Intake valve leak question

[Lugnutz65]

Question: other than the intake valves, are there any other passages that can leak air pressure from the crankcase and up through the heads into the intake manifold?

I’ve established considerable blow by in cylinders 2, 5, and 8 using a professional cylinder leak test gauge.
I have rockers completely loose so the valves MUST be closed during the test. When I add 100psi to the cylinder I definitely feel and hear air flowing at the oil fill cap. The intake manifold is properly sealed to the heads. I know that passages in the head allow drainage of oil from the head to the crankcase but to my knowledge none of them would communicate with the intake or valves unless the valve seals were bad.

Comments?

[AussieinNC]

Broken or missing piston rings will allow air to bypass from the combustion chamber into the crankcase, which you could here thru the oil fill.

If you can also here air escaping into the intake manifold, and all rockers are off the valve stems, all I can think of is there are bent valves not sealing correctly or the heads are badly cracked between the chamber and the intake port....have never seen that happen on an LS.

Is this the same engine that has all the oil issues in the intake?

On the bad cylinders, 2,5 and 8, remove the pressure feed attachment, squirt some heavy oil like 90 grade into the chamber and retest bypass...

[Lugnutz65]

Yes, this is the same engine. I know that the heads are interchangeable for either bank of cylinders. I also know that oil added through the fill cap must pass through the head into the crankcase, so there are passages in the head for that but I wanted to be sure there were no air passages from the head to the intake manifold except for the intake valve.

I understand and appreciate your comment. Leaking valve stem seals, bent valve stems, cracked head or poorly seated intake valves could cause a leak of air up into the intake.

I might try the 90W oil test to see if the blow by diminishes. Good suggestion.

[71cadc10]

question, are you taking the leakage test with the cylinder under test at TDC ?

I ask because your lowest readings show 5 & 8 which are sister cylinders and both would be at TDC (or BDC) at the same time.

like you noted, the main way air can get from chamber to intake manifold, is intake valve path. However, if the PCV hose is hooked up to intake, this could allow some more to get to the intake from the crankcase. Air bleeding by the ring gaps would then push up thru the PCV and you may hear it in the intake. If this is the case, you should hear it on all 8.

Another check could be remove the intake manifold...pressure up and spray some soapy water directly down the port to see if the intake valve is sealing. You'll get bubbling if the valve seat isn't sealing.

air rushing out the oil fill tube, will happen slightly just from the ring gaps. Large amount of air would indicate larger ring problems. I've done these type of tests before and if the piston was at bottom vs. top I'd get different results.

[Lugnutz65]

Originally Posted by 71cadc10

question, are you taking the leakage test with the cylinder under test at TDC ?

I ask because your lowest readings show 5 & 8 which are sister cylinders and both would be at TDC (or BDC) at the same time.

like you noted, the main way air can get from chamber to intake manifold, is intake valve path. However, if the PCV hose is hooked up to intake, this could allow some more to get to the intake from the crankcase. Air bleeding by the ring gaps would then push up thru the PCV and you may hear it in the intake. If this is the case, you should hear it on all 8.

Another check could be remove the intake manifold...pressure up and spray some soapy water directly down the port to see if the intake valve is sealing. You'll get bubbling if the valve seat isn't sealing.

air rushing out the oil fill tube, will happen slightly just from the ring gaps. Large amount of air would indicate larger ring problems. I've done these type of tests before and if the piston was at bottom vs. top I'd get different results.

Your points are valid. Feeding air pressure to any cylinder with bad rings will cause air pressure to build in the crankcase. That air could then travel from the crankcase, past the bad rings in another cylinder and up through that second cylinder intake valve if it were open.

For all of my leak tests, the PCV and fresh air lines were disconnected from the intake.

I tested cylinder leak by 2 methods.
First method was with rockers properly set. I then rotated the crankshaft and tested leak while the piston was midway up on the compression stroke (valves closed).
Second method was with all rockers loose so all valves would be simultaneously closed at all times and with every test. Results were almost identical.
The second method would be the most reliable way to test for any valve leak.
I suspect out of round cylinders or broken rings, or both.

[71cadc10]

broken rings, yes lots of air rush sound should occur.

I still hear some air flow with good rings, due to each ring has a .015 to .020" gap...depends on how quiet the room is when you are listening LOL