'98 Stratus misfire

[J-Ri]

One of my co-workers and I (between my appointments), have been trying to find the reason for a misfire on a Stratus for several days now. I'm hoping we can find the answer.

The customer purchased the car used while it was running poor. It appears the previous owner had replaced the catalytic converter, distributor and PCM already.

The car is a '98 Stratus with the 2.5L V6 and auto trans with "autostick".

It runs fine at idle, but as soon as you give it any gas it starts missing, it misses worse under load and will occasionally die. It has a P0300 code. The majority of the misfires are on cylinder 5, most are on bank 1. There were a few misfires on cylinder #2. After the car warms up it runs fine. It takes about 20 minutes at idle or 5 minutes reving it up. If you shut the car off once it runs good, it runs good if you start it back up right away. With the engine running at 2-3k RPM, the tachometer jumps quickly between 3-4k RPM with very little change in actual engine speed. With the engine speed at 2-3k RPM, the check engine light will go off for several seconds, flicker, or flash. That made us think PCM, but it has already been replaced, and it's an expensive guess.

We have checked the following and all are good:
ECT, IAT, TPS, O2, CMP, CKP sensors
Plugs, wires, cap &rotor, distributor
there are no vacuum leaks (checked with smoke machine and intake cleaner)
The timing is correct
The EGR valve works properly
Fuel pressure and volume are good
The compression is good

Thanks in advance for any help.

[shorod]

Since it seems to reliably misfire, have you tried monitoring the air/fuel ratio during a misfire occurance to see if the mixture appears to go rich or lean during a series of misfires? This may help you determine if it's fuel or ignition related.

-Rod

[pcjunkie7]

Kind of sounds like my 98 Stratus. It would run crappy and miss when first started, but when warmed up it would run great. Mine was caused by oil leaking into the cylinders and getting on the plugs.