Waitwutmyname

OK I'm updating the post with lots of info and this first paragraph is the most important:
Background: bought a 97 z28 M6 about 2 years ago and I haven't been able to fix this random misfire and thus haven't driven it more than ~200 miles. It misses pretty badly at idle and some but not too crazy under load. It smells rich and idles at about 1100 rpm forever. The most interesting part is throughout all my time testing it occasionally it will run without misfiring. I can't find any correlation between the few times it hasn't missed and when it does, no temperature change or anything just a couple times when I turned it on it didn't miss. It's happened maybe 3 times, and just happened the other week. I even took a video for proof. I turned it off and on and ran it for 10 minutes and it didn't miss! Then the next day I turned in on in the morning and back to normal missing. I only get the code p0336 which basically means a misfire detected from comparing the CPS signal to the opti low res signal. The car does in fact misfire, so getting this code makes sense as that is what the CPS is for in this car. But I ended up replacing that anyway because of a mechanics concern. When I got the car the guy just had the engine rebuilt, new water pump, new refurbished PCM, but it had a bad opti and blown head gasket cause the fan relay broke and the other was missing. New heads too, or at least they were lol. I replaced the head gasket, ran better and compression test shows all cylinders are good. Since then in trying to fix the miss I replaced the stuff in my signature, the whole ignition system is new. Now since the code is common for failing opti and I replaced the auto zone opti with new auto zones 3 times, it's always ran the exact same. I checked the low res, high res signals, and the signal form the pcm to the ICM with an oscilloscope and they're all perfect. No vacuum leaks that I can find.
Compression: Compression is good as tested with a compression tester, biggest difference is I think 8 lbs for a pair of side by side cylinders, something like 8-15lbs biggest difference between all the cylinders on engine.

Vacuum: Looked for leaks, can't find any with lots of starting fluid because I'd rather die than not solve this, even around the intake manifold. I have a vacuum gauge but the problem is since the misfire is detected the engine supposedly retards the timing and that lowers the vacuum, so if I recall correctly about I'm getting about 17 inches of mercury at 1200 rpm. I don't know if that's too low for that rpm and I can't find any solid info on how much the vacuum will decrease from a miss. Even with the miss, the needle doesn't fluctuate, it's steady.

Fuel: I checked the fuel injectors in a basic way by pulling them out and putting power to them and pouring gas through, not very scientific but doing that combined with visual inspection is the best I could do at home. Injectors are all 12.1-12.6 ohms, and have good harness connections for the pcm and power. They were replaced when the engine was rebuilt I believe, they're some GM injectors for a buick or something but they have the same flow rating so I think they should be fine. Fuel pressure is good at the rail and doesn't bleed off too fast, no fuel leaking into the vacuum line for the regulator. New fuel filter anyway. Pump was replaced at some point before too. Maybe looking at the injector pulses to see if they're firing right could lead somewhere?

Spark: Ignition system basically all new as I said before and in my signature. I know I have an autozone opti and they're very often bad, and you can get 3 bad in a row. That's why I checked the signals with an oscilloscope. Since I checked the signals with a oscilloscope I know they're good, but since it really still sounds like an opti related issue, is it possible that the signals appear good but actually aren't? Maybe I'm getting too desperate here. I can't see any arcing on the plugs or wires, plug gap is correct, right type of plugs.

Harness: I'm decent with electronics. I checked all the sensor wire connections to the pcm that could cause a miss, just found an idle air control pin that was bad to the pcm which made the idle a little better when fixed. Then I checked all the pcm connections to the ground and power. Then I checked every pcm pin to make sure none were shorted that shouldn't be, and yes every single one. Besides that one bad pin no issues.

PCM: Tried a 96 PCM no tune just stock from a known good car no difference.

Time left: Every time I cycle the engine and it misfires, I lose a few months of my life from the stress of frustration. I can probably only live to be 70 now.

I'm sure there's plenty I've done that I'll add as you guys ask and I remember. Is there any way to test and isolate variables? For example it's easy to narrow a misfire down to not being an O2 since the engine doesn't use them in closed loop mode, so if it still misses in closed loop it's not an O2. What else can I do? Can I make sure the plugs are firing in correct timing and be 100% that's not the issue? Or the injectors? Is it a good idea to have a shop with a hydrocarbon tailpipe analyzer take a look? If one of you helps me find the issue I'll buy you a 6 pack or 2. I have lots of tools and I'm willing to do pretty much whatever it takes.

TPIZ92

iTrader: (10)

Could be an injector if the car smells rich. Did you try ohming them? If it smells rich then im guessing its that. Ohm them, its free. I always implore people to get A vcm scanner to imitate a tech2. Lots of features, for instance you can shut off the injector on each cylinder for a balance test. You shut it off and there is no change...boom you found your problem AND you know the cylinder too.

In terms of unplugging the maf, on the ls1 it will go into SD mode. On the LT1 i am unsure, i am not knowledgeable on this platform yet to determine that.

If you find its the injector, replace it or try building an injector cleaning mechanism. I have had phenomenal success saving injectors using an old manifold/rail and pressuring a cylinder filled with liquid (usually seamfoam or anything really) feeding it to the rail (@90psi) and then firing the injector.

BTC

The LT1 also goes to Speed Density with the MAF unplugged.

Phoenix'97

Check your spark plugs for any cracked casings. When I ran into a bizarre problem with misfiring that was not attributed to my optispark years back, I ran the car at night to see if she might be arcing. Sure enough, I had a cracked casing on a spark plug. I run NGK Iridium and somehow there must have been a fine crack in production and it split after enough heat cycles. It is worth a check if you are going nuts trying to figure out what the problem is. I hope this helps.

Waitwutmyname

I've checked all these and more to no avail. Sorry for the long time no post.

Roadrunner96

Years ago i got a bad random misfire problem a few weeks after i installed headers. I found out my O2 sensors were causing the problem even tho they wouldnt throw a code.

Waitwutmyname

I've thought about that but the car still misses when it's in closed loop and the O2s aren't used.

ACE1252

You may have to get the car to a good tech who can scope the ignition system and isolate the issue rather than chasing it with parts. Autozone opti's are always suspect....but you'd think with 3, odds are you hit one that would run fine for a while.

• Is it misfiring at idle or under load(or both)?
• Manual or automatic?
• Can you log the car from a cold start to warm up(190*).
• Did anything happen to the car before this problem started? Or has this problem just slowly developed?
• Has the engine been apart to where the rockers were adjusted?
• How many miles on the engine?

BTC

Do you mean it misfires in open loop after it’s started? When the car warms up and goes into closed loop is when it uses the O2 sensor input.

Waitwutmyname

Yeah you're right I meant open loop, my bad. It misfires in open loop and closed loop.

Waitwutmyname

Sorry I should have added my info too. It's a manual.
I don't have any way to log the data and I rather not spend a lot on a scanner which look expensive to be able to scan the 96-97 years.
This problem has been with the car since I got it 2 years ago. I've made the miss better by fixing a broken Idle Control wire to the pcm, but it's always had the same miss throughout every replacement part and every opti. I replaced the head gasket cause it was overheated and we adjusted the rockers, checked the compression after and it's good furthest off is like 8lbs.
126k on the engine, less than 5k I think since the rebuild.

Waitwutmyname

Ok everyone I updated the OP with tons of info please reread it changes the angle of the whole issue. Thanks for the feedback thus far even though I really was stupid not to give more info!

ACE1252

Without a way to data log the car you are working blind. If you plan to fix this problem on your own, you are going to have to buy the hardware/software to data log the car. You also need to get software with GM enhanced parameters. I'm on CZ28.com as well and saw your post on this same issue. Injuneer is excellent at reading data logs....if you can get him one. I'm fair at it, but nowhere near as good as him at looking through them.

Scannerdanner has several good vids on Youtube on how to track down a misfire. I suggest you have a watch through them. When dealing with a misfire, when I say scoping the car, I mean the secondary side of the ignition....the high voltage stuff. This....

ACE1252

The high idle also screams vacuum leak to me(or something wrong with the IAC...maybe even throttle blade adjustment has been screwed around with....)....you really need to data log the car....

• Are you 100% sure the rockers are lashed correctly?
• What were the compression test readings?
• What vacuum reading are you pulling at idle?

SS RRR

iTrader: (2)

You have that backwards. O2's are not used in open loop, and are indeed used while in closed loop. Unplug them, drive around some more and see what happens.

Waitwutmyname

I'm currently going to take some time to check some stuff Injuneer informed me of over on the sister thread I made on camaroZ28, be back soon.

SCLT1

I would definitely find someone trustworthy to go through the computer like the guys mentioned. On another vehicle I pulled some codes it was throwing and replaced the sensor it said was bad. Then same code again so throw parts at it since it was a general code. One of the o2 sensors actually needed replaced and never threw a code but nothing I changed was the main problem. Sluggish behavior ended up being a failing timing chain but said sensors were bad.

Waitwutmyname

Ok guys the cam pin is definitely far enough in the correct slot in the opti to be fine. I found out the P0336 code was because the wire to the CPS sensor was bad somewhere and half the time working half not. I suspected an issue with the harness for the primary half of the opti also since its reference signal wire is shared with the CPS, so I ran new wires for the 4 opti data wires and for the CPS. Then after I fixed that I had codes for a bad ICM, I think because I didn't have enough thermal paste on it, it went bad. Put a new one on and no codes anymore but it still misses. Now I've ran it 100+ miles doing the diagnostic process in the manual to get codes to show up and no codes! Now it only misfires a little bit less when it's cold but as soon as it starts getting a little warm, and I don't mean up to temp I mean like 1-2 minutes after starting with a broken thermostat, it goes back to missing as bad as before. Still does a hard start where it instantly drops rpms to like 200, then revs itself up to like 1500, then slowly goes down to 1000 and stays there. I took your advice and found a mechanic that will scan my car for free! with a tech2 tool, but before I do that I'm going to do more research on these symptoms with no codes, and if you guys have any new suggestions feel free to throw them in here. I appreciate the help thus far and I'm making progress! Also I've been watching lots of scannerDanner videos recently, amazing channel!

Waitwutmyname

Speaking of scanning, what should I be looking for now that I just have a high idle and no code miss? Anything specific to the LT1 I should look for on the scanner with the mechanic?

SS RRR

iTrader: (2)

Regarding O2's, did you at least try this?