Thread: Intermittent misfire

Hello everyone, I'm new here so this may not be the best place to post this but it seemed fitting. I've have a 98 grand prix gtp, all stock, no mods. I keep getting a p0304 cylinder 4 misfire. Now before we jump through the usual hoops, let me explain. The light will only come on if I let the car idle for 10 minutes or more. If I start it up and leave immediately I can drive the entire day without the code returning, once it comes on it stays one until erased. It does not affect the performance of the vehicle I average 19.6 mpg as it always has and it runs smooth under load and otherwise. I have been through the usual suspects. I have replaced the coil packs, spark plugs, plug wires, I've replaced the fuel injectors as well, yes they were flow matched and all the parts are original GM parts. I have also replaced the lower intake gaskets with the zzp aluminum ones, I am getting very good compression on all cylinders and I've replaced all the engine/transmission mounts. Light still comes on, super annoying to look at lol. What am I missing here that could be doing this. I have a stage 2 zzp performance kit ready to go on this vehicle but I would like to track down this issue first.

Check the spark plug wire routing back there and make sure #4 isn't laying on the O2 sensor or anything else for that matter.

What type of spark plugs are you running?

Originally Posted by FordMan77

Check the spark plug wire routing back there and make sure #4 isn't laying on the O2 sensor or anything else for that matter.

What type of spark plugs are you running?

The spark plugs are ruthenium ngk plugs, heat range 4. I have also ran a set of the autolite 104 plugs heat range 4 as well, and the acdelco OEM copper plugs but I cant remember the part# issue remains. I can verify all plug wires are secured in the plastic factory retainers and do not appear to be touching each other or the o² sensor. I have also checked plug wire resistance and swapped the wires around but code always shows cylinder 4.

I gave my boy my '06 GT and he just started getting the same code on cyl 5. For the '06 however it does it when accelerating hard. Changed pretty much everything Kenny did to no avail. He did however remove the mufflers just before it started. Hope this does not take from Kennypowers original question.

Removed belt from the super charger and codes went away. Figure it is a back pressure issue.

Have either of you replaced the cat yet? a 98 and an 06 with a stock cat are likely well past the useful mileage and it's probably clogged up at this point in life.

Originally Posted by FordMan77

Have either of you replaced the cat yet? a 98 and an 06 with a stock cat are likely well past the useful mileage and it's probably clogged up at this point in life.

I was teetering on that, mine is a 98 but only has 122,000 miles on it so I never that it would be the issue. I have a new down pipe, high flow cat and O² sensors, I should have them on by the end of the week. I will be back with an update.

Originally Posted by tonic

Removed belt from the super charger and codes went away. Figure it is a back pressure issue.

Does your supercharger rattle at all at idle? When I first got my car I would get a multiple misfire code and the car missed badly when on the accelerator really hard. But ran fine otherwise. Supercharger coupler was bad, it solved quite a bit of issues. You might try that, get the green one from zzperformance if you do replace, much better than OEM, they have a whole kit for the job. I think if backpressure was the issue it wouldn't matter if the belt was on or off the supercharger, under load it would still be a problem even with no boost. Also if your car has more than 150k on it, change the lower intake gaskets, they're junk plastic and prone to sucking coolant into random cylinders, common 3800 problem especially if you've been using the orange DeX cool coolant, it's highly acidic if not treated properly.

Thanks Kenny, intake gasket is less then 2 years old. Tried running with straight manifolds, still misfire under load. I will try super charger coupler.

These cars only misfire if they need a coil or a lot tune up, occasionally bad injector.. if you know it's cyl 4 switch 2 of the coils and see if the misfire stays on cyl 4 or if it moves. If you switch the coils and cyl 4 is still the one they misfires then give the car a tune up.. acdelco plugs and wires only! These cars are known for needing tune-ups at 100k and coils go out around 140k because people don't ever do tune ups and the high resistance of bad plugs and wires over heats the coils and they burn out..

So give the car a tune up with acdelco parts and if you over 100k at least replace the coil that runs the cylinder that has the misfire so you don't end up fixing the misfire just to get it back in 10k cuz the coil finally went out..

Originally Posted by mark97564

These cars only misfire if they need a coil or a lot tune up, occasionally bad injector.. if you know it's cyl 4 switch 2 of the coils and see if the misfire stays on cyl 4 or if it moves. If you switch the coils and cyl 4 is still the one they misfires then give the car a tune up.. acdelco plugs and wires only! These cars are known for needing tune-ups at 100k and coils go out around 140k because people don't ever do tune ups and the high resistance of bad plugs and wires over heats the coils and they burn out..

So give the car a tune up with acdelco parts and if you over 100k at least replace the coil that runs the cylinder that has the misfire so you don't end up fixing the misfire just to get it back in 10k cuz the coil finally went out..

Are you responding to the other guy or me? If you're responding to my original post, I'm 100% convinced you didnt even read what I posted.

The thread didn't post my entire message.. I think it might limit the amount of chsracters.I was putting all the causes so people had a list to go thru.. my response was also supposed to say check compression, ignition control module and grounds.. in your case you might have a bad icm, ground, weak compression or a pcm issue.. and if your lucky and you happens to have replaced the crank sensor about the time your misfire started, a crank sensor variation learn procedure might be all you need...

It's an old car, how did you check compression? Did you hold the throttle to the floor? What's the numbers? I added compression to the list of what could be wrong with your car because some people don't know how to check it properly, but I did see you say you did.. also beleive it or not in my 2005 comp g I had a misfire the pcm didn't even detect, I'm just ocd like that and it turned out to be the maf on my gtp .. and the series 1 and series 2 are well known for maf issues. So bad actually that after market ones come bad in the box time and time again..

Also, when the light comes on do you feel a misfire? If I can detect a miss the pcm can't then I would like to know if the car is actually misfiring? For it to set a check engine light it would have to be pretty noticable. And if it does misfire does it do it so bad the cel starts flashing? There are 2 stages of misfires.. small ones the pcm sets a code for and bad ones that actually get the light to start flashing..

I know these cars well so I can help.. if you have a 2 channel oscilloscope it would help in diagnosing a faulty icm... Also some part stores can check them. And the ground wires for the icm and coils are right on the icm bracket and could be corroded and crappy

Originally Posted by mark97564

It's an old car, how did you check compression? Did you hold the throttle to the floor? What's the numbers? I added compression to the list of what could be wrong with your car because some people don't know how to check it properly, but I did see you say you did.. also beleive it or not in my 2005 comp g I had a misfire the pcm didn't even detect, I'm just ocd like that and it turned out to be the maf on my gtp .. and the series 1 and series 2 are well known for maf issues. So bad actually that after market ones come bad in the box time and time again..

Also, when the light comes on do you feel a misfire? If I can detect a miss the pcm can't then I would like to know if the car is actually misfiring? For it to set a check engine light it would have to be pretty noticable. And if it does misfire does it do it so bad the cel starts flashing? There are 2 stages of misfires.. small ones the pcm sets a code for and bad ones that actually get the light to start flashing..

I know these cars well so I can help.. if you have a 2 channel oscilloscope it would help in diagnosing a faulty icm... Also some part stores can check them. And the ground wires for the icm and coils are right on the icm bracket and could be corroded and crappy

I have replaced maf sensor already, it made no change. I have not checked the grounds. I have not messed with crank sensor at all. The CEL does not flash and there is no noticeable drivability issues, as long as I dont let the car idle for more than 10 to 20 min. The light never comes on, so whatever is causing it would appear to be an idle condition. For the compression test I just used your run of the mill Matco tester I tested throttle wide open, I know you get lower numbers if you dont. I also tested with and without a little bit of oil to see if there was any loss from the rings. Compression from #1 to #6 accordingly...142, 136, 140,140, 142,138. I'd also like to add I've replaced the pcm as well so we can eliminate that I'm thinking at this point I need a camera so I can look in there and see what's happening. I dont know if it's relevant but I could start this car and drive across the United states and the light will not come on, but if I go in the gas station and leave it running idle for a little the light will be on lol. Otherwise the car drives great, no power loss or hesitation, no rough idle, averages 20mpg , no spark knock of any kind. I've owned this car for 3 years and it has always been the same before and after the light.

Have you ever felt it misfire? Like sat in the car while it idles to see if it's actually misfiring when the light comes on? Like instead of letting it idle while in the gas station you sit in the car and wait for the light to come on? Knowing if there is an actual misfire would help narrow things down. The crank sensor on these cars are dual hall effect, so if one part of the crank sensor is intermittently failing the pcm could think it's a misfire.. I would do a crank sensor position variation learn procedure on the car to start with.. when you remove, replace the crank sensor or replace the pcm it needs to be done. It teaches the pcm the attributes of the crank sensor so it can properly detect misfires..

Also connect a scan tool and watch the rpm readings and graph them and see if the 3x or 18x rpm's become different than each other or off one glitch's.. crank sensor, ignition control module or one of the plugs on the icm or crank sensor could be bad and intermittently cutting out, or the learn procedure needs to be done..

But let me know what the engine does when the light comes on.. that would be invaluable information, knowing if it's actually missing or not

By the way, I live in Wisconsin.. if you live around me and don't have a scan tool I could do the learn procedure with you.. also look at the freeze frame data of the P code in the pcm to see what exactly the cars sensors were telling the pcm at the time it happened

Originally Posted by mark97564

Have you ever felt it misfire? Like sat in the car while it idles to see if it's actually misfiring when the light comes on? Like instead of letting it idle while in the gas station you sit in the car and wait for the light to come on? Knowing if there is an actual misfire would help narrow things down. The crank sensor on these cars are dual hall effect, so if one part of the crank sensor is intermittently failing the pcm could think it's a misfire.. I would do a crank sensor position variation learn procedure on the car to start with.. when you remove, replace the crank sensor or replace the pcm it needs to be done. It teaches the pcm the attributes of the crank sensor so it can properly detect misfires..

Also connect a scan tool and watch the rpm readings and graph them and see if the 3x or 18x rpm's become different than each other or off one glitch's.. crank sensor, ignition control module or one of the plugs on the icm or crank sensor could be bad and intermittently cutting out, or the learn procedure needs to be done..

But let me know what the engine does when the light comes on.. that would be invaluable information, knowing if it's actually missing or not

Sorry it took so long to get back, I had to buy a nice scanner and learn how to properly read it lol. After much studying I have determined that the engine was not actually misfiring on any cylinder as the light was illuminated. There were no changes in cylinder conditions. I am happy to announce that you were spot on with the crank position sensor. I was getting an intermittent signal failure from the sensor only at idle, data shows true above 1200 rpm, intermittent failure below 1200. I believe as you stated it was getting proper signal to run but failing on its other function. I replaced crank sensor, did a case relearn and have successfully idled several hours with no return. I believe this problem has been solved at this point and I thank you so much for sticking it out with me. I honestly thought this thread was dead and sometimes you just need a little push in the right direction. If condition returns in the future I will update accordingly but I'm pretty confident you've solved my issue. Thanks again

No problem.. I been thru allot with the 3800... Be happy you didn't have my problem on my 2005 comp g gtp... All scan data, oscilloscope data looked right.. I had to diagnose purely on symptom because the pcm was lying to me. 2 parameters for the EGR didn't make since, as in 2 P codes should have set that weren't and I had a rough idle, and under light acceleration I could feel power surging. Times the car would be slow, sometimes quick and now and then I would hear an insane amount of spark knock at 40% throttle.. I knew the EGR was the issue but it tested fine on the bench. Opened and closed all the way when I applied power to it, position sensor was working properly. So I decided the pcm was bad and sure enough it was .

2 weeks later I'm looking thru tsb's for the car and find a gm voluntary recall on the pcm for the 2005 grandprix and many other gm models. Symptoms EGR issues or fuel pump wouldn't come on. I miss the vin range by 100 cars though. Gm definitely needs to expand their voluntary emissions recall to all 2005 grandprix and other gm cars. Not only was it controlling the EGR improperly and lying about it but I keep getting a P0621 generator L terminal code and as p0703 code for the brake switch circuit 2 code.. the brake switch code would come while I stopped in park when the test conditions to set that code can't happen while stopped with the car in park. The generator code would set after the car was warmed up and I turned it off and started it back up either right away or after coming out of the gas station, I would get charging system error on my dic until I blip the gas pedal then it would charge the battery properly.

Also these 3800s will have a bad crank sensor, no start but when you check for spark it will have it . That one threw off many many people.. normally you don't have spark with a bad crank sensor but with the 3800 allot of the time they will

So I'm happy I was able to help you.. I hope your car is fixed.. if not the cam sensor can cause your problem too, they intermittently work but when they don't the pcm guesses at when to fire the injectors based on crank signal and will get it wrong at times