Possible vacuum issue

[JGood]

My engine seems to idle pretty smooth at about 800RPM's or so. It's an s50 cammed m50b25tu. It has new vacuum hoses and a new intake boot. The only thing I didn't touch yet is the ICV and the hose that connects the ICV to the intake manifold.

However, I can remove the oil cap, and the engine won't react at all. no bump in idle or anything. Same thing when removing the dipstick.

Shouldn't that be a HUGE vacuum leak?

[M-technik-3]

Yes it should be, no tell tale hissing sounds?

[Gnarles]

I have that same problem, my car idles fine, but it does bounce around slightly before settling around 800rpms when taking the car out of gear. All new vacuum hoses and intake gaskets, but I also can pull the oil cap or dipstick out and nothing happens.

[JGood]

Yes it should be, no tell tale hissing sounds?

It makes a loud sucking sound when I open the throttle body, right from the ICV/TB area. But my friends S52 does the same thing. It's too loud to be a vacuum leak, I think it's just the noise these motors make.

[M-technik-3]

mine is not stable when I open it, but doesn't stall.

[Adrian_Visser]

I haven't pulled the oil cap with the engine running to check yet but is it normal for the rev's to dive down to 500rpm or so before coming back to idle with a light weight (11lb JB racing flywheel) when you put it in neutral coming to a stop?

I notice a very slight oscillation in the idle sometimes when I step on the brake which I assume is the brake booster sucking some air, my eta with an m20b27 does the same thing

Engine in question is an s52 BTW

[hoveringuy]

The motor (crankcase) should be under vacuum at idle though the PCV. Removing the dipstick may not do anything because the tube extends down into the oil, but removing the oil cap should defininetly induce a vacuum leak that will make it run lean.

So, either your PCV isn't drawing a vacuum on the motor (shut) or you already have a massive vacuum leak into your case.

I would play with the hose before and after the PCV to trouble shoot. Does disconnecting before the PCV cause a lean surge? After?

The PCV is supposed to modulate the flow based on vacuum on one side and pressure on the other. Could just be closed all the way at idle and opening under load.

[JGood]

http://forums.bimmerforums.com/forum/newreply.php?do=newreply&p=13232908

According to the folks at BavAuto my M50TU does not have a PCV valve the part number ($5.95 at BavAuto) above is just a valve assembly that routes air out of the crankcase. If you buy that part you also should pick up the O-ring and hose with it.

That part, listed above, serves the same function as a PCV. It's purpose is to create suction inside the crankcase for better ring-seal among other things. It is supposed to separate the oil via a venturi effect and rarely, if ever "wears out". More often than not - it is the o-ring at the base. BMW engines are in large-part, negative-pressure creatures. The engines run with an internal vacuum - even the oil pan. If there is the slightest leak - they don't run as well as they should.

The M50 and M50TU have the aforementioned valve in the same spot - on the cam cover.

The ICV controls the idle (thus the title) via regulating the airflow. If your weird idle is a 'hunting' idle, or one that bounces - you probably have an air leak somewhere. Perhaps a better description would held decode the problem.

[JGood]

The reason I brought this all up is because my engine has an intermittent issue where it will run rough. It goes into a mode where it has a lumpy idle and acts like the TPS is disconnected (bucks when you go on/off the throttle), low end power is uneven and weak, and it backfires a bit. It might last for 10 minutes or so, then it goes back to normal.



Something else to add: If I disconnect the vacuum line from the brake booster, it has a ton of suction. If I cover the hose with my finger, the engine does not run very smoothly, and seems to idle high. If I leave it open, it almost stalls.

But if I disconnect one of the vacuum hoses from the intake boot, it has virtually no vacuum and no effect on the engine.