Why is there vacuum at the oil filler on my 1999 3800 V6?


Asked by Dec 13, 2012 at 07:24 PM about the 1999 Pontiac Bonneville 4 Dr SLE Sedan

Question type: Maintenance & Repair

Engine runs fine
A pulley bearing noise at the front of the engine goes away when oil filler cap is removed
Upper intake manifold was just replaced

13 Answers


PCV valve should keep the crankcase pressure pretty neutral. Are you certain its a vacuum, sometimes the crank feels like it has vacuum but its actually pressure. Only way crank would be under vacuum is if the head and/or lower intake was cracked. If that was the case then removing the oil cap would cause the vehicle to run super rough.

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I would think that if the PCV valve is stuck open, you would get a constant vacuum in the crank case.


It's definitely vacuum at the oil filler. The PVC appears to be fine. The lower intake/head sounds, alas, like the direction we're headed. The engine runs well overall so I"m holding out a hope that the heads are intact at least though the lower intake is surely a serious and expensive bit of work. Thanks John


The PCV valve tube shouldn't be under vacuum unless the air filter is severely clogged. Positive crankcase ventilation not negative. The PCV tube goes to the air cleaner tube or the top of the throttle body where there is only venturi vacuum and that only affects liquids and it only affects liquids when there is a massive flow of air. If the air filter was clogged enough to cause the PCV to have a vacuum then it would run horribly. Any way it goes I don't see how removing an oil filler cap causes a bearing noise to go away. I know fords make an obnoxious whistling when the PCV filter gets clogged. Gotta clean it out with Carb cleaner.

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I have had this problem as well on my 1999 Grand Prix. It sounds like a rough bearing sound from the front of the engine but it is actually air being sucked past the seals. It is a problem with the PCV system. I am having this problem again but I don't remember how I fixed it about 10 yrs. ago. I don't remember if there is a crankcase breather on this engine. If there is I can't find it. I think I ended up replacing the PCV valve and O-rings on the PCV housing and it fixed it. It creates enough vacuum that there is quit a bit of resistance taking the oil filler cap off.

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If anyone else has figured out a solution please let me know.


Yes the 3800 engines have a pcv valve on the top of the intake manifold, it is a tube inside the intake that shoots through the upper and lower intake to the crankcase

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The pcv valve was not installed or the O-ring that goes around the valve is missing. Remove the pcv valve cover by twisting it counterclockwise about 20 degrees. There is supposed to be an o-ring about 3/4" diameter that goes around the bottom part of the pcv valve. If the valve/o-ring is missing, it allows all of the intake vacuum to be pulled through the crankcase.

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The PVC valve was missing the O-ring that goes around the bottom. I replaced it and it is good now.

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would like to comment on the situation I had the same problem today I'm trying to locate why it's doing this when I remove the oil cap to fill it there is a resistance on removing the cap once I remove the oil fill cap the noise goes away and it sounds like a grinding like mild grinding noise. its coughs it runs like s*** then it clears itself out. I changed the intake and intake gasket, PCV valve the pcv valve spring the complete set of gaskets and o rings still has the problem can someone please help me that vacuum sucking pass the Rings or valves are seals wherever it's coming from makes a horrendous noise and my customer doesn't like it I solved the problem I drilled 2 holes in the oil cap so it could breathe the noise goes away it runs good. is this going to cause a problem in the future for my customers.

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I'm thinking it's a problem with the pcv valve. Did you make sure that there is an o ring on the bottom side of the pcv valve? It should be about 3/4 inch diameter.

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Drilling the holes in the oil cap will make the engine run lean and will probably set a lean code,(P0171), because unmetered air is entering the engine, i.e. air that has not gone thru the MAF sensor.


Drilling holes in the oil cap will have absolutely nothing to do with the air entering the combustion chamber. If you have a vacuum in your crankcase then you have an issue with the intake, either it's a cracked plenum or bad gasket. During the power stroke, every engine has what's called blow by, it's combustion/exhaust gases that pass by the rings causing the crankcase to have a small amount of positive air (pressure). The Pcv valve (positive crankcase ventilation valve) is a one way breather that shows this positive air to exit the crankcase and enter the intake to be re burnt. The reason your car runs like shit when you remove the cap is because the entire crankcase becomes part of the intake due to the vacuum leak and removing the cap exposes this leak to the atmosphere. So the car runs like it has s vacuum leak until the computer sees the problem and adjusts the injector pulse width. Check the intake vacuum while the cap is installed and then remove it while watching the vacuum gauge.

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