Engine virating\Exhaust chugging

[Gonk24]

I have an 03 GT with 136,000 that I'm having trouble with. I've had issues previously with the top plenum (replaced last summer) leaking coolant into the engine. That clogged the cat also. Had the plenum changed out and did the cat replacement myself (bolt on). The cat is a higher flow and of course has since kept the SES light on. Soon after that was taken care of the 2 plastic coolant 90 deg elbows leaked and were changed. Car has run great.

This past weekend while on the interstate going across town the a/c stopped cooling and the temp started to go up. It didn't go into the red or kick on the temp light but there was a loss of power. I get off the road and to a station (very close, again no red zone) and the SES light flashes 3-4 times then stays constant like it's been for a while. This time it's the water pump going out. I let it cool down and filled it back up with water to get it to my in laws about 1 1/2 miles away. Got there with no trouble and no overheating. I changed the water pump there the next day and it's working perfectly.


Now when at idle the engine is vibrating in a front to back motion. The exhaust is chugging also. The vibration is stronger in park and neutral than in gear. I've noticed also that just before a complete stop and when letting off to roll forward there's a fairly strong front to back shake. There's no change in power or driving at regular speeds on the streets or highway. There's no vibrations at normal driving speeds.

I changed the oil yesterday and didn't notice anything out of the ordinary. I've looked around for vacuum leaks and haven't found any. In park the vibration goes away around 1400 rpm. This may be a separate issue but I also found that it won't go over 4000 rpm. This is in my driveway not on the road. At 4000 the exhaust chugs pretty hard and it just won't go any higher. I want to say this was in gear but I just don't remember. Right before I quit for the night I did rev it a bit in gear and around 1300 I could hear a hissing noise from the engine. It went away when letting off.

Any ideas of what to look at to start eliminating things? I plan on changing the plugs in the next couple days mainly because it needs done.

This is my cat converter 1997-2003 Pontiac Grand Prix - Eastern Catalytic Converter - Direct OE Replacement, High Flow OE Replacement

[BillBoost37]

Being that it ran warmer than usual, I'd suspect a possible cooked plug.

[Gonk24]

Disregard the rpm over 4000 issue. While driving this morning there was no problem whatsoever getting past it.

[Liquifire]

If it was in your driveway, you were hitting the rev limiter when revving it up. this would have caused your chugging issue with the exhaust as well. This would make sense why it is OK when you actually drove it above that RPM point.

[Gonk24]

Chug is there without reving

[Gonk24]

Had the code pulled this morning. P0303 misfire on cylinder 3. I changed out the front 3 plugs and wires last night (includes #3) and will do the rear tonight. There's been no change in the vibrating but the exhaust did change. It no longer chugs in a consistent pattern. The tone sounds normal with an intermittent chug here and there. If the issues is still there after the other plugs/wires are changed where to?

[Liquifire]

Icm, maf, iac....

[Gonk24]

Got the plugs and wires all changed last week and still having the same trouble. I ran a can of Seafoam through the gas on a 220 mile trip Saturday. SES light is still on so I had the code pulled thos morning and it's P0303 still. The catalyst code that been there since the cat change is now gone...Seafoam maybe fixed it? Anyways I have a misfire in cylinder 3 that was present before and after the plug/wire change. Just need to track it down. I could swear that the vibration is worse now after new plugs/wires too.

[FRESCOLIO]

Auto zone can be suspect. Wish I had more help.

[Gonk24]

Auto zone can be suspect. Wish I had more help.

That's why I only have them read the code and make sure I see the screen also. I switched coils around hoping the misfire would move to another cylinder with the #3 coil but nope, still misfire cylinder 3.

[Gonk24]

So on a drive today across town to get my daughter the car ran great with no vibrations or chugging exhaust. Back to normal. Started the car back up to leave and there it was again. Once I got home I poked around a but and decided to pull the plugs on the injectors. I started with cylinder 3 (where it's been misfiring) and bam, no change in the engine with the plug off. Plugged it back in and did the others one by one and got the expected sputtering. tried it several times on #3 and always made no difference. I had been through all injectors with brake cleaner when checking for vacuum/o ring leaks and didn't find anything. Now I notice on my can of brake cleaner that it's NON - FLAMMABLE. That obviously won't do. Picking up a flammable spray in a bit to try again.

[Gonk24]

Tried carb cleaner and found nothing. Since it seems the #3 injector is fouling up at idle/low rpm I held it at a higher rpm via throttle cable then pulled the plug. It bogged down and sputtered like it should. So, the injector is working correctly there, just not at idle. Hmmm.

[Liquifire]

is it the injector then? or the wiring to it?

[Gonk24]

If it was the wiring I'd say it would have issues regardless of rpm but cars can be strange. Probably the injector. I think I'll switch 2 of the injectors around and see if this happens on a different cylinder. It could also be something else besides these 2. A reply to my post over at 2carpros suggests maybe a weak spark causing the plug to gas foul. Ignition module if that's the case?

[Liquifire]

ICM, the plugs themselves, the wires, or the coil pack...it could be any of them. I would swap the injectors and see what happens. Inspect the plug on that cylinder for any cracks, etc.

[Gonk24]

Plugs and wires are new. I used Delco Platinum plugs. I've switched the #3 coil pack with the one behind it with no change. I'll be swapping the injectors Wed. to see what happens. I did pull the new plug from #3 to inspect it and it's already black.

[Liquifire]

Have you had autozone or someone like that check the ICM? They will do it for free... If you do have them do it, have them check it several times back to back.

[Gonk24]

Went to visit my parents this weekend (5 hrs away) and had the car looked at. My Dad has owned and operated a shop forever. We were able to get to the bottom of what's going on and it's not what was expected. Turns out to be a couple bent valves. Intake and exhaust. We didn't have time to do the repair so I left it with him and he's going to fix it and only charge me for parts. Cylinder 3 compression was about 55 psi lower than the other 5.