I swear I am the KING of having strange things happen to my motor...

Anyways, upon teardown I noticed the number 2 intake runner was carboned up and it went up into the manifold as well. All the other runners were fine.

It was my first impression that the valve was bent, just enough to not seal completely. I pressurized the cylinder and filled that port with gasoline. Sure enough it started bubbling with the bubbles coming from the valve.

I pulled the head and took it to my machinist today and before he pulled the valve he hooked it up to a vacuum test machine and the port held vacuum just fine, just as good as the rest of them.

He pulled the valve and the seat was fine, and free of debris. He checked the valve and it was not bent, he lightly touched up the seat and the valve and reassembled it.

We then pressure tested the combustion chamber and filled the port with water. Sure enough there was some air coming up from the valve. There are no cracks in the combustion chamber and the valve seat checks out.

We tested the #4 cylinder and it did the SAME thing. The pressure in the chamber was 120-140lbs so it was significant.

The car ran fine with the exception of that intake runner having carbon on it. It ran an 11.8 two weeks ago in pretty bad air (6000' DA).

Is it normal for some pressure to escape the chamber? I would think not, but I have not tested any other heads to check.

I am about out of ideas, and will welcome yours!

 

Intake gasket or there is a small hole in the intake manifold. It's sucking oil out of the galley.

 

Maybe a broken ring??? A leakdown test will tell you the condition of the cylinder right away. Then a compression test -with a squirt of oil - will eliminate either rings or vlvs.
Now you should do these at normal operationg temp - hot. But i can't get near my headers for quite a while after shut-down to remove the plugs. So i just do both tests cold and look for a big difference between cylinders.
Hope this helps. cardo0

 

Carbon tells me that it very well may be the intake valve is slightly open during combustion, allowing spent products to escape past the intake valve into the intake plenum. It could be caused by a rocker that wasn't adjusted properly, holding the valve slightly open during combustion (lite off). It also could be preignition, detonation in the chamber that could explain it while the intake valve was closing.

 

If rocker/lifter is so, then a leakdown with compression test would show it. Pre ignition in only 1 cyl (#2) unlikey - but possible.
cardo0

 

Yes, the very thought ran through my mind regarding preignition. I think the tests will reveal the problem too. The rocker adjustment seems more possible. A cylinder that is lean could have preignition based on heat.

 

Holy Resurrected Thread Batman!



The valve was not too tight as I did not have any rocker arms installed when I pressurized the cylinder.

I did pull the head and took it to my machinist, he pulled the valve and it was straight and the valve seat was good too! The port passed a vacuum test with flying colors.

We touched the seat up and reassembled the head. We then bolted it to his new pressure tester and pressurized the chamber to 160psi. With water in the port we could watch little bubbles seep past the intake valve! It was very little and with the speed of combustion I don't think it will be a problem. He said he had actually never tested a head like that so it may be "normal" as it passed a vacuum test fine.

I have it back together and running and it's running great. No carbon in that runner so far!

Thanks for the ideas.

Oh, Victor Jr. vs. RPM Air-Gap - Track Results will be posted tomorrow!!! Just got back from the track, interesting results.

 

Yeah, I didn't really pay attention to the date as I should. Glad it is running great!! If you have any other things to add, we will be here same bat time, same bat channel hehe.

 

Heck my reply was less than 20 days old. :o And thats faster than i can remove and replace a set of heads.
Anyways leakdown is a simple test that gets overlooked a lot. Aircraft engines live and die with it while thier mechanics swear by it. And if i can do it in less than 1 day most anyone can. cardo0

 

I forgot this was in Engine Tech. The threads age slower here.

I do not see the need to do a leak down test as I have already identified the leak. The cylinder head was off the car and hooked up to a pressure tester. The leak was around the valve.
Lost in translation I suppose.

 

So what happened to the results of the Vic jr vs. air gap test??? I'm guess'n the air gap is a better match for your combo.

 

ralph Resuls are HERE: http://forums.corvetteforum.com/show....php?t=1189286