This morning my car started really rough and would not idle. I could get the car started by giving it some gas and it would stay running if I gave it gas. I have a digital voltage gauge. When I let the RPMs get below 900 RPMs the voltage gauge would drop down to 12.0 volts. At 1000 RPMs the voltage was 14.6v. I let the car warm up and it still did the same thing. If I took my foot off of the gas the car would stall. It would start hard and only stay running with gas applied. At low RPMs the voltage gauge would drop to 12 v and stay there. Just when I was giving up on work today and using two feet to drive the car into the garage, I got out to move a trash barrel and the car stayed idling. It ran fine and the volt gauge was at 14.6 v at idle. I drove 25 miles to work and tried restarting and it ran fine. I had a check engine light on before this happened and have not read codes yet. The check engine light had come and gone a few times in the last two months. The low voltage reading is strange, as if it wasn't getting a turn on signal from the computer oir something.

Helps if we know the make and year. The voltage drop is because the RPM is getting low and is likely not the cause of the idle problem. I would suspect a vacuum leak. Perhaps a problem with the idle air control sticking open. Also, have the car scanned and post up the code. That may help.

I am sorry. It is a 2005 Buick LeSabre. The voltage drop is not because of the low idle. When the car is operating normally the voltage is 14.6 at idle. I could keep the car running by pushing the gas and at a constant 800 RPM the voltage was 12.0 volts for a long time not just when the engine is dying. This confuses me the most. I think the alternator is controlled by the computer and not just a voltage regulator but I am not sure. I am not sure how the rough start won't idle is connected to the alternator not working. This car was hard to star this morning. I had to feather the gas to get it to start. Even when fully warmed up it would not stay running. I am going outside now to see if it will start.

Really sounds like a vacuum leak. The idle air control opens a plunger to let air in. If that'* stuck you will have a big vacuum leak.

It started and ran fine at lunch. I think that you are right and the IAC was stuck closed. I might replace the IAC. I found out that the charging is controlled by the ecm and may not turn on the alternator when it is cold and low idle.

The IAC could be dirty. Carbon builds up and it can't close.

The car has fixed itself for now. I checked the cel code and it was 128 ( thermostat) not related to the no idle problem. I might change the IAC as it is only 29.00 at advanced auto.

make sure you clean the TB side where it seats too.