number 3 misfire

[Spartanzz]

Ok IV been stuck trying to find what's causing a misfire with me cylinder 3. Its only at idle and when giving any gas it just goes away. I also found out that when the AC is turned on it goes away again. That makes me think idle air control valve or tsp what do y'all think? Car is a 98 gtp with 148k miles

[BillBoost37]

If it's always one cylinder, it's not TPS, IAC or sensor related. It'll be plug, plug wire or coil. ....once in a while..ICM, but that's a last resort.

[Spartanzz]

well now see thats the thing plugs, wires, ICM, coil to that cylinder are all new. And, i swapped injectors around to see if it was that and it stayed in number three. It has a new fuel filter iv run fuel injector cleaner through it we replaced all the vacuum lines and even sea-foamed the car. The only thing that stops me from thinking it is mechanical is teh fact that it goes away when the ac is on or when i hit the gas. What else could it be?

[BillBoost37]

Swap plugs between 3 and 5, see if the misfire moves. maybe a cracked plug.

New parts can be bad. If none of this cures it..I'll send you under the valve cover.

[Spartanzz]

ill do that today will be sitting at home most of the day anyway but...just cuz it wasnt too expensive i got a IAC just to be sure didnt seem to change but dont i have to unplug the battery for 10 mins? cuz i herd with a new iac the computer has to relearn the idle is this correct?

[BillBoost37]

I've never relearned one...Not sure...unplug it anyway. But for 30 min.

[Spartanzz]

ok i just went outside and swapped the 3 and 5 plug and unplugged the battery. Is there any possibility it could be the egr or the tps?

[BillBoost37]

Things like the sensors..only sense. The EGR and IAC control exhaust and vacuum into the motor to make it run differntly. They do this before the SC or right at the TB basically. That's why one cylinder misfiring is never caused by them. Because all cylinders are pretty much fed the same by the stuff.

The only things that effect one cylinder are injector, injector wiring, valves, piston, plug, wire, coil, icm.

Now you know all my secrets of diagnosing and I'll have to silence you..want a bribe?

[Spartanzz]

LOL well i thank you for all the info just exploring all the possibilities. Its just been driving us nuts for a while now car runs and drives fine and the mifire i have at idle isnt really huge and doesnt cause any lag or bogging down. Mainly you can feel it while sitting at a light and if parked and you stand behind the car you can hear the puffing from the exhaust. But, like i said before its just odd cuz the roughness at a light and the puffing from the exhaust completly goes away once the AC is turned on. And i have the torque app for my android phone and i watched the RPM with and without the AC and it was a 40 RPM difference (AC on being the higher RPM at around 800 and without being around 760)

[Spartanzz]

ok well, went back outside connected the battery and the 3 and 5 plugs are swapped i cleared the codes so if the misfire moves ill see the code pop up for it. But, i can still hear the misfire so i guess i just wait then?

[QUICKSILVER462]

do a cranking psi test on the suspected cylinder, and compare with all other cyliders.

[BillBoost37]

Ya know..something else that might be doing it is a weakening spring. I've often found when modding cars that the cylinders leak a bit when you put air to them. A few taps with a mallet to the top of the valve stem seals them decently and new springs (usually stronger) with one or two taps...they seal almost completely if not 100%

[Spartanzz]

interesting, well im going to see if i can do a quick test on the cylinders this week if i get time and just hope it wont be too extensive to fix...

[EJOUIE]

one thing that hasn't been mentioned is that the PCM can also cause misfires. Probably not in your case though.

[Spartanzz]

yeah one guy said that could be a possibility only way to tell though is get my hands on a completly stock PCM or find a buddy with a gtp or something.