HELP! My S55 has a rough start and rough idle but drives fine.

[AlpineF82brrr]

Hello,

I’m writing here that in some off chance someone might have experienced something similar and has some useful feedback. I’ve seen a few posts about rough idle but none of them have helped me in my case so far.

The car turns on and starts at rpm 1.2k or so then will dip to like 500 rpm’s, back to 1.3k to 700 to 1.2k and just misfire through the whole cat heating sequence. The rpm’s are always random and sometimes it doesn’t do it as bad. It chops like a cammed vett, and revs itself up sometimes during cold start.

The car drives fine but in idle it def misfires a bit and on cold start it’s the worst.

The only codes I got were all cylinders misfiring and just some codes saying all the cylinders : Combustion misfires, cylinder : harmful to exhaust gases after starting. Other than it telling me all cylinders are misfiring it doesn’t tell me what’s causing it. And again it only misfires on idle the car drives completely fine.

The car was stage 2, front mount intakes, downpipes, charges pipes, J pipe, mid pipe, stock muffler, crank hub is done, and it used to run CJ E85 tune. This issue persists both in stock and stage 2 tune.

Things that my mechanic has tried are
New set of sparks (old owner had 5 NGK and a Bosch????)
Not new injectors but injectors that were fine on another car put in
Removing front mount intakes and replacing them with stock intakes
Cleaning the MAF sensors

Things that he’s planning to do
Replacing downpipes with stock cats
Checking O2 sensors

But that’s where we’re at.

[wheela]

Do you have any logs of this happening? Also you mentioned you have new (used) injectors, were they coded on installation, and are you certain they are the correct injector?

[AlpineF82brrr]

I think they’re OEM injectors off his own M3 that he totaled, he didn’t code them just put them in. He’s also getting my own injectors cleaned to put those back. As for logs I’m not sure how to do that. Do I just start logging on BM3 and start the car and also log me driving?

[wheela]

Okay, the injectors definately need to be coded, as they all flow slightly differently, and the DME needs each injectors specific flow value to control them properly. Can't say for sure that's the only thing going on, but I'd start there before doing anything else.

[AlpineF82brrr]

After doing some research I’ve heard a lot of people say different things but majority says you don’t need to code s55 injectors.

[wheela]

Well, I just researched a bit and you seem to be right. Older eu5 cars need coding, but apparently newer eu6 cars just need small mixture adaptation reset:

https://f80.bimmerpost.com/forums/sh....php?t=1534478

https://f80.bimmerpost.com/forums/sh....php?t=1795072

[Wan15bad]

Originally Posted by AlpineF82brrr

Hello,

I’m writing here that in some off chance someone might have experienced something similar and has some useful feedback. I’ve seen a few posts about rough idle but none of them have helped me in my case so far.

The car turns on and starts at rpm 1.2k or so then will dip to like 500 rpm’s, back to 1.3k to 700 to 1.2k and just misfire through the whole cat heating sequence. The rpm’s are always random and sometimes it doesn’t do it as bad. It chops like a cammed vett, and revs itself up sometimes during cold start.

The car drives fine but in idle it def misfires a bit and on cold start it’s the worst.

The only codes I got were all cylinders misfiring and just some codes saying all the cylinders : Combustion misfires, cylinder : harmful to exhaust gases after starting. Other than it telling me all cylinders are misfiring it doesn’t tell me what’s causing it. And again it only misfires on idle the car drives completely fine.

The car was stage 2, front mount intakes, downpipes, charges pipes, J pipe, mid pipe, stock muffler, crank hub is done, and it used to run CJ E85 tune. This issue persists both in stock and stage 2 tune.

Things that my mechanic has tried are
New set of sparks (old owner had 5 NGK and a Bosch????)
Not new injectors but injectors that were fine on another car put in
Removing front mount intakes and replacing them with stock intakes
Cleaning the MAF sensors

Things that he’s planning to do
Replacing downpipes with stock cats
Checking O2 sensors

But that’s where we’re at.

Seem to have same issue here, did this get resolved for you?

Many thanks

[summernightruns]

I would check the PCV plastic hose that attaches the valve cover to the front turbo inlet. They can be brittle and break

[AlpineF82brrr]

Bumping this post because I have an update

I haven’t checked the PCV as the above comment says yet

But
I had brand new injectors put in
Brand new coil packs
Brand new sparks again
Brand new MAF sensors

The issue gets a little better on startup after cleaning the MAF sensors then it deteriorates progressively and starts revving itself randomly and chopping a little bit.

After it started getting worse again(it had been good on startup suuper smooth the RPMS didn’t bump up and down at all then it got worse again after about 20 days)

I replaced the MAF sensors this time which made it better again just not totally smooth like last time. I wonder what could be causing this issue?

Shops aren’t sure either but has anyone ran into this issue before?