I'm going to have a giant post because I want to lay everything out and figure this thing out.

Background: Well where do I start, first time posting after (literally) dozens of hours of research on the web. I 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.

 

The fact your first post was so long caused the site system to put the post in the “Moderation “ queue, requiring approval from a moderator before it will appear.

You mention P0336. Did replacing the CPK sensor eliminate that code? That code does not indicate a misfire - a misfire would be P0300 to P0308. P0336 means the crank position indicated by the CKP data is different from the (pseudo) crank position derived from the cam position sensor in the Opti, and the number of degrees deviation exceeds a normal limit.

I have had one case where P0336 set because the cam dowel pin was too short and had not fully engaged the correct index hole in the Opti, allowing the dowel pin to slip past the index hole, throwing the ignition and injection timing off. That would appear to be a possibility here if replacing the CKP sensor did not eliminate P0336.

Tried to scan the lengthy post quickly, and didn’t see any mention of a misfire code, which would not set with P0336 active. There is also a misfire count stored in the PCM by individual cylinder. That data can only be retrieved by a scanner capable of GM “enhanced” diagnostics. Again, that data may not be collected with P0336 active. But if available may determine if it is a misfire specific to one or more specific cylinders.

 

First off I forgot to add the most important piece of info: 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.

No replacing the sensor didn't fix the code still pops up after running the engine for a while. I could have sworn I read somewhere that the comparison of the crank and cam timing is used for misfire detection. I'm going to try to get a scanner to it and datalog too if I can. I also found the low and high res signals from the opti to be fine, and the one from the PCM to the ICM was good too. Could a problem with the opti not show up there? Thanks for the advice!!!

 

Just in case I do need an opti what are all the methods of getting a good one? Hard to find OEM re manufactured ones now.

 

Petris

 

Yes, the CKP sensor was added only for the purpose of detecting misfires. Yes, misfire detection involves comparison of the CKP data to the low res data from the Opti.

What you aren't understanding is that P0336 does NOT indicate a misfire. It indicates that the angular difference between the crank position indicated by the CKP and the Opti is outside the expected/allowable range of deviation of -10 degrees to +7 degrees. Hence, the data comparison cannot be used to detect misfires. There are many possible causes.

You can determine if your CKP and Opti readings are outside the allowable deviation range using a scanner, but that scanner has to have the ability to read GM enhanced parameters. Not every scanner can do that, because they require GM proprietary info that GM will only sell them for $50,000. You could use a Tech-2 tool,, and look for the parameter "CKP:LoResolutionAngle". That parameter is the number of degrees difference between the CKP and Opti low res signals. If it is outside the range indicated above (-10 degrees to +7 degrees), it sets code P0336, and disables misfire detection

Read my post #4, in a thread on another site:

http://www.ltxtech.com/forums/showth...3-Opti-Problem

You may also find post #18 in that thread useful.

Might want to read through that entire thread, because the data was used to resolve the problem - misalignment of the cam dowel pin in the Opti drive mechanism.

I'm NOT saying with absolute certainty that this is your problem, but your issues seem VERY similar to what is described in the thread on ltxtech.com. And a scan with a Tech-2, looking at "CKP:LoResolutionAngle" will tell you whether you have the same problem or not.

I would also suggest that since you seemingly do not want to believe my comments about P0336 vs. the true misfire code P0300, that you download (free) a copy of the 1996 factory service manual (identical to 1997), and review those codes in Volume 2 of 2, Part 1 of 2,

http://www.mediafire.com/?40mfgeoe4ctti

 

Depending on year make and model (every car made) only P300,301,302,303,304,305,306,307,308 are ‘misfire’ codes.
P300 is a general misfire
p301-304 on 4cyl
P301-306 on 6cyl
P301-308 on 8cyl
those tell what cylinder/cylinders are misfiring.
most likely your P0336 is a bad opti.

 

It could be as simple as the cam dowel pin not being in the correct index hole.

I mentioned the specific misfire codes, including the misfire counts per cylinder in my first post.

 

Sorry it's not that I don't want to believe you I was confused about the difference between the CPS being used to detect misfire and P0336 being a code that itself has nothing to do with misfires.
I appreciate the help and my next task is to read up on all those resources and look at the data to figure out what's going wrong. Thanks guys.

 

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!

 

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?

 

Well I just came back from getting it scanned. Right now it misses at idle and a little above that maybe about 1200 rpms really bad, then when you go above that it doesn't miss. When it revs down from any rpm it backfires a lot out the exhaust. It also does this when starting
I've driven it 50+ miles and the computer had 0 recorded misfires. Even as we ran it for 10 minutes and could see and hear is missing in front of our eyes the whole time the tech2 picked up in total 3 misfires in cylinder 5 and that's it. My fuel trims were -3-5% so taking fuel away, even though every other time I've driven it they've been 5-15% adding fuel. The only thing that was even slightly out of the ordinary was the vacuum is at 16 inches which is a tad low since I'm at 3000ft and the IAC counts are 16 which I read here should be 30-40 (32 ideal). It idles at ~820 rpm. I couldn't find a vacuum leak with engine start fluid or water. Tested for an internal vacuum leak, no problem there. I can't just drive it like this because it gets like 10mpg and it runs so rich it's gonna destroy my new cat. 2 mechanics said it's gonna be practically impossible to find the problem so they don't try to and I don't blame them. The IAC counts are the only thing really our of the ordinary but I can't find any vacuum leaks and today the fuel trim was trying to remove fuel not add it like what happens when there's a vacuum leak. Anyone got any idea of what I should do now?

I can get the data saved with a tech2 tool since obviously letting you guys look at the data is best but before I do that, would the data from only the tech2 be enough to you guys to be useful? I'm reading the manuals and tutorials about the tech2 but I can't actually find it's sample rate and a full picture of what it gathers and what it doesn't.

 

Have you cleared P0336? The misfire count system can't function if that is still set.

Having low IAC counts will not cause a misfire. The reason you want the value in the range of 20-40 counts (the recommendation from my LT1 scanning guide stickied in the "Computer...." forum) is so that the PCM has headroom to lower the counts to control the idle RPM. As long is it doesn't go to zero at any time while idling, it's "OK". Its just that 20 (or 30, wherever you read that) increases the probability that the PCM has adequate headroom to control the idle speed. A low IAC count may infer a vacuum leak, but yours doesn't seem to be far enough below the guidelines to be of concern.

In what cells were #'s were the LTFT's negative? The fact that it is getting only 10 MPG would seem to indicate it is running extremely rich. Misfires (or exhaust leaks before the O2 sensors) can cause that.... but would typically be accompanied by positive LTFT's.

What is the variation in idle speed? Is it still exhibiting the wild swings you described in post #10? What was the scanner indicating as the "target" idle RPM? While fully warmed up idle for the M6 is 800 RPM, if the coolant temp is below 111-degF, the PCM elevates the targete idle speed. Typically, idle speed is not constant, and will normally vary +/- 25 RPM.

Rather than vacuum readings, what were the MAP and barometer readings? MAP - BAR = vacuum

I have no idea what data the Tech 2 can log, or how you set it up to produce a useful report. I have never seen a data log from a Tech 2. Probably best thing to do is download GaryDoug's OBD-2 scan/logging software:

https://www.firebirdnation.com/forum...nner-software/

I get a lot of logs from his LT1/OBD-1 version (Scan9495) and it has 55 sensors and parameters logged. I generally hide a few columns that are not relevant to the problem I am looking for, but the list of what I find useful is going to contain 35 to 45 items.

 

Yes I cleared the P0336.
I datalogged the car with HPTuners so I only got basic data, I'll get more when the adapter gets here on Monday so I can use the GaryDoug software you recommended. For now, the fuel trims short and long term both fluctuate wildly as I drive the car, but always go rich at idle. You can also see the idle RPM is pretty stable now and seems within normal range. I want to clarify that it does seem like it will misfire also at higher rpms than ~2000, but not very much. MAP readings are on there too.
Here's the CSV of the data of me driving then revving within the badly missing rpms and just idling at the end:
https://drive.google.com/file/d/1iBB...ew?usp=sharing

and here's the HPTuners file
https://drive.google.com/open?id=1zR...JxgAVJ5GoAuxrg

 

Are they 2 versions of the same file? I can't open hpl files..