1998 Firebird Random misfiring at different times

Hello and thanks in advance for any guidance in this issue.

We have a 1998 Firebird with 55,000 miles on it. Prior to a month ago it had never broke down or had any issues short of a battery and altenator a few years back. The only maintance we preformed over the years was oil/transmission fluid changes and injector cleanings.

The only modifications we made are cold air intake and flowmaster muffler and that was 3 years ago. It does use a K&N Filter and it is oiled/clean.

About a month ago we decided to put on new brakes, ironically as I was driving to my brothers workshop with the new pads the car broke down. First time ever breaking down and while on the way to do something to it really! It gave a computer code for cyn 3 misfire.

It would run but was misfiring quite alot. I remembered that feeling from a 95 camaro I used to have knowing it was probably a plug wire being burned by the manifold. I replaced the wires and plugs( and brakes!) And while before I got a computer error that said cyn 3 and would misfire at low throttle but went away at WOT now after new plugs and wires it was excellent and smooth low throttle and misfiring at WOT! Completely reversed.
The plug wires I was not sure on as they were a old but unused set so I went and got a autilite platnium set and put those on to be sure. I also replaced the fuel filter which had never been changed and cleaned the dirty K&N filter.

Now with all that done when it starts and decides it is going to be rough it seems to be that regardless if it is idle or throttled and just missing all around. However most of the time it doesn't misfire at all and is very smooth idle and running.

Over the last several weeks it has continued to do the very rough idle. Most of the time it is fine and runs great. When it decides to run bad it usually continues to do so until it is turned off. Sometimes it does it when it is cold, sometimes it does it after it warms up. Even though it feels like it is misfiring be it very rough idle about to die or in the driving range it does NOTgive a code anymore. I have not had a code since the day the initial cyn 3 misfire code and was cleared before the new plugs and wires.

What is making this hard for me to diagnose is how most of the time it starts/runs fine be it at idle/acceleration or even hitting the gas hard. Then out of the blue you go out and start it and it runs like it is about to die, sputters in acceleration and cannot be drove hard if hardly at all. We have been stuck out at places and had to drive home like that and other then the sputtering in acceleration it drives and makes it home just fine.
To give a idea in the 3+ weeks since it has had the stuff done, it is drove 4 times daily( start/drove to a place ) and this has happened only 3 times out of the probably 30-35 times it has been drove.

I'm really leaning towards either O2 sensors or ignition coils but other thoughts are welcome to explore. I really din't want to get into replacing thousands of dollars of stuff that didn't need it so hopefully you all can help!
Do you
have the same problem?
Tuesday, June 26th, 2007 AT 3:31 PM

1 Reply


We have a 1998 Firebird that had the same symptoms namely running poorly, shaking, stalling and even leaving my wife stranded and having to be towed. There were no codes. Good mechanics cound not find the cause. Like you the puzzle was that the problem would actually go away for long periods and the car would run great, full power and all. Then when the problem returned, it was not always a totally stalled or dead car, it would be very poor running, getting worse or better. Sometimes it even stopped and after half an hour would start right up and run perfectly. Sometimes I would come out to where she was stuck and drive the car home pumping the accelerator to keep it from stalling as it shook like it was missfiring. As I write this I am looking at the old part the mechanic gave me that caused all this trouble. IT IS A CONNECTOR that is at the end of a main wiring harness, which has 7 female pins to one side of the center screw and 7 female pins on the other side. This connector is gray in color and on the other side of the pins it has the 14 color coded wires that had to be cut in order to take this faulty conector off of the wiring harness (these wire ends are about half inch and from the way they are pulled you can see the tension they were under). The repair required a section of new harness with a new connector to be SPLICED into the main harness (I hope they actually soldered the wires and not just taped them). My mechanic was reliable, but even he needed the help of a friend of his who worked at the Pontiac dealers. His friend got the portion of harness with the conncetor, and saved me the three hundred dollars the dealer would have charged just for an entire new harness. The connector is TWISTED (I believe under the dash board or just outside the firewall) because the wire harness is too short and the END PINS are pulled at an angle and after time the electrical connection becomes INTERMITTANT (especially with heat and vibration). After this was replaced with a new CONNECTOR and harness section, the problem was solved. Good Luck :-) PLEASE SEE MY QUESTION ON MY OTHER 1997 FIREBIRD HAVING "STRANGE SYMPTOMS" POSTED JULY 07 AND NO REPLY YET. CAN YOU HELP OR DIRECT ME? THANKS, CAPPI45.
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Sunday, July 29th, 2007 AT 7:52 PM

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