Engine misfire help

[SeanC]

Hey guys,

I'm getting a code in my Trans Am after recently replacing a blown head gasket. I bought a scanner and am getting P0300, "random engine misfire". When you rev the engine, it seems to have a tiny shake and then all of a sudden smooths back out (same during driving; seems weak at lower RPMs and then all of a sudden picks up and the slight shake goes away). I checked all the plug wires on both the opti and the spark plugs, but everything seems to be tight. I haven't replaced the wires, but the spark plugs are brand new. The previous owner said he replaced the wires somewhat recently (although I don't know how true that is). I know that there are many things that could cause this problem, but does anyone have any suggestions on how to go about trouble shooting this? My scanner can record a limited amount of live data, but i'm trying to figure out how to narrow the misfire down. I've never used a scanner before so i'm not sure how everything works or what everything means yet. Any help with this would be greatly appreciated. Thank you!

[Joe 1320]

I'm not saying that this is your problem, but I've seen several things that could contribute.

1. Do you still have the metal spark plug boot covers? I've seen those things cause arcing that gives the exact symptoms you describe. I ultimately had to remover the metal boot covers and used taylor Firesleeves...... a braided fire resistant cover to keep out the heat and no problems since. (they don't conduct electricity like a metal boot cover) I also use dielectric grease in the boots to seal out moisture and prevent the plug boot from sticking to the plug from heat.

2. Oxygen sensors are not tolerant to silicones that are present in antifreeze or silicone RTV. there is such a thing as 02 sensor safe RTV. Usually a blown headgasket will foul 02 sensors at some point, the silicones coat the electrode and don't burn off. That's probably not the source of the misfire, but it might be a contributing factor.

[SeanC]

Thanks for the response Joe! I do still have the metal boot covers on the spark plug wires (I notice that there are only shields for the two rear spark plugs on each side). I definitly understand what you are saying, but why would something like that cause a problem now, when it really hasn't for the past 100,000 miles? If the oxygen sensors did get messed up because of the blown head gasket, wouldn't there be a code showing that the sensors were inaccurate? Apparently I can test the oxygen sensors with my scanner somehow; i'll try to look into that a little more.

[Joe 1320]

Sean,

if I had an answer for that one, I could get rich. Mine (and others) worked fine from the factory and the minute we changed plugs and wires, the misfires started. Put the car into a dark garage, open the hood and have an assistant start her up. Throw a few revs and if they are arcing, you will see the spark jump fromt the boot shield to the plug or the block. As soon as the boot covers were removed, the misfires stopped. Pull the boot covers and if the problem goes away, there ya go. It would be wise at that point to get taylor firesleeves to prtect the boots from the heat.

The 02 sensors would start showing signs of slow response, you would have to observe their operation in real time using a scanner, not a code reader. Even so, doubtful that is the cause of the misfire, the 02 sensors would make it run with terrible throttle response, lack of power and most likely it would run pig rich.

[SeanC]

I'll definitly check the plug wire shields first; even if they don't look like they're arcing I think i'm going to replace them. The more possible problems I can get rid of the better. The car is in my friends garage, so I most likely wont be able to find out before the weekend. I believe that my scanner has the ability to read oxygen sensors in real time; i'll have to find out. Like I said, I really just started to figure out how the scanner works, so i'm still pretty unsure about what its capable of. Anyways, when my friend drove it, he said it didn't really feel like it had much power below like 3500 RPMs and then all of a sudden he would feel it pull; he didn't say anything about bad throttle response.

I noticed that there are numerous engine misfire codes that can pop up, and the one I got is a "random miss". Does this literally mean that it can be missing on any cylinder? Sorry if it's a dumb question, but I would assume I would get a like P0301 if it was missing only on cylinder 1. If it is truly a "random miss", wouldn't the problem have to be in the opti or somewhere that affects all cylinders, and not just a spark plug wire? What else should I be looking for/testing? I know that this is an extremely broad topic; i'm just trying to figure out how to narrow down the possibilities.

Thank you very much for your help so far Joe, I really appreciate it!

[Injuneer]

If your scanner has "GM enhanced parameter" capabilities, you can look at the misfire count for each cylinder. That might point you in the right direction. Could be cross-firing on a couple of cylinders, may be sign of a bad valve adjustment. Probably a lot of other possibilities. "Random" does not necessarily mean equally distributed among all cylinders. Could be anything from an ignition to a fuel to a mechanical problem. I don't think its time to jump to the conclusion that its the Opti.

[SeanC]

So if we take the heat shields off, and determine that the oxygen sensors are giving accurate readings, what do you think the next step should be if it is still misfiring? Unfortunately, it doesn't look like my scanner is capable of telling which cylinders are misfiring. Of course not, what did I expect for $160

Anyways, what are some of the more obvious problems we should look for? How should we start to test the ignition? Sorry i'm asking so many questions, im just so anxious to get my car running right, and am really not sure how to begin trouble shooting the misfire. I only got to drive it for like 2 weeks after buying it before it died on me

Thank you very much for the help guys; I can't even tell you how much I appreciate it!

[RAF7789]

have you replaced the opti yet?
is that the only code you are getting?

there are a few ignition troubleshooting guides in the forums here, i believe JOE has a good one up. will try to find it.

[SeanC]

I can't say that I'm a hundred percent sure that the opti hasn't been replaced, but I'm pretty sure it is still the factory one. The misfire is the only code that I'm getting.

[BrdWAtti2d]

I'm not saying it's the opti, just that it could be. When mine first started going south it only showed symptoms when the car got up to operating temps. It ran fine cold. I think it has something to do with the way the cap is cross routed.

[SeanC]

The misfire is happening whether the engine is completely cold, or up to operating temperature. The miss does not seem noticeable at idle (no engine shake), but as soon as you hit the gas a little you see a tiny "studder" and then it smooths out and continues to rev what seems to be normally.

I definitly realize that this problem could be opti related, but at the same time, it could be a hundred other things. I would much rather find a bad spark plug wire or something than have to replace the whole opti!

My friend started the car in his dark garage, and didn't see any obvious arching (although it is nearly impossible to see under the lousy exhaust manifolds).

[Joe 1320]

Pull of the boot covers. If your problem goes away, there ya have it.

[SeanC]

Hey guys,

I just thought of something that MIGHT have something to do with my misfire. After not being able to run the car for a couple months, I didn't realize how little gas I had in it. We ran it for awhile, but then it wouldn't start back up because it was out of gas. I realize that this isn't good to do, but it was just a dumb mistake. Could this have clogged one of my injectors or fuel filter? I have no idea when the last time the fuel filter was changed; could this cause a "random misfire"? How should I clean the injectors if they might be a possible problem? Is there any way to test the injectors without being able to monitor them with a computer?

[Joe 1320]

you're giving alot of "what ifs". Start with the most simple and systematically eliminate things until you find the culprit. You won't know what it is until you dig in and start messing with things. Until you do that, everything is just guessing and nothing is getting solved.

[97LT1Z28]

My metal spark plug boots didn't "act up" until 130,000+ miles. It just started misfiring really bad at idle and at low rpm's it was almost undriveable. Yanked the boots off and it was like a new car again..... hope this works for you!

If it does turn out you need the opti changed, it's not actually too hard to do. With your "f-body brothers" on this forum, you're in good hands support-wise!