Unstable rpm at idle

[3rdgear]

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Im rewriting this post with more details hoping someone can advise, this problem occurred after I started modding my ride, could be a coincidence, who knows. Mods includes Cobb, Afe intake, forge intercooler and ak mid/muffler
At times when my car is in neutral, rpm will go up and down then stall on me.
I understand this could be caused by several reason including intake leak, bad maf, dirty filter, vacuum leak, weak battery, spark plugs etc and from what I read, it's quite difficult for dealer to detect this problem. If you have any experience with this problem, please kindly share it with me. Thank you!!

[moveswiftly]

Get the car scanned for codes. Is this only when in Neutral?

[3rdgear]

No codes were shown on my Cobb, not sure if BMW dealer scan codes like Cobb Accessport does. Yes only happens at neutral, jumps from 500 to 1500 rpm. Twice it went so low it stalled. It's Chinese new year holiday here, so dealer won't open for another 10 days but eager to diagnose what the problem could be.

[MDORPHN]

What is mileage and have the intake valves been cleaned?

Neil

[3rdgear]

Hi Neil. Only 12000km and my intake is 2 months old, is the valve a part of the intake?

[3rdgear]

Have returned to stock map a few days ago but the problem is still here, guys at the dealership thinks is intake related but since there is no code, they don't know where to start fixing. They screened out a lot of possibilities since the car is very new. Might install my stock intake and see if the unstable rpm is related to that.

[robertm]

Really doubt it's OEM parts related since the car is so new. I'm guessing either the intake or intercooler plumbing has a leak. Since you've removed the Cobb that's already been ruled out. I would first replace the intake with stock and see what happens. If still there then replace the intercooler with stock. Get the car 100% back to stock and see if there is still a problem. If there is then drop it off at the dealership and let them throw parts at it on BMWs dime until its fixed.

Also, check the charge pipe and just confirm it's not cracked.

The midpipes and muffler won't have any effect on anything so don't worry about them. You didn't mention downpipes so assuming those are still stock.

[Dackelone]

IF it only happens in neutral I'd say its a software issue. A lot of N55 DCT's stall when coming to a stop in drive. Why are you even driving around in neutral? The fix is a newer SW level.

It could also be a vacuum leak. I would check your Afe intake. Also look for any oil leaks around the valve cover(you might even remove the plastic cover and check for leaks). Even a bad oil cap can cause a vacuum leak. But you would see some oil residue. G.L.

[Karmic Man]

I suspect it's the intercooler.

My Evo had this issue when I changed to a larger and smoother intercooler piping.

My mechanic just raised the idle limit to solve the issue so I don't have to re-tune the car.

I believe if you get someone to custom tune your car then it should be fixed.

[3rdgear]

The Cobb stage 1+ tuning map which I was using was programmed for car with larger intercooler like mine and I was in neutral at red light or traffic jam and Im not sure if it is a software issue as it happens on stock sw and Cobb sw but I never had it before I started modding my car, will install my stock intake and see if it helps. Hope it isn't the Cobb related as I really enjoyed it

[MDORPHN]

Highly unlikely that it's the Cobb tune. I'm guessing that you have a minor vacuum leak.

Neil

[3rdgear]

I hope so too but the unstable rpm does seem to happen more often when I'm running Cobb map, both times I stalled was running their maps. Stock map still unstable but not as frequent

[fastauto]

maybe your fuel pump is going to crap out on you. I had a similar issue with the surging but it never stalled. it did come pretty damn close to doing so sometimes. it was a fuel pump that was on its way out. when the fuel pump is in the process of dying it will not show a code.

[headsup7up]

it wont be a leak in the intake causing your problem since our cars use MAP sensors rather that MAF sensors, i blew a hole in my chargepipe that was over 1" and the car still ran fine. My car had a simular issue and even stalled out on me. it was a misfire on cylinder #2 caused by a bad injector. The problem was more noticable on cold starts but got worse over time.

[3rdgear]

Hey fastauto but my car only has 12000km on it, if it is the fuel pump then the quality must be pretty crappy.

As for the maf or map sensor, I'll bring this up at the workshop. Thanks a lot

[headsup7up]

Originally Posted by 3rdgear

Hey fastauto but my car only has 12000km on it, if it is the fuel pump then the quality must be pretty crappy.

As for the maf or map sensor, I'll bring this up at the workshop. Thanks a lot

Its not unheard of to have a pump go that early, i know of a guy that has gone through 3 of them by 60k miles, my car has 75k miles and its still on the original pump. I guess its just luck of the draw.

As for MAP sensors, our cars read the amount of air going into the motor from the intake manifold. this means if theres a leak anywhere in the system before the manifold, the car will not notice. It will measure the amount of air actually going into the motor and adjust accordingly. If the cars used a MAF sensor, the car would measure the air from the actual intake pipe. so if there was a leak the car would be reading more air than is actually making it to the manifold. someone in an earlier post mentioned having this problem with his evo, thats becuase evos use MAF sensors so a leak will effect how the car runs.

Good luck getting it fixed, in my unprofessional opinion, its fuel pump or injector related.

[3rdgear]

Thanks so much headsup, so with all these possibilities, how do I determent what to replace. Fuel pump for example, can the workshop test it in anyway since there is no codes. Just want to make sure incase I order something not broken

[robertm]

Seriously. Remove cobb completely, test. Then remove intake and test. Then remove intercooler and test. If its still a problem after everything is back to stock drop the car off at the dealership and let them solve the problem.

[M3 Adjuster]

Originally Posted by robertm

Seriously. Remove cobb completely, test. Then remove intake and test. Then remove intercooler and test. If its still a problem after everything is back to stock drop the car off at the dealership and let them solve the problem.

[fastauto]

Originally Posted by 3rdgear

Hey fastauto but my car only has 12000km on it, if it is the fuel pump then the quality must be pretty crappy.

As for the maf or map sensor, I'll bring this up at the workshop. Thanks a lot

It's a weak link on our cars. Mine went at 15k.

[FIRSTM]

2 of my previous cars use accessport, take it off or get a proper tune, stock ap map is very rich

[3rdgear]

But I heard Cobb is very reliable and conservative I only had it installed for 2 months and I like it a lot and unfortunately there ain't many tuning companies in shanghai, what issues did you run into with your accessport?