94 540i rough idle

[ammo00]

Was wondering if any one can help me out. Just bought a 94 540I about 6 months ago. It has developed a rough Idle over the past few weeks. It started out idling rough at start up and then would idle smooth after about 2 to three minutes. It has gotten progressively worse. Now it idles rough at start up and continues to do so. Here is something else I have noticed about the situation. When I first start it it runs rough, if I shut it off right away and restart it will idle smooth for 30 seconds or so then go back to the rough Idle. After it warms up it runs alittle better but it is still rougher than i think it should be or when i first bought it.

Willie

[Johntee540]

2 Questions first. 1) How many miles are on the car? 2) Did it just recently turn cold where you are?

It sounds like 1 of 2 things. o2 Sensors or Intake Manifold Gasket. There is a Vacuum leak in the IMG that when the weather is cold gets exposed. The cold contracts the gasket opening up the leak. This causes rough running until the engine warms up and the heat expands the gasket closing the leak - allowing the engine computer adjust the engine to run normally. - JT

[bpattan]

Nikasil. you probably have a Nikasil engine block, as i do on my 530. ther's lots of info on this formum about Nikasil. To confirm this have it checked out by a bmw dealer or other shop. if you have less than 100,000 miles you may still qualify fora new engine, made of Alusil. some bmw dealers have a inventory of new blocks, i just priced one here in Houston, $3,800, just for the shortblock. good luck, if that's your probelm you have two options. 1) live with it, 2) replace the block. bob

[632 Regal]

everyone on this board with Nikasil blocks have that same problem.

I dont think so.

Check for vacuum leaks from the air box to the intake, all the small hoses too.

[ammo00]

150K mi. It just turned cold but I noticed the problem before it did turn cold. The later of the problem has been going on since it got colder. The intake manifold gasket has been replaced recently. I was in the process of getting the O2 sensors to replace them. I will let you know when they come in if that was the problem. Thanks

Willie

2 Questions first. 1) How many miles are on the car? 2) Did it just recently turn cold where you are?

It sounds like 1 of 2 things. o2 Sensors or Intake Manifold Gasket. There is a Vacuum leak in the IMG that when the weather is cold gets exposed. The cold contracts the gasket opening up the leak. This causes rough running until the engine warms up and the heat expands the gasket closing the leak - allowing the engine computer adjust the engine to run normally. - JT

[632 Regal]

thats why I say to thoroughly check for vacuum leaks first.

[SharkmanBMW]

when my 540 had rough idle for the first 2-3 minutes - then it would smooth out.... it was the pcv plate on the back of the intake manifold, now it is fine!

[Johntee540]

Not everything points to Nikasil. Sheesh. This sounds more like a vacuum leak than anything else. Lets start the diagnosis cheap and work towards the expensive. - JT

[Johntee540]

The rear of the intake Manifold is also a great hunting ground for Vacuum leaks. Also - all of your hoses are old and are biodegradeable. They will start to leak over time as well. At this point in the life of your car you will need to invest some time in making sure that there are no leaks whatsoever in the vacuum channels. Good luck - JT

[DueyT]

Nikasil. you probably have a Nikasil engine block, as i do on my 530. ther's lots of info on this formum about Nikasil. To confirm this have it checked out by a bmw dealer or other shop. if you have less than 100,000 miles you may still qualify fora new engine, made of Alusil. some bmw dealers have a inventory of new blocks, i just priced one here in Houston, $3,800, just for the shortblock. good luck, if that's your probelm you have two options. 1) live with it, 2) replace the block. bob

I'm wondering why is this (Nikasil to blame) often the default? A Nicky block (if he in fact has one) doesn't just suddenly turn in the space of a few weeks. Like John and Jeff note, vacuum problems are generally the blame, especially as the pliability of plastic/foam is reduced with the colder temperatures...I as well would check the rear vacuum valve...it's in here in the archives under "M60 vacuum plate, etc..."

Cheers,
Duey