Crankcase ventillation

[hoveringuy]

Can someone explain this to me:

My car has been sitting in the garage while I wait out this snow storm. I wanted to let it run for a bit and it ran like cr@p! Blowing black smoke, not revving, missing, etc.

I finally figured it out that if I disconnected the crank breather on the head it cleared up and ran fine. Reconnect, run like cr@p.

So, someone explain that to me. Why would crank ventillation do that? I thought it was an innocuous system.

[e30serg]

Are you getting some massive blow-by? The crank breather is supposed to exhaust the combustion gases that get past your rings and put them into the intake stream to be re-burned.

[hoveringuy]

At idle with the crank ventilation connected, if I pull the oil cap off it sucks. That is, there is a nice vacuum at the oil cap.

[DaveSmed]

When you say you disconnect it, are you removing the hose from the manifold and plugging the manifold, or are you inducing a vacuum leak?

[hoveringuy]

When you say you disconnect it, are you removing the hose from the manifold and plugging the manifold, or are you inducing a vacuum leak?

Yeah, I didn't think of it that way but I guess it is a vacuum leak. Why would it run so cruddy without the vacuum leak? Not just at idle, but stumbling and missing everywhere with very dark, obviously rich exhaust?

[Money$hit]

springloaded tube of death? why would you take that out?

[SpecM]

springloaded tube of death? why would you take that out?

its not an m20

hoveringuy: your motor must be running rich, and the vacuum leaks just smooths it out

[DCColegrove]

What engine? Does it have an Oil Separator (Vent Valve)?

[DaveSmed]

Yep, allowing "false air" thereby leaning out the car. Check out the coolant temp sender first, then move on to fuel pressure. Make sure it holds pressure for a reasonable amount of time with the car off to check for leaky injectors or a leaking fuel pressure regulator.

[jlevie]

An M50 or S50/52 engine that runs better with a a large intake leak, as appears to to be the case here, would make me think of a problem with the MAF. How does the engine run if you reconnect the crankcase breather and disconnect the MAF?

[hoveringuy]

It's on my M54, which has the oil separator. It's been cold here in Seattle and the car hasn't been driven in a week. The garage got so cold that I had coolant leaking out of the radiator end cap and it's a new radiator!

The MAF is one of the things I thought about. When I unplugged it the car stumbled badly. It ran better connected.

I also unplugged the intake air temp sensor. Same thing.

Couldn't reach the coolant temp sensor but that would be a logical guess.

I would love to drive it and see if it clears up, I may have ice in my crankcase vent system for all I know!

I also have the fuel pressure dialed up a notch because it was running lean with the VANOS at 20 degrees advance, but it never idled poorly like this.

I hope it's just a weather thing.

[DCColegrove]

Could be the Oil Separator/Vent Valve they do weird shit like this when the diaphram breaks.

[jahnaboi]

Hopefully its just ice within your engine that needs to melt. Cars really don't like freezing temperatures, just cool temps. =) Also, kinda ot but, did you ever get your dual vanos running or just single vanos on the intake side?

[hoveringuy]

I'm working diligently on the dual part of dual VANOS. See my "M54" thread for an exciting update!

[hoveringuy]

Solved?

Opening the oil cap leaned it out which only cured a symptom, not the problem. It's still been running badly and after checking EVERYTHING I finally tried replacing the spark plugs.

Bingo!

The old ones were fine in warmer weather or when I started it and drove immediately to work. But when it got colder the car started running richer during warmup which is what cars tend to do. It certainly couldn't tolerate idling in the garage while I took O-scope readings off it!

The plugs could no longer fire properly in the richer mixture and started fouling.

So, if your car is running like cr@p in this colder weather check your plugs.