1991 Grand Prix "Service Engine Soon" Light

Hey all. I own a 1991 Pontiac Grand Prix SE, 4-door sedan, with the 3.1V6 and all options other and sunroof and leather. Yesterday, I took the thermostat out to check if it was alight, because the car does not heat up like it should. It will heat up while sitting, but when driving, it cools right down, and does not blow out hot air. Well, the thermostat seemed fine, and it is not even a year old, so it should be fine. That is question #1. Anyone know why the car is not heating up properly? As I said, if you leave it sitting for a long time, it will heat right up, but cool RIGHT down when driving.

Question #2. Put the thermostat back in, and tightened everything back up. Started the car, and it runs like garbage. Stalls when you first start it, if you do not give it gas, and from 0-40KPH, it is really rough, and spits and spudders. Above 40KPH it runs fine. Any ideas from anyone out there? Oh, and the "Check Engine Soon" light comes on, and goes off when ever it wants too. NEVER seen it on before.

Thanks all.

That is GREAT!!!! Thanks for the info!!!! I know absolutley nothing about cars. The extent of repairs I can do, is changing the thermostat and I also did *most* of the work on changing an alternator on my Dad's Pontiac 6000. lol. Well, thanks again!!!!! Do you think this would cause the engine light to come on aswell? Or is this more of a solving the heating problem? Or could it solve everything? As I said, I know diddly about cars. lol

Thanks again!!!!!!

BTW, this is quite the board, yes? I love it. I post, and you replied within 20 mins!!! Sorry it took so long for me to reply. I just checked my e-mail. I had no clue someone would respond so quickly!!!!! I only hope I put the message in the right place. It never asked me where I wanted to put it, I just typed it out, and it put it here. lol. Well, sorry to anyone out there if it is in the wrong place!!!! Thanks again!!!!

Hey 80knight,

First check your wires from your coils. You could have a bad coil pack. I think that year it had coil packs. If not, a distributor cap check for hairline cracks on the inside of it. Corrosion on the points inside the cap. If you indeed have the coil packs one way of testing this on your own is to take off one of the wires leading into the spark plug. Now with that end off stick a srewdriver inside that wire and ground it to chasis ground. Now have someone else crank the car. Look at the screwdriver while they are cranking the car and you should see a spark between the screwdriver and ground. That is saying that, that wire is working properly. If you do not see a spark then that coil pack is bad. As far as your check engine light you could have a bad O2 sensor. This sensor is found on your exhaust. The studdering could be what I described to you. Post your results.

Hello djfeanry2 and Sound Pressure. Just wanted to give a big thanks to both of you for your help. I finally figured out what was wrong. A vacumm hose had broken off the engine, and was causing the "SES" light to come on, and the spittering and sputtering. I fixed the hose, and the car runs like a top, with the exception of the ECM having not totally re-set itself yet, due to the battery being disconnected for 10 mins.

As for the not heating up part, I will definatley try flushing the coolant, and replacing.

Thanks again guys!!!!!!!!!