Thoughts on fixing misfire
Thread Starter
Drifting
Joined: Jun 2000
Posts: 1,856
Likes: 55
From: Largo Misfit, Florida
St. Jude Contributor
Cruise-In III Veteran
Thoughts on fixing misfire
Hi all! One morning my '01 developed a noticable misfire, mostly noticable at idle, or ubove 3,000 rpm. I had put in the NGK TR55's myself with MSD wires a couple years ago. Took them out and noticed a couple plugs had faint dark areas on the insulator tip. I figured maybe as I was fishing them into their holes, a little anti-seize might have got on them. So, I changed all the plugs,no change. I switched places with the wires, and it's better, but when it's hot, it will still miss, and the exhaust pops at shifts. I've ordered new wires, if that doesn't fix it, what else can cause misfire? Injectors, or something else? Thanks for any input!
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
The DIC DTC codes will not pin point what specific cylinders are mis-firing! All you will get is a flashing check engine light and if the mis-fire is severe enough, it will set a P0300 DTC code. If you happen to be checking the DTC when the mis-fire is occurring, you may see the P0300 code in the DIC but if the mis-fire isn't occurring continuously, the code will clear shortly after the mis-fire event stops. At least that how it happens on my 02 Z06. Other year C5 may be a bit different. The Tech II and some tuner soft ware have the ability to pin point what cylinder/s are mis-firing.
You can measure the resistance of the wires. OEM wires should read 250-750 ohms end to end. The boots should be clean and playable with no burns and tears. The spark plug insulators can crack and cause a miss that is sometimes difficult to locate. If your wires are good, check the plug insulators. You could also have a bad/broken valve spring.
You can measure the resistance of the wires. OEM wires should read 250-750 ohms end to end. The boots should be clean and playable with no burns and tears. The spark plug insulators can crack and cause a miss that is sometimes difficult to locate. If your wires are good, check the plug insulators. You could also have a bad/broken valve spring.
Le Mans Master
Joined: Nov 2005
Posts: 6,993
Likes: 25
Not entirely true! A RANDOM MISFIRE will many times show up as a P0300, a SPECIFIC cylinder misfire can show up as a P03xx where xx is the cylinder misfiring. I've seen this behavior with clogged fuel injectors, bad plug wires and bad plugs. There are other possible reasons for misfires that may NOT show up as a P03xx code but it's a place to start looking. Things like hung up EGR valves can also lead to what feels like misfire but will show up with different codes. The P03xx code will stay in the ECM for several restarts. It takes 2 misfire codes in a preset time period to set the MIL lamp, a single one wil log in the ECM.
Last edited by dougbfresh; Jul 24, 2006 at 03:08 PM.
Melting Slicks
Joined: Mar 1999
Posts: 3,326
Likes: 0
From: Taylor TX
NCM Sinkhole Donor
My 98 had intermittent misfire, and I got no codes.
A friend with a Tech2 drove the car and saw misfire predominantly on one cylinder. When one cylinder misfires, the computer will also show misfire on neighboring cylinders as well, but it will be smaller in magnitude.
So we looked at plugs and wires first. They were fine except a couple plugs were pretty dark, probably from unburnt fuel in the misfiring cylinders. Then we swapped coil packs, and it didn't change. Then we swapped injectors, and the problem tracked with the injector.
We pulled the problem injector back out, he slapped it into the palm of his hand, and some crap came out. It amounted to about 4 grains of sand.
Put it back in, and the car has run fine for about 5k miles since.
A friend with a Tech2 drove the car and saw misfire predominantly on one cylinder. When one cylinder misfires, the computer will also show misfire on neighboring cylinders as well, but it will be smaller in magnitude.
So we looked at plugs and wires first. They were fine except a couple plugs were pretty dark, probably from unburnt fuel in the misfiring cylinders. Then we swapped coil packs, and it didn't change. Then we swapped injectors, and the problem tracked with the injector.
We pulled the problem injector back out, he slapped it into the palm of his hand, and some crap came out. It amounted to about 4 grains of sand.
Put it back in, and the car has run fine for about 5k miles since.
Thread Starter
Drifting
Joined: Jun 2000
Posts: 1,856
Likes: 55
From: Largo Misfit, Florida
St. Jude Contributor
Cruise-In III Veteran
Misfiring
Good to see some discussion on this! I changed all the wires, and again, not much help. It seems to mis a lot more when it's hot. I did check the codes, and got the one that says "misfire detected". Not much help! I was about to buy a new coil and start replacing them one at a time, but the injector idea sounds good. If I can only find the cylinder that's misfiring. A friend gave me a thing that looks like an ink pen. You hold it against the plug wire and it lights up as the spark goes through. It's hit and miss, tho, and doesn't really do the job. Maybe I can find a clip on version. Thanks again everyone for your help, I will have to take it to the dealer eventually if I can't fix it myself.
Le Mans Master
Joined: Jul 2002
Posts: 7,370
Likes: 8
From: Slave to the evil empire
Cruise-In V Veteran
St. Jude Donor '04
Originally Posted by dougbfresh
Not entirely true! A RANDOM MISFIRE will many times show up as a P0300, a SPECIFIC cylinder misfire can show up as a P03xx where xx is the cylinder misfiring. I've seen this behavior with clogged fuel injectors, bad plug wires and bad plugs. There are other possible reasons for misfires that may NOT show up as a P03xx code but it's a place to start looking. Things like hung up EGR valves can also lead to what feels like misfire but will show up with different codes. The P03xx code will stay in the ECM for several restarts. It takes 2 misfire codes in a preset time period to set the MIL lamp, a single one wil log in the ECM.
I had a misfire on the #8 cylinder and my DIC showed a P0300 code but when I hooked up a code scanner it showed a P0308 indicating the #8 cylinder was the culprit.
And to Jim terry, you can also use a timing light just clip the lead on to the plug you want to check and pull the trigger and watch the strobe light up, although unless it's pretty bad you won't be able to tell much.
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