different length plug wires for headers, or should a stock LT1 plug wire kit be fine?
 

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LT1 specific wires routed through the factory looms & brackets are what you want.

I encourage you to pinpoint cause of issue first as "shotgun" response can be costly in $ & time....and may not resolve your problem.

if you hear a ticking sound look at that cyl. you can mist the wires/plugs with water in dark condition and look for arc. this includes looking from underneath the car while it is running.

confirm your header bolts are tight and your gasket is good.

if it turns out to be a bad wire, then replace wires but first confirm wtf the problem is.
 

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Discussion Starter · #5 ·
Now that you mention it, I bet the exhaust leak is coming from one of the headers. Ever since I got the car (already modded, about 15k ago), I'd get an exhaust smell with the heat or AC on if the car was stopped or moving slowly, so the leak must be close to the engine and the blower motor.

My mechanic wants to check stuff like valve lash and do a wet compression test, but I'm hesitant to have him invest labor time in that stuff without going through all of the external stuff first. I've already thrown more than a grand at an Opti, coil and a PCM. (PCM was because he thought the injector driver was bad--two of the header tubes were a lot cooler than the others. After changing it, header tube temps seem to be comparable across the board but the misfire persists.)

I feel like if it was a valve lash issue, the problem would have started out small and slowly gotten worse. The car ran great one day, and like **** the next. The misfire is very pronounced--the entire engine shakes. When the torque converter locks up, the whole car shakes.

Misting to look for the arc sounds like a good idea but the car is at my mechanic's so at best I can relay the suggestion to him.

Is there anything else likely to cause this issue besides the plug wires, since the likely culprits--the opti and the coil--have been replaced?
 

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Damaged injector wiring inside the insulation can easily cause these symptoms. Wiggle each one near it's connector,and it should reveal itself.
 

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Discussion Starter · #7 ·
Damaged injector wiring inside the insulation can easily cause these symptoms. Wiggle each one near it's connector,and it should reveal itself.
My mechanic found two injectors that didn't appear to be getting a consistent signal to fire, tested by a noid light. After replacing the PCM, all seem to be firing properly.

But maybe one of those cylinders was lean while I was running it hard and I damaged something internally. Then again, the car ran fine the entire time I drove it the day before. I'd think if I'd hurt something internally, it would show up right away, not the next day.
 

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PCM's rarely go bad.....but if replacing it resolved 2 injectors not firing, great. Typically faulty wiring, as noted by 1 Slow, is the reason injectors individually don't work.

if you have a exhaust leak pre-02 than the 02 reads lean and PCM dumps fuel. This should throw a CEL for whatever bank (PS is #2) that has exhaust leak. a misfire code 0301-0308 (end # = specific cyl) would show what cyl(s) are misfiring so it would be prudent to look at that particular cyl for the "why". your mechanic should have a scan tool to look for and read codes.

if you hear a "tick" and have a "misfire" I would focus on fixing cause of leak and look very close at that cyl plug wire, plug & injector.

do you have the factory heat shields on the plug wire boots?

since you are not doing the work and a "mechanic" already has car doing whatever he feels needs to be checked...you need to decide what path to follow. if you are not capable of doing this work yourself than the mechanic will...and you have to folllow his advice or chose another mechanic.

asking for advice on the internet is fine but you have to take it for what it is since the well intended responses, even with knowledge of B-body LT1's, may not be accurate because we are only responding to "words" conveyed by you.

I doubt your mechanic is anxious for input from you about possible fixes you found on the internet. He may however be happy to take your $ and check whatever "you" feel may be the problem

when the pcm was replaced, do you know if you had a aftermarket tune in the old one?

is the car modded with a cam?

a bad valve lash....it would run bad all the time if that was the case. if it is this "intermitant" thing as you say it ran fine one day and not the next that ignition or wiring would be what I would focus on.

the exhaust leak, tick, noise, missfire,whatever you have is what I would focus on fixing and see if that resolves your problem
 

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Discussion Starter · #9 ·
Thanks for the detailed response. No CEL, so maybe the leak isn't pre O2, but it's audible from the passenger side rear of the engine bay. The exhaust smell when the HVAC is on has been a constant issue, but now that I think about it, the noise is new--I don't remember it before the car started misfiring.

The car has a PCM4Less tune. I was able to extract it from the original PCM and I emailed it to the guy who sold me the replacement PCM and he (said he) installed it on the PCM for me. I forget the guy, but he had a website detailing LT1 tuning. Guy seemed very knowledgeable and professional.

Car has a cam, ported aluminum heads, LT4 knock module, RAISS, rockers (1.6, I think), 30lb injectors and some other go-fast mods, all done by the previous owner. I've got a thick stack of receipts, if specifics would be helpful.

I believe the factory heat shields are on the plug wires. At least several of the plug wires connect to the plugs at a 90 degree angle, and there are little metal shrouds around them.
 

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you might want to carefully check the plug boots that have those heat shields. I chased a misfire for some time and found arc between plug boot end and heat shield. This was VERY hard to see as arc was between the boot shield and boot. I had to be under car, while running, to see it.

you could simply remove the heat shields to see if this is the problem

there may be a crack in the plug boot end allowing this arc...if that is your problem
 

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Discussion Starter · #11 ·
That sounds similar to a few other stories I've read about hard-to-find misfires. I'll just have my mechanic put a new set of plug wires on it--probably cost me close to the same in labor to have him try to hunt it down. Thanks!
 

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Most likely the ticking isn't a leak it's the cylinder misfire, check all wire connections at the opti, then pull all ur spark plugs to make sure non are cracked as this will cause a misfire. Since you said it's mostly down low sounds like an ignition problem. Does it back fire at all?
 

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Discussion Starter · #13 · (Edited)
I'm not positive my mechanic replaced the plugs, but I think he did.

No backfiring. Opti has been replaced with no change, so presumably all of the connections, if they weren't good before, they are after the Opti was changed.

Plug wire sets range from $50 on up to $130 (at Summit). I'm having a hard time deciding--the only quantifiable difference is the mm of the wire itself: 8, 8.5, 10.5. Previous owner used to drag race the car; I just street drive it. I'm happy with the power, just want it to be reliable. Is there any reason to pop for the expensive wires, or should one of the $70-$80 sets work well? Thanks!
 
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