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

misterswizzo
25

Asked by misterswizzo 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

9 Answers

John Saffrahn
12,935

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.

TonytheWoodsman
755

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

misterswizzo
25

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

John Saffrahn
12,935

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.

Ineedapepsi
20

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.

Ineedapepsi
20

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

John Saffrahn
12,935

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

stephend103
30

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.

1 out of 1 people think this is helpful.
Ineedapepsi
20

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

2 out of 2 people think this is helpful.

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