M30 intake vacuum and removing the oil cap

[Jeff N.]

Checking some things on the 3.7 today and was curious if anyone had any input.

Vacuum at idle is just about 17.5 inches. This is taken from the 3mm fuel pressure regulator gauge. I haven't checked TIS or the manual but I was wondering if anyone knew what vacuum ought to be on a stock motor.

Second item - waaay before all the upgrades started, if I removed the oil filler cap when the engine was idling it ran like crap. Now, with the rebuild, it runs just the same (again - at idle) cap on or cap off. If I put my hand over the filler hole, I don't feel any vacuum in the motor.

Sooo...anyone have any ideas why this is? I'm thinking the cam changes the intake vacuum somewhat. Martin and I compared a while ago and his was more like 18 or 19 inches of mercury. As the valve timing changes with the new cam, I can see how the intake manifold idle vacuum changes.

I'm curious why the engine doesn't mind having the filler cap removed. This I can't quite figure out. Anyone have any ideas?

Thx

Jeff

[Bill R.]

really need for a street car... this is a common problem on american v8's with long duration high overlap cams..sometimes they don't even have even vacum for the brake booster at idle or low rpm's.. On a number of them they have to have a separate vacum pump for idle speeds to provide enough vacum to run things like brake boosters.. This would also explain why you don't have the torque and power you want at lower rpms, but it comes on suddenly above a certain point..

Checking some things on the 3.7 today and was curious if anyone had any input.

Vacuum at idle is just about 17.5 inches. This is taken from the 3mm fuel pressure regulator gauge. I haven't checked TIS or the manual but I was wondering if anyone knew what vacuum ought to be on a stock motor.

Second item - waaay before all the upgrades started, if I removed the oil filler cap when the engine was idling it ran like crap. Now, with the rebuild, it runs just the same (again - at idle) cap on or cap off. If I put my hand over the filler hole, I don't feel any vacuum in the motor.

Sooo...anyone have any ideas why this is? I'm thinking the cam changes the intake vacuum somewhat. Martin and I compared a while ago and his was more like 18 or 19 inches of mercury. As the valve timing changes with the new cam, I can see how the intake manifold idle vacuum changes.

I'm curious why the engine doesn't mind having the filler cap removed. This I can't quite figure out. Anyone have any ideas?

Thx

Jeff

[632 Regal]

Right on all points Bill BUT he is stating that he is getting 17.5 lbs at the intake port. This to me sounds like perhaps the PCV is not hooked up correctly? Shoot 17.5 is a great reading in an over cammed american POS, Can am gets 5lbs in gear and you wouldnt even want to know what it was in the old race car engine (>3). Any vacuum at idle should appear at the oil filler unless the rings have not broken in yet and there is conciderable blow by at idle (common in rebuilds).

really need for a street car... this is a common problem on american v8's with long duration high overlap cams..sometimes they don't even have even vacum for the brake booster at idle or low rpm's.. On a number of them they have to have a separate vacum pump for idle speeds to provide enough vacum to run things like brake boosters.. This would also explain why you don't have the torque and power you want at lower rpms, but it comes on suddenly above a certain point..

[Jeff N.]

What I don't understand is how vacuum is generated in the engine block. From the intake on the M30, there are 5 take offs if I recall:

- 3mm line to the FPR; setup as stock
- 3mm line to the valve cover vent; setup as stock
- fixed line to the brake booster; setup as stock
- line to the ICV; setup as stock
- electric valve controlled line to the evap system; capped at manifold

That's it. There is no PCV as I understand it on these cars. The only line that connects the intake vacuum to the engine is the 3mm vent line. Everything else is to a fixed system so it shouldn't "leak".

So, how exactly is crankcase vacuum generated in this motor? I understand how a cam change would effect the AMOUNT of vacuum in the intake at any given point. But no crankcase vacuum at all? Hmmmm

I can see that IF you have crankcase vacuum and open the lid, the engine run will change. It's essentially a huge leak in the system.

I'm missing something here (hehe besides vacuum)....?

[Bill R.]

photo.....First off i think some confusion exists... pcv stands for positive crankcase ventilation. Positive meaning it has an outside source creating a vacum to help ventilate and remove all the nasty combustion byproducts and moisture from the crankcase.
Some of the confusion may exist because most of you are used to seeing whats called a pcv valve which is a spring loaded check valve that is used to allow manifold vacum to suck out the crankcase on most cars that are american made or japanese made. These have a very lite spring loaded valve and a metering orifice in them to regulate how much vacum is allowed into the crankcase.
Back to your bmw m30, since this has bugged me for awhile too as to how they regulate it. You have a small vacum line from the intake manifold tapping straight into intake manifold vacum which is high at idle, in your case 18 inches of vacum, not lbs as regal suggested..18'' is a little lower than i would like but it still should provide some vacum to keep the crankcase sucked out...
Now assuming that your front and rear seals are sealing tightly on the crank and that all your gaskets are sealing tightly and the rings are now sealing as well, the crankcase should be fairly airtight with the exception of how much blowby comes past the rings when the engine is running..
So since its fairly tight the small line that connects into the larger hose that runs to the valve cover should provide enough vacum at idle to suck out whatever gases are present at idle...
Now also if the throttle plate is closed the amount specified then that will have an effect on manifold vacum, also the icv or idle control valve bypasses the throttle plate and will also make some difference on manifold vacum, all of these things , the throttle plate opening, the small vacum line to the bigger hose leading to the valve cover and the hose connecting the afm and the throttle body and last but not least the icv... as i said all of these things will pull the afm metering plate or door or whatever you call it open very slightly at idle if your afm air bypass is adjusted correctly , so when you break the vacum by removing the oil filler cap, the afm plate moves ever so slightly and changes the engine speed and mixture...
Thats my story and i'm sticking by it.. Also along the same lines, the small vacum line combined with the vacum from the icv and the throttle plate opening help to create enough vacum to keep the crankcase cleaned out, at higher rpms the larger hose that goes into the boot connecting the afm and tb has enough airflow rushing through it that if creates a constant vacum at the larger hose going to the valve cover and meeting the boot at a 90 degree angle.... Make sense to you? Anyway here's a pic i just took of the larger hose between the boot and the valve cover which has the smaller vacum line that helps to vacum the crankcase at idle


EDIT, also as an additional note, if you look at the eml equipped m30 without the icv it has a check valve which you could think of as a pcv valve an it has a volume control valve as well..

What I don't understand is how vacuum is generated in the engine block. From the intake on the M30, there are 5 take offs if I recall:

- 3mm line to the FPR; setup as stock
- 3mm line to the valve cover vent; setup as stock
- fixed line to the brake booster; setup as stock
- line to the ICV; setup as stock
- electric valve controlled line to the evap system; capped at manifold

That's it. There is no PCV as I understand it on these cars. The only line that connects the intake vacuum to the engine is the 3mm vent line. Everything else is to a fixed system so it shouldn't "leak".

So, how exactly is crankcase vacuum generated in this motor? I understand how a cam change would effect the AMOUNT of vacuum in the intake at any given point. But no crankcase vacuum at all? Hmmmm

I can see that IF you have crankcase vacuum and open the lid, the engine run will change. It's essentially a huge leak in the system.

I'm missing something here (hehe besides vacuum)....?

[Jeff N.]

...here's what I'm wondering.

In your scenario, you mention the AFM. The AFM has a spring loaded door and bypass setting. Essentially, this is an intake restriction that sits between the throttle plate and the 3mm hose valve cover vacuum line. That flapper door serves to create a vacuum in that section of intake bellows which inturn creates a negative pressure in the valve cover by way of the hose pictured above. Vacuum in that section would be less than manifold vacuum as it's regulate by the spring door, bypass screw, ICV and general engine seal. When you open oil cover cap...bingo...huge leak thru the valve cover hose around the AFM.

Now....hehe...(I think we are figuring this out). I don't have an AFM. I have a MAF. No spring door. No bypass. MUCH less vacuum in that hose section between the MAF and throttle plate. With less vacuum, there's less evac thru the valve cover. With less evac, less crankcase vacuum. Less crankcase vacuum means very little impact when you open the system (ie: remove the oil filler cap).

I think we can safely say that the amount of vacuum in the crankcase is equal to the spring pressure provided by the AFM door less whatever gets by due to the bleed screw setting.

Hmmm...seems to make sense, no??

[Bill R.]

valve cover hose at idle, I just think since you don't have a flapper valve to be affected by the opening of the oil filler cap then it won't make as much difference , but even with a maf if its set up correctly you would still be bypassing air that would normally go through the maf all the time and allowing it to suck through the valve cover instead when you remove the oil cap... unless your maf isn't that sensitive at low volumes like the stock flapper on the afm is.

...here's what I'm wondering.

In your scenario, you mention the AFM. The AFM has a spring loaded door and bypass setting. Essentially, this is an intake restriction that sits between the throttle plate and the 3mm hose valve cover vacuum line. That flapper door serves to create a vacuum in that section of intake bellows which inturn creates a negative pressure in the valve cover by way of the hose pictured above. Vacuum in that section would be less than manifold vacuum as it's regulate by the spring door, bypass screw, ICV and general engine seal. When you open oil cover cap...bingo...huge leak thru the valve cover hose around the AFM.

Now....hehe...(I think we are figuring this out). I don't have an AFM. I have a MAF. No spring door. No bypass. MUCH less vacuum in that hose section between the MAF and throttle plate. With less vacuum, there's less evac thru the valve cover. With less evac, less crankcase vacuum. Less crankcase vacuum means very little impact when you open the system (ie: remove the oil filler cap).

I think we can safely say that the amount of vacuum in the crankcase is equal to the spring pressure provided by the AFM door less whatever gets by due to the bleed screw setting.

Hmmm...seems to make sense, no??

[Jeff N.]

Martin:

At idle, if you pull the oil filler cap and put your hand over the hole, do you have vacuum or not?

How does it run if you pull the filler cap? Same or funky?

Jeff

[Martin in Bellevue]

Pulling the dispstick on my m30 will cause a stumble. The vacuum gauge on the map sensor reflects a big dip in vacuum.

Martin:

At idle, if you pull the oil filler cap and put your hand over the hole, do you have vacuum or not?

How does it run if you pull the filler cap? Same or funky?

Jeff

[Jeff N.]

We should check 'em side by side. Do you recall home many inches yours pulls at idle?

I'm having a hard time figuring out why just my cam change would impact the vacuum in the engine case that much. If I had like 5 inches of manifold vacuum at idle, that'd be one thing. 17 to 18 seems to be another.

...and your setup with no AFM flapper AND vacuum in the crankcase pretty much puts cold water on my hypothosis.

Jeff