ok, I have posted this twice in the regular forums. I got hardly any responses to my problem. I have taken my car to two mechanics and they are both stump. My car tends to missing fire, and back thru the exhaust when you drive around 15 minutes. Diagnosis so far. We put the car on the machine, no codes on any of the sensors. Everything on the computer checks out ok. I have replaced the sparkplugs, wires, coil, fuel filter, and the fuel pump. One mechanic says to replace the rotor and cap, and the their is a crack in the cap causing the miss fire). The other mechanic says it is still leaning out and to replace the 02 sensors (which the computer says is ok).

Honestly, I think both don't have any idea what is wrong. Where should I start? I will replace the 02 sensors this weekend. Then the rotor and cap next pay check. I hate to say this, but I may have to take a loss and sell the damn thing. Even the dealer is stump, all my mechanic friends come back with....."I don't know" THis is my last chance. Could the 02 sensors be bad even without giving a bad code on the computer? Anyway thanks with any response. The computer is suppose to give a bad code when it missfires or leans out, but no code at all. That is what is stumping every mechanic I take it to....Oben Wan's "you are my last hope"

 

I had similar problems with my 305 TPI setup. Took it to 3 dealers as a last resort and they couldn't find anything! ANYTHING! The bvest answer out of one of them was that the cat was plugged and they aren't allowed to touch that part of the car because of legal reasons...thanks GM So I ditched the TPI

Make sure the spark plug wires are not touching each other or anything else around them. Does the car run good in the first 15 minutes of driving? or does it just start of kinda bad from the get go?

 

The car runs ok when it is cold. When it warms up around 1500-3000 rpms it starts missing and backfiring. One mechanic says the rotor cap is causing the missfire. The other mechanic states it is missfiring because it is leaning out. Again, I have replaced so far.....spark plugs, wires, ( no difference when replaced, still missfired before replaced), fuel filter, fuel pump, coil.

You have to get the car REALLY hot in order for it to start to missfire.

Funny thing is it doesn't missfire when it is cold. So when I purchase it, I drove it around, and it was fine. The car heats up....pow, papow, back thru the exhaust. I am changing the 02 sensors this weekend (even thought the computer says they are working ok). The cap and rotor next pay period. So the cat convertor can act up when it gets hot? I am in the country so getting rid of it is not a problem.

 

If this is a 93-97 f-body, this sounds like the start of an optispark problem. On the other hand, if the misfires start as soon as you get into closed loop, then it could easily be a sensor problem. Before you start throwing new sensors at it, maybe you should get monitoring software or tool (AutoTap, Diacom, Scanmaster, depends what car you have) and SEE if any sensors are acting weird. (Go ask on the appropriate forum - LS1/LT1/3rd gen - if you're not sure what to minotoring tool get.)

 

I would get the car to were it is missfiring and then hock up to the scanners to see if any codes are thrown!

 

I think you would have a better chance of a meaningful response if you provided ALL the relevant data.... what engine, what year, displacement, mods, etc.

It is almost impossible to give a detailed response when no one has any idea what engine/car you are talking about.

 

1994 z28 lt1 Sorry guys, after posting this subject about three times and getting no response in the regular forum, I forgot. I am going to do what was suggested, me and a mechanic are putting in a new rotor and cap for around $120.00. Thanks guys for all your help.

 

this could be a long shot, but is the car in open loop or closed loop when this happens? i say long shot because you say after 15 minutes it starts happening. the car should be in closed loop by then. other then that i say get it on auto tap. you may not be throwing codes, but that doesnt mean youve got all you sensors working correctly. Trey

 

Thanks for all of the responses that I have got. I am going to replace the opti spark this weekend. One of my mechanic friends tends to 100% sure that this will fix the problem. I have tried dissconnecting some of the sensors, example maf, map, etc. It made no difference in the missing. The other mechanic tends to think the 02 sensors are going bad (after the computer told him that were working ) are leaning it out. Again, thanks to everyone for the help. I hope NOT to use guys again for a long time but who knows.... Thanks Rich

 

I just had a similar problem. It turned out to be a bad coolant temp sender. If it weren't for datamaster, I never would have found it as the sensor on the motor was new. It would report the correct temp until gradually it would go down to -40 degrees. You need to get some monitor software and data so that you won't be arbitrarily changing parts. Just my suggestion.

Curt Sr.

 

Me and my mechanic friend removed the opti-spark. I order the ac delco (cap and rotor) from advance auto and everything was the same except the back part of the distributor cap. How come every after market part you buy is different than the orginal manufacturer? We made a home made gasket and used some rtv.

We had to stop after 7 hours, his chevy busted a brake line as I started to help him with his brakes first thing this morning. Chevy's were just falling apart today. We will have the opti-spark in tommorow. The cap and rotor didn't look bad.


So far this is what I have changed and have checked on the camaro....no engine codes, coolant temp sensor, spark plugs, wires, coil, fuel filter, fuel pump, opti spark. If it misses after this I am at a lost. Anybody need a used camaro??????