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1998 pontiac grand am 4cyl

My car starts great. when I start my car and take off right away and go about 100 feet to a stop sign it starts to chug and when i give it gas it will run great. Yesterday the same thing but the car killed so I cranked on it and nothing so then i gave it some gas and it started but chuged and blew black smoke for about 3 sec and after that it ran great. It only happens when the car sits for about 8 hours. What do you think causes this?
Thank you for your help, John

Cleaning the throttle body might be the best first effort. Any McParts store will sell several cleaners made just for such a job. The old carburetor cleaners are actually the same product, so take your pick. With the engine off, remove the flexible snout connecting the air filter to the intake, push the throttle partially open and spray the cleaner around the circumference, cleaning the ‘bath tub ring’ of sludge away.

When was the last time your car got new spark plugs? How about the last time the car got a new air filter? The car is supposed to run “rich” when the motor is cold. Old bad plugs can have a problem firing a rich mixture. When the car stalled the plugs were wet with excess gas and your stepping the gas added more air to the mixture and that got the car running. The black smoke was the extra gas burning off.

An old and clogged air filter could also cause a too rich mixture on start up. There are lots more things that could cause your problems, but let’s start with the simple stuff for now.

I’d clean the idle air control valve while I was at it. However, you might also be having a fuel issue. I’d check the coolant temperature sensor since if it is telling the PCM that the engine is colder than it really is, it will send too rich a fuel mixture on cold start.

Ok I put new spark plugs in it and cleaned the throttle body and also put a new fuel filter in it and they cleaned the fuel system and the air filter was good but still have the problem. It does not do it all the time. I put about 350 miles on in a week and and it wont do it for about maybe 3 weeks. Any other Ideas? Also how much to replace the moror mounts in this car? I think they are bad because my motor moves quite a bit and also i can move it wth my hands. thanks for the info. John

If you can move the motor by your hands then yes you have at least one bad motor mount maybe more. I am going to say that you have a bad fuel injector on one Cyl, I would bet it has a small amount of dirt in it that is preventing from closing tight all of the time, at which point it leaks fuel. The dirt is too large to be blown through the injector tip so it just keeps bouncing around… Or you have a internal seal that is on its way out and sometimes it seals, sometimes it does not. First thing you can try is removing the fuel rail and cleaning each injector individually with carb cleaner spray (trying to flush the dirt out). This is almost free and just takes some time, otherwise you have to figure out which injector is giving you a problem and this is very hard unless your car is throwing a code and telling you which cyl is misfiring…

“I’d check the coolant temperature sensor since if it is telling the PCM that the engine is colder than it really is, it will send too rich a fuel mixture on cold start.”

I would check the fuel pressure regulator for leakage. Remove the vac line going to FP reg and check for presence of fuel/fuel vapors.