Help! low rpm miss

Joined
6 July 2003
Messages
143
I'll try to include all info, but I will probably forget something

Basically, I have an engine missing at idle and low rpm (sub 4000). The car is a 1991, manual trans, with 150k miles. The car is perfect when cold, but when warm, it will begin missing slightly when pulling away from stopped at <1500rpm. The miss seems to get worse, until after 15min of driving, it will have a lumpy idle, and miss up to 3000+ rpm (maybe higher, it seems to have less power.

So far, I have tried:
No engine light codes
unplugging both O2 sensors (to force alt map)...still missing
unplug coolant temp sersor (TW)...no change
shorted the fuel pump relay...no change
unplug coil packs on front cylinders one at a time to see if any change...no change.
(at that point I noticed that the miss was not regular enough to be a single cylinder failing to fire, and I decided coil packs and plugs were not indicated)

The first time I noticed any problem was ~2 weeks ago...a few little odd misfires at low rpm 1st gear starts, and it has deteriorated each time I have driven the car. I've run through 2 additional tanks of gas, thinking that I could have gotten bad gas...but no luck

So, I'm open to suggestions...again, when cold in the morning, the car is fine. When I first see the thermostat open, the car is still ok...but at some point, perhaps some component gets hot, resistance goes up, and life gets hard.

Thanks in advance!
 
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Pull your spark plugs and look for color/deposit differences (sure sounds like a coil pack to me). Have you changed your fuel filter or had your injectors cleaned or balanced (highly recommend RC Engineering)? Sounds like your problem goes away at higher RPM's. That is indicative of our 2 stage fuel pump (yours may be marginal). Let us know what you find so we can brain-bank it. Good Luck and Happy Motoring!
 
I agree, pull your plugs. Might want to check the gap on them while you are at it. Excessive plug gap can cause a miss under load. The problem would be even more noticeable with a weak coil pack. Check the resistance of the coil packs to make sure they are all within spec.
 
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