Thread: Rough Idle after Rainstorm

Not sure I'm in the right place, but I hope someone can give me some advice! I have a 1993 Pontiac Grand Prix SE, with the 3.1. Up until a few days ago, it was running fine. This weekend we had a really bad rainstorm, and some of the roads flooded. I had to drive through an area that was "flooded," there was about 6 inches of water on the road. I was going a little fast when I hit the water, so it splashed up, and then the car sputtered and died. I pushed it until it was out of the water, let it sit for about 15 minutes to dry out, and then started it. It seemed to be running fine, and I got home okay. I then left my dome light on, and killed my battery. This was about three days ago, and we had another round of rain during that time.

Today I charged up the battery, started it up, and it's idling very poorly. The best was I can describe it is that it sounds like a drag car at idle, super rough. I check the air cleaner and it was still damp, and looked in rough shape, so I replaced it. It seemed a little better, but it's still not right. If I drive it, it runs fine, but idles pretty fast, if thats the right word for it. It seems to stay close to whatever speed I accel to. If I bring it up to 40, and then take my foot off the gas, it'll stay at forty, I have to brake to bring it down. The slowest I can keep it at is around ten, any lower and I have to hold the brakes. When stopping, I get just about to 0, fighting the engine, and then it "slams" down into the rough idle again and stops, but doesn't die (not yet anyway). A few times the check engine light has come on while stopped, and then goes off after a bit.

Any ideas on what could be wrong?

could be your plug wires are shot, and the water finished them off, not sure if you have coils on a 93 but one could be cracked and got wet and shorted out. or if you have a cap and rotor they could need changing as well. water makes the rotor corrode.

If it was an ignition problem would it still run fine above idle? I'll take a look at the wires as soon as I can. Also, as an update, I've noticed that if I throw it in neutral while moving, and then slow to a stop, the RPM stays pretty high until I stop, and then it drops down to the rough idle.

After some research, I found out how to jump the OBD1 connector so that it would flash out the codes, and I got a code 33. From what I found this is a MAP sensor error, that the signal is high, possibly indicating low vacuum or a bad sensor. Would this cause the problems I am experiencing? The check engine light only came on a few times, when I had stopped after driving, and the engine returned to the rough idle.

a vac leak could make it idle rough, i guess if you find no leaks, ( spray some carb cleaner around the vac lines, if the idle jumps you found the leak area) then change the map sensor.

Sorry I don't mean to bump an old thread, I just wanted to post what ended up being wrong. I took it in to a local shop and they found that the idle control motor was stuck too high, so they replaced it and now it's running fine.