Random missfire

William Jones · Posted: Wed Feb 15, 2012 11:45 am Post subject: Random missfire

I have a LT1 I built for a customer a few weeks ago he is 3 hrs away so trying to help him diagnose the problem. It runs fine then every once in awhile when he tries to accelerate it sputters then the check engine light flashes and stays on, the code says random mutiply missfire. This is in a 96 Impala SS the opti spark, plugs, wires, o2 sensors are all new. The crank trigger and coil are old but I would think it would always do it if they were bad. He can drive it like 20 to 30 mins and it will be fine then he resets the computer and it does it again. Kinda has me stumped without actually seeing the car.

Bam

Paul P · Posted: Wed Feb 15, 2012 12:57 pm Post subject:

Opti spark is the issue. My friend has had 2 of these cars and issues are common with them. The aftermarket replacements are better but not cheap.

William Jones · Posted: Wed Feb 15, 2012 3:14 pm Post subject:

We put an accel optispark on it new and there is less than 20 hours on it.

10sec.et · Posted: Wed Feb 15, 2012 4:16 pm Post subject:

problem with the Opti is that it only takes one good puddle in the road to ruin your day. new or not, that Opti HATES water. has he been through any drive through car washes ? how old is the water pump ? those will leak and soak down the Opti. also seen issues with corrosion on the sensor pigtail at the Opti. are you positive its "random misfire" ? could it just be bad gas ?

William Jones · Posted: Wed Feb 15, 2012 11:49 pm Post subject:

10sec.et · Posted: Thu Feb 16, 2012 1:52 am Post subject:

if its been dry then its time to look elsewhere. realistically, the reason the water kills the Optis is... 1. it causes crossfiring in the cap or 2. it has enough time to take out the little sealed bearing the shaft rotates on and the sensor reluctor hits the sensor itself.

i have had MAF cause misfires on mine. is the air intake still stock with that weird "home plate" looking thing on top ? mine would work its way loose and cause an air leak (didnt fit worth a damn to begin with).

silly question but, did all the ignition cables get pushed onto the dist with at least a couple "clicks" ? if the wires are missing the little metal clips it wont contact the terminals very well and ultimately burn the end of the wire and cap terminal off.

William Jones · Posted: Thu Feb 16, 2012 2:12 am Post subject:

squeeezer · Posted: Thu Feb 16, 2012 1:48 pm Post subject:

check fuel trim under load too,when you get the chance
ive seen too many+25

Paul P · Posted: Thu Feb 16, 2012 4:37 pm Post subject:

Was there an issue like this with the previous engine in the car?

William Jones · Posted: Thu Feb 16, 2012 9:31 pm Post subject:

I just got off the phone with him he said the MAF was a little loose and before we worked on it that there was a check engine light but wasn't sure what it was for. When we got it at the shop it had a cracked head and two bad rod bearings he had overheated it when the radiator tank cracked. So I have him checking for any vac leaks and then reseting the computer and see what happens if it continues he said he would take it and have a diagnostic check done. It only happens like once everytime he drives it. He said he will be cruising along then he tries to accelerate and it will not respond then sputter and the CEL flashes and it goes away and runs fine. It is also hunting for idle when he comes to a stop sometimes.

Bam

10sec.et · Posted: Thu Feb 16, 2012 11:49 pm Post subject:

definitely sounds like a vacuum leak somewhere after the MAF.

Paul P · Posted: Fri Feb 17, 2012 8:20 pm Post subject:

Get rid of that stupid intake muffler right in front of the motor and put an aluminum puck in it's place. That could be a source of leaks. I will post a pic of my buddies as he had issues with that too. Simple elimination of a point of failure.