Rough Idle

Z

Z4JAY

Member
 Doncaster
Good day all,

I have a 3.0i that I care for very much and drives like an absolute dream. I've recently put Asymmetric 6's on and I can't even begin to tell you how well the car handles and feels now, but that's a story for another time.

An issue I'm having at the moment is that of a rough idle. This only appears to happen from a cold start when the car has been sitting overnight. The car is my daily driver and 6 out of every 10 days it's on the motorway on my daily commute which is about a 40 mile round trip. After 30 seconds of being on, the idle is steady as and there's no issues. It's literally just the first 30 seconds from cold, it hunts very slightly.

In the last 6 months the car has had the following under the bonnet:

- New DISA Valve (From BMW)
- New MAF (Continental Unit)
- Idle Control Valve cleaned out
- 6 new spark plugs (BOSCH)
- 6 new ignition coils (BOSCH)
- Rocker cover gasket
- New alternator

I'm thinking this could be linked to the VANOS seals, potentially PCV. The battery was replaced early 2022 by a previous owner, it's a reputable brand however I have wondered due to all the optional extras in the form of heated seats, electric wing mirrors etc could they be drawing power from a poor battery causing a rough start.

Any thoughts would be appreciated.

Jay
 
Hi there,
Have you tried taking the plugs out when it’s cold to see if any are wet?
It could be a leaking injector and it’s taking a little while for the fuel to clear from that cylinder.
Does a code reader show any misfire codes?
 
Good morning,

Thanks for your reply. That's a good point, I haven't removed the plugs just yet, I hadn't thought about a possible leaking injector. There's no misfire codes at all which is all the more confusing. However when there was a proper misfire on this car when the MAF required replacing there was still no code!
 
Checked the intake boot for holes? (Sometimes a whistling noise can be heard signifying this)
Could also be a varnished/sticky ICV.
 
Hi Z4JAY,

I'm inferring that your 3.0i has an M54 engine, correct ? Chris_D's suggestion is a good one - check for vacuum leaks, even uncommon ones.

I once owned an M54 engine 3-Series sedan, with a slight erratic idle. It went away when warm. No check engine light. MAF, DISA, and ICV were good. I changed out intake boots, CCV (PCV), vacuum lines, - all to no avail. The culprit turned out to be a badly sealing Oil Dipstick O-ring - and until it got really bad, it would seal itself better as the engine warmed up.

Not saying that this is your problem, but a slight vacuum leak is all it takes. Good luck on your quest, and post what you find.
 
I don't know how large a leak needs to be to trigger the logic and set a trouble code on an E46 or our E85/E86 cars.

Things I should have made clear: 1) My E46 with M54 engine had a manual transmission and the idle instability was slight and only when cold. 2) The leak was the O-ring sealing the dipstick to the engine block, not the O-rings on the dipstick. I had checked DISA, replaced vacuum hoses, intake seals, intake boots, CCV, rubber plugs, vacuum valve, etc. Then my daughter's older SAAB 9-5 Turbo, which I was maintaining for her, threw a code for a significant vacuum leak. I traced that to a worn out O-ring on her dipstick, and "the light bulb went on". I found the dipstick O-ring to the block of my E46 was stiff and loose - a new O-ring fixed my cold idle problem.

Probably not the exact problem for the OP. But older cars need things we never thought of as normal maintenance, and groups like this help keep them going.
 
Thank you all for your replies and apologies I am responding late to your tips, work has been very busy but I'm now able to throw some more time at helping the Z4 remain in top condition.

Upon thorough inspection, the intake boot did appear to have a leak! I found this by placing the boot in a bucket of water and surprise surprise, there were bubbles right where the very faint split was. This would have been very hard to see without removing the part, but for now at least replacing the boot seems to have resolved the issue!

Thank you for all of your help and again apologies for the late reply!
 
Z4JAY said:
Thank you all for your replies and apologies I am responding late to your tips, work has been very busy but I'm now able to throw some more time at helping the Z4 remain in top condition.

Upon thorough inspection, the intake boot did appear to have a leak! I found this by placing the boot in a bucket of water and surprise surprise, there were bubbles right where the very faint split was. This would have been very hard to see without removing the part, but for now at least replacing the boot seems to have resolved the issue!

Thank you for all of your help and again apologies for the late reply!
Click to expand...
Excellent result. Back to full fat again with a relatively simple fix :thumbsup:
 
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