Or lack thereof I should say. I've finally mustered the energy to get back to work on my 94 Z28 and had some interesting findings yesterday.

A little background for those that might not be aware: The engine appears to have blowby. .40 over 385 LT1, big cam, less than 500 miles on the engine. I expect that the rings should have set by this point so I was dismayed to continue seeing some blue smoke from the tailpipes and a steady stream of smoke from the valve covers when the oil cap was removed. I am running a PCV system with the lines set up like a factory LT1 with a vented opti.

http://shbox.com/1/pcv_pipe.jpg

The only exception to the above picture is that I'm using a hose instead of a hard line. I don't think that should make any difference. PCV valve is new and appears to be pulling hard. I also have the vent line going from the throttle body to the passenger's side valve cover present.

Before I resigned myself to a rebuild of the engine I obviously wanted to do a cylinder leakdown test. Yesterday I went to my buddies house and we pulled all of the plugs to try and find one that was obviously burning oil and to my surprise found none. All of the electrodes were light tan to white with no soot or sludge buildup anywhere on the plug. The worst looking one was #8 which was dark tan. If anything, the engine needs cal work because it's running lean. So, we then did a leakdown on every cylinder. Not one of them were out of spec. They all registered between 6% and 10% leakdown which according to the manual is "excellent".

Now I'm stumped. Where is this smoke coming from and why is it coming from the valve cover and the tailpipe? Am I missing something with the PCV setup?

 

It's time to hook a manometer to the crank case. Be careful to cap it while starting the engine if you use a water type.

 

Thanks Speedy. That's what I was thinking too. What could cause such excessive pressure buildup? I'm thinking that this huge cam with 24 degrees of overlap is just not pulling enough vacuum.

 

Gauge indicated 3 in H2O which comes out to about 0.1 psi. From my understanding, anything less than 1 psi is acceptable.

 

Did you measure a pressure????? Based on the way you said it, it was. You should have a vacuum.

 

Yes, there was a positive pressure of +3 inches of H2O (0.11 psi) as measured at the dipstick tube.

 

So obviously your PCV system is not functioning since you proved it's not blowby. Crankcase has to be a vacuum NOT pressured.

 

I'm wondering where the restriction is. I can feel strong suction at the valve itself. There doesn't appear to be a restriction in any of the lines. Should I be feeling a vacuum at the vent line on the passenger's side?

 

Just for the heck of it, I measured vacuum at idle and came up with a measly 11 in. Hg.

 

The valve is a check type. It flows in one direction by my understanding. It lets positive pressure come out of the crankcase created as the piston moves down the bore. But then it seals to not let air back into the crankcase as the piston moves back up the bore. Thus creating a mild vacuum. Without this vacuum level, all sorts of problems arise.

If your feeling a vacuum on the outside of the valve, then it is not functioning the way it was intended. Or it's installed backwards.

Incidentally, the amount of vacuum can be an indication as to the health of an engine. And you can see that this vacuum has nothing to do with the cam. That has an influence on intake vacuum.

 

Are we now talking crankcase vacuum?

 

No, still the same +3 in. H2O in the crankcase. This was just a fun test to see how much manifold vacuum this cam was or was not drawing.