Q.
I’m asking this question about my sisters car, it’s a 1999 Grand Prix GTP, supercharged 3.8L. my step dad has no idea whats wrong with it. The ignition control module was tested, and they said it was fine. It starts fine when it’s cold, but after about 15-20 minutes it will die. We just tested the coil pack/spark plugs after getting it hot, and they all spark. Other symptoms are– when we were testing the spark plugs, when it would start, it would idle really low, and then spike up to about 2k, then drop below 1, almost stall, and then spike back up. My step dad said it has done that before, and does it even with all the spark plug wires are connected, but when cold it runs perfectly fine. When trying to start it, if you push on the gas, it’ll run, but really horribly. It blows a bunch of black smoke… but not when cold. you wouldn’t even know anything was wrong with the car, until you drive it until it gets hot. Also, according to my step dad, the temperature gauge doesn’t move at all. What are some other things we could test? My step dad is in the process of swapping the ignition control module, and coil pack out of a different Grand Prix we have sitting in back. I don’t have any hope that this is what the problem is, so what else could we try?
A.
The black smoke is the telltale sign of either an O2 sensor that’s bad, or a Mass Air Flow sensor that’s plugged or bad. The temperature gauge not working will also cause havoc, as the sensor for the coolant temperature also helps the computer make decisions on air/fuel ratios, mixture, timing, and more.
Clean the MAF sensor with MAF cleaner, available at any parts store. That may or may not fix the problem, as sometimes the sensor wire inside the MAF gets broken. A new MAF for that car is around $200, and is a common problem in 90′s and early 2000′s GM vehicles. If you’re using an oiled air filter (most “performance” or re-usable filters use a oil to capture dirt), get rid of it and replace it with a standard air filter (the oil in the filter often get on the sensor wire for the MAF, causing it to malfunction. The O2 sensor(s) should be replaced every 50k miles, and I’ll take a guess they’re probably the original one(s). Only use quality O2 sensors (my preference is Bosch), not a cheap or “universal” sensor. Replace the coolant sensor(s) (there may be 2, be safe, replace both), they’re cheap and easy to replace. After replacing the parts, unplug the battery for 10 minutes to reset the computer, reconnect the battery and drive the car like you stole it (the computer learns faster after a good flogging). This should solve the problems after about 50 miles of driving.
By the way, the reason for it running fine when cold but not when warm is due to the ECM (engine computer) – when the coolant is below 160 deg., the computer operates in “closed loop” mode, ignoring all the data from the engine sensors and running on a pre-programmed information table. After reaching temperature, the ECM goes into “open loop” mode, and relies on the information from the (presumably faulty) engine sensors (MAF, MAP, O2, etc…).