Vacuum at oil cap

Condor232

Registered User
Maybe a stupid question but...I finally got my smoke machine put together and have a few small leaks here and there:

1.) TPS on the sensor side
2.) Throttle Body plate shaft (heard there is nothing you can do about this and it is more or less normal)
3.) Bypass valve shaft on the back of the plenum
4.) Oil cap

The only large leak is the oil cap, and this is because for some reason the Mustang oil cap does not fit properly and doesn't seal. Do SC's take vacuum at the oil cap, and could this be my problem? I am facing some wicked vacuum issues.

Thoughts on the other leak places are also welcome, please. Need your guys' help!

EDIT: I also pull relatively low boost (5 PSI WOT) and low vac (-5 to -8 at idle, sometimes it hits zero after driving for a short time). The car may also die occasionally when putting it into reverse or drive after idling or driving.
 
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Two things.

1) There's a feed from the air intake to the inlet (before the supercharger) to allow for air to enter the PCV system, and for it to be monitored. If your cap doesn't seal right, that's a vacuum leak all right.

2) The PCV valve itself is plumbed to vacuum, so when the conditions are right, yes, there's some vacuum on the crankcase system. It's very small, but still there.

RwP
 
Two things.

1) There's a feed from the air intake to the inlet (before the supercharger) to allow for air to enter the PCV system, and for it to be monitored. If your cap doesn't seal right, that's a vacuum leak all right.

2) The PCV valve itself is plumbed to vacuum, so when the conditions are right, yes, there's some vacuum on the crankcase system. It's very small, but still there.

RwP

For your first point, is this enough of a leak to cause my vac readings to be so low?
 
EDIT: I also pull relatively low boost (5 PSI WOT) and low vac (-5 to -8 at idle, sometimes it hits zero after driving for a short time). The car may also die occasionally when putting it into reverse or drive after idling or driving.

That would have to be quite a vacuum leak!

- What RPM does the car idle at? Does the idle RPM hunt up and down?

- Does the car "pop" while running?

- Have you checked the timing?
 
That would have to be quite a vacuum leak!

- What RPM does the car idle at? Does the idle RPM hunt up and down?

- Does the car "pop" while running?

- Have you checked the timing?

It idles around 800 right now. It doesn't tend to go up and down too much, but I have a big cam so it does lope quite a bit.

It does not have a distinct popping noise.

I have not yet checked timing, as the smoke test revealed some leaks I wanted to address first.
 
It idles around 800 right now. It doesn't tend to go up and down too much, but I have a big cam so it does lope quite a bit.

It does not have a distinct popping noise.

I have not yet checked timing, as the smoke test revealed some leaks I wanted to address first.

RPM doesn't sound too bad.

I would check the timing if you have access to a light, just to be sure that the harmonic damper hasn't slipped or something. Although now that I think of it, I forget if the crank pully bolts to the inner hub of the damper or the outer ring. So would the timing read correctly if it had slipped? I wonder.

I wouldn't worry much about the shaft leaks (throttle and boost bypass), unless they are getting ridiculous. They are fixable, by the way, but it takes a little doing to get to the seals, and it's not 100% foolproof. So I would say, try the propane or carb cleaner test with the engine running, and see how much difference it makes in the engine idle. If it's not a lot of RPM rise, then I would look elsewhere for the moment.

I would say that the oil cap should seal, for sure. Isn't there an o-ring or rubber gasket on the stock cap? As RalphP and sts70004 said, just seal the oil filler with anything for now. Heavy Ziploc bag and rubber band, anything. See how it works.

Lastly, the low boost is a surprise, and would seem to be totally separate from the oil cap and throttle leaks. It could be related to the boost bypass leak, but I find it hard to believe it is leaking that badly. But you did mention that you have swapped the cam. How recently did you swap it? Did the car ever run right since then? My train of thought is heading toward improperly adjusted rockers hanging open a valve or two.
 
RPM doesn't sound too bad.

I would check the timing if you have access to a light, just to be sure that the harmonic damper hasn't slipped or something. Although now that I think of it, I forget if the crank pully bolts to the inner hub of the damper or the outer ring. So would the timing read correctly if it had slipped? I wonder.

I wouldn't worry much about the shaft leaks (throttle and boost bypass), unless they are getting ridiculous. They are fixable, by the way, but it takes a little doing to get to the seals, and it's not 100% foolproof. So I would say, try the propane or carb cleaner test with the engine running, and see how much difference it makes in the engine idle. If it's not a lot of RPM rise, then I would look elsewhere for the moment.

I would say that the oil cap should seal, for sure. Isn't there an o-ring or rubber gasket on the stock cap? As RalphP and sts70004 said, just seal the oil filler with anything for now. Heavy Ziploc bag and rubber band, anything. See how it works.

Lastly, the low boost is a surprise, and would seem to be totally separate from the oil cap and throttle leaks. It could be related to the boost bypass leak, but I find it hard to believe it is leaking that badly. But you did mention that you have swapped the cam. How recently did you swap it? Did the car ever run right since then? My train of thought is heading toward improperly adjusted rockers hanging open a valve or two.

Thanks alot for taking the time to reply in detail.

The cap I am using is off a Mustang, so it does not fit properly. It will sit in the cover somewhat firmly, but just pops out and it does not seal.

The low boost is really my main concern. The cam is a 224/230 (its been awhile, I may have to re-check that) and was installed on a brand new motor. The engine is not that old and has about 500 miles on it, but no, it has never really run all that properly. Any tests I can do for a stuck valve or poorly adjusted rocker?
 
Ok, I just took the car for a drive with a new, proper oil cap installed. Here is what I noticed:

1.) Vac is still low. When put into drive, it looks to be 1-3" according to my gauge.

2.) Hammering it to 30mph or so, I seem to have more boost. It jumps above 5, and even broached 10psi briefly. I have not been able to take it out on a major road to try it out at higher speeds yet. Can I expect more boost at higher speeds?

3.) During one run, I hammered it, boost shot WAY up to about 20, and the motor stumbled. Almost like it cut out for a second. Is this belt slip occuring?

In addition to the questions above, what are the normal scenarios you see under boost? For example, when you hammer the gas do you see boost immediately jump up to its peak, or does it climb as speed increases?
 
The low boost is really my main concern. The cam is a 224/230 (its been awhile, I may have to re-check that) and was installed on a brand new motor. The engine is not that old and has about 500 miles on it, but no, it has never really run all that properly. Any tests I can do for a stuck valve or poorly adjusted rocker?

I think a compression test should identify if there is a problem there. Bit of a pain to get to the spark plug holes (I would probably do it from the bottom), but it beats tearing anything apart.

When I go WOT in my car (10% pulley, ported stock blower and plenum), boost jumps to maybe 11-12 psi and then climbs with RPM to its peak (maybe 14.5, it was more before my blower rotor bearings got worn out!).

What blower do you have, and at what OD? If you have something very aggressive like the MPX, the low-rpm boost may be lower than a stock blower at the same overdrive. But high-rpm boost will be way higher.

In addition to the questions above, what are the normal scenarios you see under boost? For example, when you hammer the gas do you see boost immediately jump up to its peak, or does it climb as speed increases?

Your vacuum is awfully low. I wonder if your gauge is working right? Or if you have a leak in the gauge line?

If you hit 20 psi, you could have had some hard detonation, or spark failure ... boy, lots of things! I wouldn't go there again until you get it running better. I think the quick spike is most likely related to a misfire occuring under lots of boost.
 
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