I have been trying to search for the answer on this one. Anyone seem to know what kind of problems would exhibit a lot of vacuum inside the valve covers?
We will need some details on your motor. Any mods? (a signature is great for this)
Where did you plug the vac gauge in?
Have you made any changes to the car lately?
How is the engine running?
I have had this problem before myself. It turned out that the lower intake manifold gaskets had slipped down when I bolted on the intake. The result was a vac leak between the intake runners, and the lifter valley.
Well this is a new engine that I have been trying to get running right. The only time it runs right is in the mid to upper RPM's. I fixed the first problem I had which was a leaking injector and another with a terrible atomization/spray pattern. The Vacuum of the engine is terrible at idle also (if it can be made to get down to a good idle rpm). I posted awhile ago asking about the low vacuum, but I am thinking this has to be a major part of the problem.
The engine is as follows:
4.020" bore, JE/SRP 8.9:1 pistons, Speed Pro rings: gapped .020" top, .018" bottom (prepped for a s/c) AFR 165 heads, Comp cams XE270HR-14 cam, 1.6 Harland Sharpe rockers, Cobra intake, yada yada...............(anything else needed, just say so).
Vacuum gauge was used with the port on the manifold where i'm assuming the equivalent to the stock, had the Evap system hooked up (first port after TB)? At roughly 800 rpms, I see 11-13" of vacuum, and the valve covers aren't far behind. However, the odd thing is when I get the oil cap off, the manifold vacuum drops another 2" or so of vacuum.
The write-up by Mr. Yount is fairly good, any further ideas to throw my idea with the excessive valve cover vacuum situation?
Opening the oil filler cap, will reduce manifold vac if you have a manifold vac leak common to the crankcase.
I would think that you have the same problem I had (the pic I linked to above).
You should be able to see the top of the intake/head gasket when looking down at the motor. If the gasket seems excessively lower than the intake manifold, then it may be time to drain the coolant and pull the intake.
I am not sure where else there could be a vac leak common to the crankcase.
I have been wanting to blame a vacuum leak on this since I put this thing in. No, that lower intake gasket did not go on the easiest for some reason (I did it twice because it DID slide down the first time, but before the engine ran). It's a felpro 1250. Anyone else on the vacuum leak idea, or any other suggestions on high vacuum in the valve covers?
Just for peace of mind though, I feel like ripping the intake back off and replacing the gasket anyway....
After I screwed up on the manifold gasket, it was suggested to me that I use some silicone or other gasket/adhesive compound to hold it in place while positioning the intake.
If you let the compound tack up it will hold the gasket in place fairly well.