On Start Up-Rough Idle/Engine Shake

[ABotheredMind]

Hello everyone! I finally pulled the trigger and joined the forum! I have a 2018 M3 F80 with 56 miles. As far as mods I have VRSF catless downpipes, AA single midpipes with stick muffler that I TIG welded together, Mishimoto charge pipes, ER J pipe, NGK plug (gap .022), carbahn keyed crank hub, and bootmod stage 2 tune.
On a cold start (Note: I turned off cold start b/c no cats) my rpms will be stable for 2-3 seconds and then jump up and back down causing a very noticeable vibration to the engine. When it starts doing this I give it a little throttle and the rpms go back into range and the vibration stops. Now it doesn’t do this every cold start and very rarely it will do it after the engine is warm. I ran the DTCs and no codes. I replaced the spark plugs 5k miles ago, checked the ignition coils and cylinder 6 looked pretty toasted so I replaced it. Other than that everything looks good so what the heck is going on? I hope someone out there can help me out because I’m stumped!

[theweebabySeamus]

Any smoke out of the exhaust? How's the fluid level of your intercooler circuit?

[Boznien]

Double check the tightness of the spark plugs. How new are the rest of the coils? I had a bad Bosch ignition coil on my N20 and it was causing rough idles and stutters through acceleration.

Also double check your chargepipe clamps and make sure everything is nice and tight. Any air leaks will cause the MAF sensor to tweak out.

[ABotheredMind]

Originally Posted by theweebabySeamus

Any smoke out of the exhaust? How's the fluid level of your intercooler circuit?

Good evening theweebabySeamus! No smoke coming out the exhaust. I will check the intercooler fluid level first thing tomorrow morning. Thank you!

[ABotheredMind]

Originally Posted by Boznien

Double check the tightness of the spark plugs. How new are the rest of the coils? I had a bad Bosch ignition coil on my N20 and it was causing rough idles and stutters through acceleration.

Also double check your chargepipe clamps and make sure everything is nice and tight. Any air leaks will cause the MAF sensor to tweak out.

Hey Boznien! I double checked the torque on the spark plugs and all are good. I only replaced the cylinder 6 coil. The rest of the coils are the originals from the factory. I checked the charge pipes clamps and all are tight and no air leaks. Thank you!

[xwilkinx]

I had a similar issue on cold days with cold start disabled. The fix for me was to simply keep cold start enabled, even with catless downpipes!

[Boznien]

Originally Posted by ABotheredMind

Hey Boznien! I double checked the torque on the spark plugs and all are good. I only replaced the cylinder 6 coil. The rest of the coils are the originals from the factory. I checked the charge pipes clamps and all are tight and no air leaks. Thank you!

Any luck on solving your issue?

[ABotheredMind]

Originally Posted by xwilkinx

I had a similar issue on cold days with cold start disabled. The fix for me was to simply keep cold start enabled, even with catless downpipes!

Hey xwilkinx thank you for your response. Yes, I did turn on the cold start and I’m still experiencing the issue. It actually didn’t make any difference which was kind of surprising.

[ABotheredMind]

Originally Posted by Boznien

Any luck on solving your issue?

Hey Boznien! No luck. I checked my charge pipes and j pipe and everything is nice and tight. I checked all my spark plugs and they’re all torqued to spec. I checked both coolant reservoirs and both are filled to the appropriate level. It’s strange that this only happens when I first start the car for the day. Today when I started it the engine had a significant rock back and forth. Enough to move the car side to side. No codes though! I have also notice a high pitch whining noise when I’m accelerating (it’s not turbos). It’s a much higher pitched sound and goes away after the engine is warmed up. This all started at the same time out of no where. Any help is greatly appreciated!

[dezzracer]

A hunting idle can often be a MAF issue as stated by Boznien. Something to do with airflow.

[ABotheredMind]

Originally Posted by dezzracer

A hunting idle can often be a MAF issue as stated by Boznien. Something to do with airflow.

Thank you dezzracer. Do you know the resistance range for the MAF sensors so I test them to see if one or both are going bad?

[dezzracer]

Im sorry I do not. I’ve experienced it with other cars though. One time because I removed the screen on a cold air intake, which I mistakenly thought was there to stop any debris from getting through. When in fact, it was there to smooth the airflow before a curve in the pipe. Subsequently it disrupted the airflow to the mass airflow sensor and my idle started hunting. Put the screen back in and all was well again.

[ABotheredMind]

Originally Posted by dezzracer

Im sorry I do not. I’ve experienced it with other cars though. One time because I removed the screen on a cold air intake, which I mistakenly thought was there to stop any debris from getting through. When in fact, it was there to smooth the airflow before a curve in the pipe. Subsequently it disrupted the airflow to the mass airflow sensor and my idle started hunting. Put the screen back in and all was well again.

Hey dezzracer. I pulled both MAF sensors and took their readings on my multimeter and based on the resistance ranges I found online they are both in range. They also had identical (near to) readings so that gave me a little more peace of mind. I’m still hunting done the issue so I will keep you updated if I find out what it is!

[summernightruns]

I had a similar issue. My issue was the hose from my valve cover PCV to my front turbo inlet pipe was cracked. Sucking in unmetered air

[follow_f80]

Ive changed all plugs+coils and that fixed the problem in my case. Got the same rough idle after cold start and i was unable to start driving without rpm drops or enginge off for the first 60 seconds.
My last set of coils held only ~6k miles

[gcant35]

I cleaned my MAF sensors and it seemed to help. Was having similar cold start idle issues with BM3 Stage 2 and VRSF intakes.