Low RPM Misfire [Archive] - GrandAmGT.com Forum


View Full Version : Low RPM Misfire

05-10-2009, 09:11 PM
This is driving me insane. My 3400 in my Grand am GT with 64,900 miles on it seems to have a misfire at lower RPMS that clear out without any hesitations or "hiccups" when the throttle is pressed. Note, however, that the car has 124,900 on it, but my engine and transmission have 60k less.

My power seems alright and I do not get any hesitation or anything at any RPMs. The car feels pretty much normal when it is above 1500 or so. Under that, it drives well, but when I press the throttle with my hand, it still misses at that RPM.

At idle, my RPMs do not surge at all; they are rock steady where they belong - at about 650. I have sprayed starter fluid on all vacuum lines to no avail. I listened around the engine for a hiss with a rubber hose in my ear (lol) and found nothing. My coils have all been checked and two were swapped out. My plugs and wires have been changed, I have checked spark only to find that all six wires produce a nice blue arc.

I also unbolted the down-pipe to see if the catalytic converter was clogged up, but the engine ran exactly the same with the pipe disconnected. I put a vacuum gauge on the upper intake manifold and the vacuum read between 18.5 and 19 in-hg; the needle swayed very very little. I have changed every fuel injector, the fuel filter, and air filter (as maintenance). My head gasket blew a couple of months ago, but this problem started about a month before it blew. I know exactly when the head gasket went because the car overheated on the highway when an idler pulley broke off causing the belt to fly off. I have performed a compression test and all of my cylinders held at least 175-180 for 5 minutes without losing any compression. My head gaskets and everything above has been replaced as required/recommended.

I disconnected my pcv hose from the valve and plugged up the end of the hose where the valve should be and started the car. It ran exactly the same so no valve cover/pcv valve problems there. Also, My O2 sensors are both brand new. I saw that my harmonic balancer wobbled a bit so I swapped it off for the other one that was on my 1999 engine. The balancer is nice and steady but the problem is the same.

This is definitely a misfire rhythm. There are no codes stored in the computer, so obviously there is no Service Engine Soon light.

I have cleaned both the idle air control valve and the MAF sensor. I also swapped the MAF sensor from another GAGT and found the problem to be identical.

My FPR has been replaced because my other one leaked.

Thank you to anyone who responds!!!!!

05-11-2009, 01:45 PM
I have verified that my lifters and my camshaft lobes are fine as well as my lower intake manifold gasket. No vacuum leaks anywhere whatsoever.

05-20-2009, 04:03 PM
Any ideas?

05-19-2010, 12:52 AM
hi,,you might want to check the tps voltage readings,,,and see in they are in spec,,,since you changed the FPR,,,and swapped coil packs,,,you may have an issue with TPS/fuel pump voltage...OR,,,maybe related to MAP sensor,,,which regulates engine load and uses info from TPS to increase/decrease fuel psi....