Random Misfire

Josh Wendler
155

Asked by Josh Mar 25, 2009 at 04:13 PM about the Pontiac Trans Am

Question type: Maintenance & Repair

As much as i like to think i know about cars this one has me stumped. I recently went and drove my car today, and it had acted funny when i started it up. It started backfiring at idle. So i let it warm up and see how it did, it started to act fine, so i went ahead and drove it. Feeling it jerk everynow and then, it also felt as if it had lost some power most hills around here i can go up in 6th gear without needing for giving a lot of gas. but I actually had to downshift at 60 to 5th and sometimes 4th to keep it going. Then after i stopped and turned it off it had a hard time starting. It also starts hard when its cold. But it had acted like it was turning over and getting ready to start a couple of times before it did start. So i had it scanned and they read up a Random Cylinder Misfire i dont know the codes sorry, a Catalytic Converter Deficiency, and an O2 sensor out of range. I'm thinking maybe a fuel pump, coil, or timing is out of wack. But right now i'm trying to get opinions. I really dont want to start just shooting at things and end up spending 1000 and 15 hours on a problem that should be relatively cheap and not so time consuming. The ignition is pretty new but i'm going to go ahead and get new wires i have one that looks pretty torn up but it ran fine before with it like that as a matter of fact for about 6 months like it.  

17 Answers

Jacob Fraser
765

try the new wires and plugs first, if one cyl is misfiring it would cause excess oxygen in the exhaust and make the o2 sensor read high, or out of range, it could also cause converter issues with fuel in the pipe. im not sure, but the backfiring could be caused by the o2 sensor maybe... either way start with the cheap things that could cause it and work your way up

Bill Flunker
265

it would help to know what year the car is and the engine it has before i can help, but whatever you do dont listen to the first guy, when i went through my schooling for auto tech there was nothing they said about going after the cheap stuff first, actually its diagnose it and dont just start throwing parts at it, and the backfireing has nothing to do with the O2 sensor it could in no way cause that. Did you have a check engine light when it misfired? fuel pump shouldnt cause it to backfire so I wouldnt be searching for that at this point. Heres where knowing the year helps, is it a DIS ignition or does it still have the disributer? that can make a difference on where to check on the ignition system. Good luck and if you get me more info it may be easier to help.

1 out of 1 people think this is helpful.
Jacob Fraser
765

its a 6 speed so its very likely going to be a 98-02, so its a coilpack system, a misfire would cause excessive oxygen in the exhaust pipe, and an o2 sensor reads what? it was already put on a obd II, got a code of a random misfire, and o2 sensor out of range, so no help there.... "diagnose it" that is what schools say is the most perfect way to do things, which is obviously true, but its not always possible. why not replace the plugs and wires when they look old, and could use replacing anyways?

Bill Flunker
265

which bank is it? is it before or after the cat? and why is it impossible to diagnose it? the tools are there, if he can pull codes its easy to diagnose, but i do agree if there are alot of miles and stock plugs its time to change them, also if the wires are ugly (wore spots, carbon trails or oil soaked) they need to be changed. but that shouldnt cause the random misfire, it would cause a misfire on cylinder whatever is missing, well hopefully its as easy as plugs and wires though

Josh Wendler
155

its a 97 so lt1 i thought it had included it that it asked for the year sorry... but it has the optispark distributor. and its bank one that really messed up looking cat. But i actually got the mis out some how i messed with the valve lash and drove it around for a while and it threw the SES light again, so i went and had it scanned again it pulled out Catalytic Defficiency bank 1 and EVAP Purge not working.

Josh Wendler
155

123928 miles as of today, wires and plugs were replaced last summer so pretty new still as well.

Jacob Fraser
765

it could be your evap, it controls air going into, and fumes getting out of your gas tank, if it was plugged up it could cause low fuel pressure.. i would ignore the cat issue codes for now, they are being caused by something, not the cause themselves usually. try pulling the vacuum line off the rear of the charcoal canister, and the throttle body, blow out the line with compressed air, and see if any black specs/chunks come out. if any are seen at all you need a new canister and purge valve

Jeremy Wanamaker
25

If it is an emissions control problem it could very well be your oxygen sensor or the idle air control. Or maybe you'r timing belt slipped and the timing is off. It wouldint be you'r coil cause it would stall all the time. First thing I would do is check the timing. The electro harmonic balancer in the distributor is probably fried and it screwed up your timing, in that case a new distributor would be the best thing. Or one of the metal pins that the rotation pin hits inside the distributor is probably worn. change the dist. cap, it might be cracked and moisture gets in, that will cause the car to missfire too. And if it dosent work i say change the whole distributor. Tell me what happens!

Josh Wendler
155

ya i would change the dizzy but i'm sure ur just as aware of how much it cost's as well as i am. I replaced it last winter so i think its still pretty new only 10k miles on it. but $300 right now is kind of a bit. I did find some more information out my vacuum on my intake mani is fluxuating spiradically at idle so im positive its something in the valve terrain more or less im thinking a collapsed lifter.

William Trute
25

catalytic converter is plugged, remove clean out and replace. that will fix it ,that fixed mine

1 out of 1 people think this is helpful.
Jacob Fraser
765

a converter plugging is the sign of of a problem, not necessarily a problem

William Trute
25

the converter is restricting air flow away from the engine if the exhaust does not leave the chamber no fresh air is introduced into the cylinder therfor the fire is only fireing on gas with no air ,and is seeking air ,causing the random misfire may be even in the exhaust pipe , get air get fire get gas ....will run thanks

Jacob Fraser
765

yes but a converter just doesn't plug by itself, and if it was it would cause low power but the engine would run smoothly at idle, and at low throttle

Chris Lundy
35

All, I have a '96 w/LT1 and had this problem. This is a widely known issue with the LT1...they don't like platinum plugs. Pontiac guys told me that after I replaced mine w/ Bosch Platinum+4s...sure enough 3000miles later I got the micelaneous misfire code they warned me about. I pulled the plugs and all were fouled. I hen replaced them with regular old champions and she has been purring like a kitten every since. Technically it comes down to a heat range mismatch. The LT1 runs relatively cool as compared to most modern engines and the platinum plugs (especially Bosch) like to run in an engine that develops more heat. I suppose that if you found a platinum plug that likes the LT1s heat range you would be fine...however, I don't have ths specs and i doubt it's woth the labor. When I went back to the parts store I found another guy with a LT1 car and he said that he has his up to a little over 750hp for drag racing and can only run the regular champions...everything else he has tried threw the Micelaneous misfire code and caused the engine tor run rough after a few heat cycles. Bottom line go get some regular plugs at the auto parts store and enjoy your weekend of pain changing the plugs. Wish I had better news...it is a freaqing bitch to get to the #7 and #8 plugs. It's a good thing no one was recording me while I was changing mine...again! --Chris

1 out of 1 people think this is helpful.
Josh Wendler
155

i actually did some research after taking of the exhaust manifold seen that the #5 port was clean compared to the rest, so i took out the spark plug and it was clean too. so i watched the wire fire and the spark plug they got good fire so i'm now assuming its the fuel injector.

Chris Lundy
35

I didn't beleive it was the plugs either, but the local experts recommended the change. I figured $16.00 for new plugs is way cheaper than injectors or fuel pumps or etc, etc, etc while I chased a phantom problem. $16.00 fixed my issue and she has run great for 15 months. Wish ya luck with the injectors.

1 out of 1 people think this is helpful.
Josh Wendler
155

well i'm writing an update, to my own stupidity on my own car... I was a fouled #8 plug. I still haven't gotten it out but, I also scanned it and saw the #4 cylinder missing as well. so i pulled the spark plug there. I found to my amusement that it had burning rich. Not only did i have a fouled plug but a bad fuel injector. I have switched the injector and it runs alot better, but to whom may read this never and i repeat never use ACDelco's in the LT1's the best shot you have is a 50% chance of them running fine.

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