1994 Saturn SL1 misfires at 1500 RPM after cooling system m

MAKETOAST

MEMBER
1994 SATURN SL1
4 CYL
FWD
AUTOMATIC
130,000 MILES

Hi,
Car is a 1994 Saturn SL-1 with 1.9L 4cyl TBI
Sorry for the long question, I do my own maintenance and it takes awhile to get me stumped sometimes.

Car has been running great for the past 5 years. I keep up on routine maintenance and that sort of thing. Replaced plugs, wires, PCV, coolant temp sensor and air filter as a tune up last month, car ran great afterwards getting 34mpg. Just last week, however, the waterpump started leaking badly and I think the thermostat was stuck open because the temp would never get past 1/8 on the gauge. After flushing the system, installing a new thermostat and pump, the cooling system works great but the motor misfires around 1500-2000rpm ONCE WARMED UP. Car runs great while cold (first 5-10 minutes). I've already eliminated so many of the variables by that tune up last month. What could be causing this misfire? I guess the car never warmed up completely before changing the thermostat, so now I'm thinking that it might be a problem that has to do with a sensor. Ideas?

2 Replies

RASMATAZ

MECHANIC
75,992 POSTS

1994 Saturn SL1 misfires at 1500 RPM after cooling system m

Engine misfiring can be caused by worn or fouled spark plugs, a weak spark (weak coil, bad spark plug wire), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even bad fuel.

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Sunday, January 18th, 2009 AT 4:10 PM

MAKETOAST

MEMBER
2 POSTS

The key here is that the engine only misfires after being warmed up. That means that when the computer switches from open to closed loop the mixture control is incorrect thus resulting in my lean misfire problem.

Plugs and wires have been replaced. Coil isn't a likely option, since the car performs fantastically when the engine is cold. Compression tests good. Fuel, and most likely fuel pressure is ruled out since it ran great right before doing the coolant maintenance. I've been checking for vacuum leaks but cannot find any. I will replace all vacuum lines and gaskets just to be sure. I think that will leave me with EGR. I will remove it and inspect it tonight, cleaning if necessary.

I am also wondering if the O2 sensor could be a possibility? I tried testing it, but it is hard to do with the engine misfiring at 2000RPM. The readings that I did get seemed to be in spec (.02-.08) but if something else is messing up the mixture, then it wont be accurate anyways.

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Monday, January 19th, 2009 AT 12:02 PM