Rough S50 Idle? Check This Simple Fix

[-Keith-]

After getting a PM from W1234 this morning about a rough idle, I thought I would share this in the Knowledgebase.

Just to let you know, I deserve no credit for the write up below (it is essentially copied and pasted with a bit of tweaking to make it read better, simply as you cannot view my other forum without registering).

After reading it a while back, I made a mental note of the problem, and it would be a shame to not offer this information up for all my fellow S50 brothers and sisters :thumbsup:

It's such a valuable check and could potentially save a lot of head scratching and wasted garage diagnosis money.


Rough Idling S50
Ever since taking the M3 to Elvington last year, I've been having major idle problems. From hot and cold it was running very rough, and was stalling constantly.

A diagnostic at BMW came up with faulty lambda sensors. After changing these, the stalling went away and the idling improved, but it was still not right. A few squirts of carb cleaner around the airbox told us that there was an air leak somewhere, locating exactly where though was proving difficult.

After doing a quick search of the forum I came across a problem which is simply caused by a small runner T-Piece that is missing.

I couldn't see the t-piece or the cap that should have been present, so decided to strip down the airbox to find out if that cap had popped off on mine:

http://img.photobucket.com/albums/v709/m3yc/IMG_4125Large-1.jpg


Low and behold, the same cap was missing on my car, just as in the other thread.

The T-piece on which the cap should sit:

http://img.photobucket.com/albums/v709/m3yc/IMG_4130Small.jpg


So it was a trip to the dealers to order the cap. Outrageously, the price had gone up by 5p :lol: While I was there, I decided to order a new coolant temperature sensor and new O-rings for the airbox. Here's the cap in question:

http://img.photobucket.com/albums/v709/m3yc/IMG_4177Small.jpg


Fitted:

http://img.photobucket.com/albums/v709/m3yc/IMG_4180Small.jpg


Although it just pops on, I put a cable tie on it to make sure it won't pop off easily again in the future. I fitted the O-rings, cleaned the ICV and fitted the coolant temp sensor. With the airbox out, it would be rude not the give the throttle bodies a good clean:

http://img.photobucket.com/albums/v709/m3yc/IMG_4186Small.jpg

http://img.photobucket.com/albums/v709/m3yc/IMG_4188Small.jpg


Put it all together and fired her up. The result :hurray: :cornut:

Idle is now rock steady at about 800rpm. Took the car for a blast to get it up to full temperature and it was perfect. In fact, the car never idled better than it is. I started it from cold again the next day... Still perfect. Job done.

So if your S50 is running a bit rough, have a quick look for this cap. I can see it easily enough as i've got a DaveF kit, but I doubt you'll be able to see it with a standard airbox. But if you remove just the airbox you should be able to see the t-piece.



So there you have it. Please excuse the fact I had to recreate someone else's thread here, but I hope this useful little gem of a check will help out others in the future :thumbsup:

[Bromlow-Motorsport]

Good call Keith, it's really common on S50 engines for this to be split, or missing altogether. It popped off a couple of times on my S50 and caused all sorts of minor running issues.

It can be reached for replacement/inspection pretty easily too on a standard car (it's a 5 minute job). Undo the two jubilee clips on either end of no 10 here, and remove the elbow.

http://www.realoem.com/bmw/showparts.do?model=BK92&mospid=47423&btnr=11_1303&hg=11&fg=40

Get a torch, and there it is. Or probably not as the case may be :lol:

Ollie

[Brewster]

Another job to add to the checklist :lol:

Cheers Keith. :thumbsup:

[kwoffa]

What a gem(he says hopefully) i have a slight misfire on idle along with a bit of a mis on accelerating,changed the throttle idle sensor..has made a little difference but this one i will do aswell...car pulls well..with no other probs.thanks Keith and bromlow!

[Brewster]

What a gem(he says hopefully) i have a slight misfire on idle along with a bit of a mis on accelerating,changed the throttle idle sensor..has made a little difference but this one i will do aswell...car pulls well..with no other probs.thanks Keith and bromlow!

If the above doesn't appear to be wrong with your car then you could try the throttle postion sensor. It caused a hesitation on mine as it was worn at roughly 10-20% throttle. Really easy to check, it's the first sensor you see on the end of the throttle rail. Just disconnect it and see if your problem goes away. I wouldn't recommend running without it for very long but it's a 10 minute job to change and about £60 from the stealer. :thumbsup:

[kwoffa]

Already changed it Brew,but thank mate

[kwoffa]

Well i had a look and after buying the cap,was amazed to find a)that it wasnt missing but b)a small bolt had been inserted along with a jubillee clip round....it all looked very secure..so still idling a bit rough ! and on acceleration. so a set of new plugs will be next !

[cragg711220]

I've toyed with the though of just taking mine to an indy and getting them to replace all under bonnet hoses. They just dry out and crack with time.

[2skiddy]

Great to highlight the problem of missing or damaged vacuum pipes. On older cars that I`ve had, there is always a split, perished to within an inch of falling off, or totally not there pipe! Even more so on engine modded cars. Worth the time to check and prevent problems even IMO.:thumbsup:

[inside]

I just replaced mine cap with a new one; the old one was present, but was torn apart, barely holding on the plastic tube (T-piece). So far looks like it resolved my cold-start-high-revs issue :hurray:

Thanks Keith and forum for another good info!

[Aceman]

I had a similar problem on my M52TU engine which has exactly the same bung on an unused port on the inlet manifold. The rubber bung had just perished and split, replacing it cured the lumpy idle.

Amazing these engines share such similar problems.