QuestionI have a 1993 Chevy Beretta with a 2.2 4cyl engine with manual 5 speed. There are approximately 155,000 miles on the car. The problem started with a surging or stumbling on acceleration but smooths out at higher revs and speeds. It does not present itself immediately on start-up of a cold engine. It usually begins within a few miles and will end when the engine is fully warm. It sounds like the ignition is being retarded and will not fire correctly. This leads to the surging and stumbling. When I am at highway speed, I do not notice any performance issues. When the stumbling begins, if I push the accelerator to the floor, the stumbling will smooth out, but I do not have full power until the RPM's get a little higher. I end up riding the clutch a little bit more than I like in first and then keep the revs higher when shifting.
I have replaced the plugs and wires. The fuel filter was replaced. The coil packs were replaced (as suggested by a friend). I have tested the throttle position sensor and it gives a smooth resistance reading throughout the throttle movement. The intake air temp sensor was also tested and gives a good resistance measurement when tested. I didn't have a vacuum pump so testing the MAP sensor was difficult. I ended up replacing it because the symptoms were close to that of a failing sensor (or so I thought). The air filter was also changed. The only one I am unsure about is the coolant temp sensor. It is a four wire sensor and I do not find anywhere how to test this. I suspect this because the cooling fan is hard wired to a hot wire and will power up when the ignition is on. (I just recently found this one.) If the hard wire is disconnected, the fan will not power on itself and I assume because of a faulty coolant temp sensor? With all of the replacements, the problem is still persistant.
Another symptom is if I drive the car until warm then make a short stop. The car will start, but will stall after a very short distance and will not start until the engine sets for about 20 to 30 minutes. After it sits, it will start up and drive normally.
Sorry about the long message, but I thought some history would be helpful. Any ideas would be appreciated.
Thanks........Mike
AnswerHi Mike,
It sounds like you may have pointed it out yourself.
The ECM is in closed loop, and doesn't use sensor values during that time, but rather depends on default values for controlling the injectors.
Then, after it warms up, the fuel mix is controlled according to the readings from TPS(throttle), MAP(Vacuum), O2 sensor, MAF(Mass air flow), and probably some more.
Engine coolant temperature is another. If it is saying it is a different temp than it actually is, the fuel delivery won't be correct for the temp that it really is.
Considering the fan problem, I would tend to think something in the temp sensing circuitry would be incorrect.
Trouble is, it could be in the ECM. But the sensor would sure be cheaper, and besides, if the ECM is bad in that circuit, it could be because a bad temp sensor damaged it, and the same fate could happen to a new ECM if it is replaced without correcting the cause.
So...although I seldom recommend just replacing stuff, that sensor might be something I would change.
A scan tool connected to the test connector can read the actual readings from the sensors, so that is what I would recommend.
Van
