Negative crankcase pressure

[dukeofpg]

I noticed a hissing sound from inside my car at idle. So when I investigated, I found that there is excessive vacuum from the tube at the back of the drivers side valve cover. I need to find why this has reversed itself from positive pressure to negative pressure. The engine runs great. I cannot find any other issues at this time.

[J Wikoff]

Intake manifold gasket leak to the lifter valley?

[dukeofpg]

Thanks for your reply. What would cause that? Everything runs well and no codes are showing. This is an L76 with 80,000miles. Also if that is the problem, maybe I should upgrade my intake manifold.

[J Wikoff]

I haven't had to mess with the gaskets, but they are a rubber piece laid in place, and don't come in some sort of plastic/metal carrier frame, correct? One could have had a defect and eventually got partially sucked in.
It might be worth pulling the plugs and looking for one that doesn't match the rest, showing signs of being leaner than the rest. Can you scan the fuel trims and see if the engine is compensating for extra air?

[dukeofpg]

Im not certain if the scanner I have will read individual fuel curves. So I will try the spark plug check. Since this is a 2 day old issue, I would be lucky to see much of a difference. Thanks again for you response. I appreciate any help I get.

I have a catch can. It has a place for a filtered vent. Currently I have it plugged off with the provided plug. I was told that the filter would cause a lean condition, so I do not run it.

[J Wikoff]

I forget, do we have a PCV, or is it really just an open line into the intake?

[STL_G8GT]

It's my understanding that the port on the back of the drivers head is the one with the restrictor in the orifice, and is therefor the inlet. Meaning it will pull vacuum.

[dukeofpg]

I forget, do we have a PCV, or is it really just an open line into the intake?

The PCV is simply a restriction and not any sort of valve. The hose from it goes directly to the intake just behind the throttle body. The catch can filters out the foul air and condenses it so that the oil does not accumulate on the inside of the intake manifold. It is positive pressure, but only a slight amount of pressure. My car has enough vacuum that if you leave your finger on for more than a few seconds it will cause a blister. (So keep cats and small children away)JK!

[Greg@PacePerformance]

The PCV is simply a restriction and not any sort of valve. The hose from it goes directly to the intake just behind the throttle body. The catch can filters out the foul air and condenses it so that the oil does not accumulate on the inside of the intake manifold. It is positive pressure, but only a slight amount of pressure. My car has enough vacuum that if you leave your finger on for more than a few seconds it will cause a blister. (So keep cats and small children away)JK!

Do you have the valley tube also hooked up? This would pull vacuum at the rear VC fitting I would think if you are trying to use both.
There should only be the passenger VC fitting hooked to the air intake tube for fresh air and either the valley cover hooked to the intake vacuum or the left valve cover hooked to intake vacuum.

[dukeofpg]

Do you have the valley tube also hooked up? This would pull vacuum at the rear VC fitting I would think if you are trying to use both.
There should only be the passenger VC fitting hooked to the air intake tube for fresh air and either the valley cover hooked to the intake vacuum or the left valve cover hooked to intake vacuum.

Greg, I have had the catch can on there for years and it has always flowed from the Valve Cover to the Catch Can and ending at the back of the Throttle Body. I have never changed the routing of these tubes. I also did not move the hoses from anything else. I just noticed the hissing sound and thought that I had an air leak. As i investigated, i pinched the the hose coming out of the Valve cover and the hissing stopped. So then I disconnected it. I actually could not pull it off until I shut off the engine because it has so much pull. When it is open to the atmosphere, the engine stumbles and dies or runs at extremely low idle and a bit rough. When the system is closed everything runs smooth. No CEL's. I do seem to be getting bad mileage, but that could be because it has been running in the shop with the hood up. I've been to 2 GM dealerships and one other LS mechanic and basically they say to pitch a tent and sell tickets because this is impossible.If anything it should be creating too much pressure.

•One other note. The oil fill is on the opposite side. I do not notice any vacuum when I remove the oil filler or the dipstick. Only the drivers side sucks (and maybe the driver too). At this point, I should offer a reward for the the answer that fixes it!