My 09 G37S with 115K miles that runs like crap in cold weather, even after the engine warms up and water temps is at normal range. I can let it run for about 15 minutes or more in the driveway on a cold day 40-60F, and it runs super rough especially at around 2000 RPMS. It will gradually get better but only after driving for about 20 minutes and is never super smooth. If it is a warm day 80 F , I can start it up without warming up, drive and all is good.

There is also a engine knock when it is cold and I take off from a stop, like I have bad gas. I have changed plugs, done gas treatments, always used super gas, etc and still the same. I have also tried resetting the ECU, and no maybe slightly better but not much.

I am not getting any error codes either.

Any recommendations / ideas greatly appreciated!!

 

I'm thinking your issue may be fluids. Check your engine oil. Is it clean? How often do you replace?

 

40-60 is not cold. That being said how is your fuel mileage? I've seen issues on other cars where the car thinks it is colder than it is and dumps fuel because of this. This could cause poor running.

 

I had synthetic oil changed about 12k ago but it has been a long time since I changed the tranny fluid if ever

i typically get about 18-19 GPM with fairly moderate city driving. I have the sport model and I think always got about the same miles per gallon

you bring up a good point. I could plug in the code/data meter and see what the ECU is showing for air intake temperature. Do you know if the MAF sensors measure the incoming air temp or is it a different sensor?

 

umm i would change your oil now. 12k is way too long. change it every 5k miles. and i would change your tranny and differential oil.

 

I thought the tranny fluid was lifetime? Is it serviceable, or does it require a trip to dealer for tranny change?

i am curious how fluid change makes the engine run very rough? Just trying to understand the connection. Also, the car will continue to run rough well after the engine temp is up to normal operating temp. Thanks for the help!!