Crankcase Vac = Engine Vac??

vristang · Aug 11, 2005

Yes Crankcase vac. As measured at the PCV. Obviously the pcv is bad, but engine vac being pulled through the crankcase?

I have 17" of vac on the motor at an idle of 1000rpm and 21" at 2000rpm

When I hooked the vac/psi gauge up to the pcv and plugged the valve cover/TB holes, I got the exact same readings. 17" at 1000 and 21" at 2000.

Also, what does the stock 50 pull at idle?

thank you very much for the help.
I am never disappointed by you guys.

TheUser · Aug 11, 2005

well a stock 5.0 pulls between 15-20 hg/vac at an idle of 675 or around there anyway. Your 408 w/ custom cam, etc isn't exactly a stock 5.0, so really you're comparing apples to oranges here - or it seems that way to me. Your idling vac would be a reflection of your cam overlap and a few other things.

Anyway, I've never measured vac at the pcv and so I don't know what it should be(I wouldn't assume there'd be any, though ). I'd be curious as to how you did that exactly.

Did you hook the vac gauge up to the rubber hose that connects to the intake? That's what it sounds like. You did disconnect the hose from the upper intake and use that end to attach the gauge right? You didn't measure vac. from the intake again did you?

If you measured it through the PCV valve at idle and got those results then yeah I'd say your PCV valve is probably broken or something.

I'm anxious to read what others say about this

vristang · Aug 11, 2005

TheUser said:
I'm anxious to read what others say about this

I was very careful to make sure that the intake was completely capped off. the only source of press/vac on the gauge at that moment was through the pcv. I am about to take a walk back out to the garage to double check this though.

I have never measured engine vac before so I was looking for some kind of reference point. I'm not trying to decide if I am getting decent vac by comparing to the stock spec, just trying to get a point of reference.

Also, it does seem like as I stack up more miles on the car, more oil is puddling in the intake.

vristang · Aug 12, 2005

No one has any thought huh? I gotta say I feel pretty ignorant right now.

Here is my current theory, let me know what you think.
I think the intake manifold to head gasket may not be seated properly. I am guessing that the mating surfaces of the head and the intake are not parallel and that this is allowing for engine vac to seap past the intake gasket and into the lifter valley. The intake I purchased new and has never been machined, but I purchased the heads used, so all bets are off there.
My current plan of action is to pull the intake and see if I can find a way to confirm/deny the parallelism of the 2 surfaces. Also, the condition of the gasket should give me some hints.

:OT:
If you have 20% blowby, and the pcv is venting all of the crankcase pressure to the intake (or it is blowing past the intake gasket), does that qualify as supercharging?

vristang · Aug 12, 2005

Well here's the final verdict if anyone is following this saga.

Yes, I truely was pulling a vac through the crankcase. It turns out that the left intake gasket had slid down when I installed the lwr intake. This pushed the bottom of the gasket out so that there was no gasket seperating those 4 intake runners from the lifter valley. End result was crankcase VAC.

So, I had locked the intake gasket into the head gasket like you are supposed to, but the head gasket tab had bent down, probably during installation.
This time I have siliconed the gasket in place with a small dab on each end. The silicone has been sitting for a couple of hours so it should help hold the gasket in place a little better this time. I also marked a line along the top of the gasket on the head, so that I will be able to tell if it shifts when I put on the intake.

My next question is if any one else has had this happen? I've never heard of this before.
I am also willing to bet that this is the vac leak that was causing my high idle.

vristang · Aug 13, 2005

Ever had this happen to you?

Click here to see first pic

Click here to see second pic

srothfuss · Aug 13, 2005

Never to me on a Ford motor... I have heard that with the aftermarket intakes the PCV can suck up a lot of oil since they don't have a baffle covering the port.

stang8urimport · Aug 13, 2005

PCV is Positive Crankcase Ventilation. It is basically a controlled vacuum leak to clear out any blow by caused by the rings, and will help seal the rings. You should pull the same vacuum through the PCV if you put a gauge on it that completely covers the suction hole in the valve itself. I see no problems with your vac at idle or at RPM. It's actually VERY good for a built engine. Comp cams did you good on a very streetable cam.

TheUser · Aug 13, 2005

PoopDawg said:
PCV is Positive Crankcase Ventilation. It is basically a controlled vacuum leak to clear out any blow by caused by the rings, and will help seal the rings. You should pull the same vacuum through the PCV if you put a gauge on it that completely covers the suction hole in the valve itself. I see no problems with your vac at idle or at RPM. It's actually VERY good for a built engine. Comp cams did you good on a very streetable cam.

Positive Crankcase Ventilation is pressure....psi. Vacuum is negative pressure...sucking. You should have positive pressure from your crank case because of any blowby that gets by the rings, which is what the PCV valve is for - to vent that pressure from the crank case so you don't blow your seals.

Cliff notes: You should not have a vaccuum in your crank case...at least as far as I know.

stang8urimport · Aug 13, 2005

http://www.2carpros.com/topics/pcvvalve.htm

"Question: What is the purpose of the PCV valve and what are the symptoms when it goes bad?

Answer: PCV means positive crankcase ventilation. This valve uses vacuum from the intake manifold to draw the fumes from the crankcase. Symptoms of a faulty PCV valve are poor idle and excessive pressure in the crankcase."

That should explain it enough. It is evacuating the Positive Crankcase pressure using vacuum from the engine.

vristang · Aug 14, 2005

PoopDawg said:
PCV is Positive Crankcase Ventilation. It is basically a controlled vacuum leak to clear out any blow by caused by the rings, and will help seal the rings. You should pull the same vacuum through the PCV if you put a gauge on it that completely covers the suction hole in the valve itself. I see no problems with your vac at idle or at RPM. It's actually VERY good for a built engine. Comp cams did you good on a very streetable cam.

I think you may have misunderstood what the problem was. The vac reading seems quite good, and I am happy with the cam I designed. However, the pcv valve was sucking air into the crankcase instead of pushing it out.

I think the primary purpose of the pcv is to pull contaminants like unburned fuel, moisture, and other volatiles out of the oil system.

More importantly, I hope this can help someone else introuble shooting their problems.
Good Luck to all the Mustangers
I Hope you never have to deal with this problem.

stang8urimport · Aug 14, 2005

So you're telling me your PCV vavle is pushing air instead of sucking? ROFL. You def got somethin' messed up then.

TheUser · Aug 14, 2005

vristang said:
However, the pcv valve was sucking air into the crankcase instead of pushing it out.

The manifold never has positive pressure unless you have forced induction (at WOT it should go very close to 0 vacuum) and you can't "push" vacuum - especially past a one way check valve. The PCV valve is a one-way check valve for just that perpose - so you can't push anything into the crankcase through that PCV valve.

I can't explain how you had vacuum in your crank case at idle - I'd have to actually gauge it myself to believe it.

vristang · Aug 14, 2005

TheUser said:
The manifold never has positive pressure unless you have forced induction (at WOT it should go very close to 0 vacuum) and you can't "push" vacuum - especially past a one way check valve. The PCV valve is a one-way check valve for just that perpose - so you can't push anything into the crankcase through that PCV valve.

I can't explain how you had vacuum in your crank case at idle - I'd have to actually gauge it myself to believe it.

I already know how it happened. Take a look at the 2 pics I have links to in earlier posts. One will show how the intake gasket slipped about a quarter inch. This is how I ended up with vac in the crankcase. The crankcase vac did not come through the pcv, it was from the intake runners!!!

After I had put a vac gauge on it i still had a hard time beleiving it.

Crankcase Vac = Engine Vac??

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