[Ven0m21]

I have a 2011 malibu 4cyl that i recently purchased, It had a few issues that ive been chipping away at. Currently i have everything ironed out except a p0300 when coming to a stop when the car is warmed up. If I drive the car around for the better part of half an hour or so and stop it will throw a p0300 pending code and rpm will rapidly go up and down, occasionally stalling out. Putting the car in Park doesnt seem to help any, only way to get it to hold idle is to either restart the car, or give it a rev in park. Worth noting the MAF and coolant temp sensor was just replaced by myself, the stalling issue was present prior to these going in though. No codes whatsoever except a pending p0300 which im assuming is caused by stopping when hot. Ive never experienced anything like this issue before. If i were to restart the car while its missing and back it to the back of the driveway and drive it back up it will start missing again when you stop. Let it cool down and it will be fine for another half hour or so. Any information would be apricated. Only thing i could think of would be a messed up throttle body not opening enough when you come down in speed, but why would temperature affect it?

 

[repairman54]

When things heat up they expand. Have you cleaned the throttle body with a dedicated throttle body cleaner?

 

[Drunken Elvis]

As repairman has said, cleaning the throttle body is a good idea. It's cheap and easy.

Throwing parts at it, is hit and miss. Can be expensive. My guess might be a CPS or crank position sensor.

This problem you have is repeatable. It won't idle well when warm.
Paying a diagnostic technician for an hour might be the least expensive route to the cure.

 

[Ven0m21]

When things heat up they expand. Have you cleaned the throttle body with a dedicated throttle body cleaner?

Yeah when I had the cover off the engine I cleaned up the throttle body with brake clean. It really wasn't all that dirty anyhow but it definitely had some carbon. No change though

 

[repairman54]

Did you use GM parts on the MAF and coolant sensor? There is a big difference on GM parts and aftermarket parts. New doesn't mean good.
What temp is your scan tool reporting when it's acting up.

 

[Ven0m21]

Did you use GM parts on the MAF and coolant sensor? There is a big difference on GM parts and aftermarket parts. New doesn't mean good.
What temp is your scan tool reporting when it's acting up.

I want to say it was acdelco? Whatever the AutoZone had at the time. The issue was there prior to the sensors though, they went in to fix other issues (the intake air temp sensor and our coolant gauge didn't work) before I believe the maf was factory

 

[repairman54]

Aftermarket sensors are not reliable new out of the box. Many posts on folks having issues with AZ or AA sensors. AZ usually does not stock AC parts, only there brand of Duralast.
You may need a shop level scan to see what is going on. There are 4 code families, P, U, C and B and a P code scan tool will not read the others. May very well be system communication issues you are unaware of for example.