Suction when oil fill cap removed??? - Page 2 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 08-11-2004, 01:44 PM   #11
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Is this suction even a problem or malfunction?Unless it'* really excessive it would seem normal to me due to the PCV system.Taking off the oil filler cap introduces a big vacuum leak into the system.I've seen cars that would shake bad and just about stall with the cap off and running.A buddy of mine who used to work on BMW'* said that just pulling the oil dipstick on some bmw'* would cause them to run bad .It may not be a problem and just normal operation.And thank you 57chevythunder for the PCV system info.
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Old 08-11-2004, 02:01 PM   #12
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I just checked the PCV valve again. There is only one o ring in the PCV housing. It'* a large orange colored one that looks like it seals the twist lock cap. There is no o ring around the PCV valve or lower in the housing.

Does this mean I should get an o ring that fits around the body of the PCV valve and put that in there too? Thanks.

Oh, I didn't see any sensor (MAP?) attached to the PCV housing either.
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Old 08-11-2004, 02:04 PM   #13
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To answer 57chevythunder'* question, Yes, I did replace the upper intake (plenum) 2-3 years ago as I stated in a previous post. So I'm sure the passages in that are clear.
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Old 08-11-2004, 04:47 PM   #14
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Yes, there should be an O-ring around the PCV valve body itself. -the larger diameter part of the valve, just under the flange. As you push the valve down into it'* hole, that is what makes the seal between the vacuum side and the vapor side of the valve. (-you may want to shine a light down into the hole with the valve out, and see if that is where it is hiding)

Looks like you have found your problem.

Also, apparently the MAP sensor lives somewhere else on your engine. (-mine is a '99, and I think your'* is a '95? I suspect there are a few minor changes with about every year, on some of that stuff)
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Old 08-11-2004, 09:00 PM   #15
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-did just a touch more research on the PCV system: the air flow path (not vacuum), after it drops down into the lower manifold, finds it'* way into the crankcase lifter valley area through the left cylinder head. And the draw from the PCV valve is through the complementary hole in the right cylinder head.

It is sort of easy to see this hole, by studying a lower manifold gasket. After you account for all of the bolt holes, you will find that there is one extra hole, fairly near one of the intake ports. That is the pcv vapor, and return air passageway.

By the way, you should never have any vacuum in the crankcase. If you do, something is wrong.
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Old 08-12-2004, 11:41 AM   #16
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I bought an o ring to fit over the PCV valve, put it back into the engine, started it, pulled the oil fill cap off and......... no more suction from the valve cover, and the engine still ran smooth. Great find guys, I appreciate all the help!!

I don't remember why I even pulled the oil cap off originally when the engine was running, just screwing around I guess. Maybe it'* a good thing I did, otherwise I never would have known about that missing o ring.

I'll have to keep an eye on my oil consumption now. The car is burning oil, and every now and then when I start the car there'* a big cloud of smoke behind the car. Maybe now it won't do that and burn so much oil. Thanks again!
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Old 08-12-2004, 04:31 PM   #17
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And Thank You Very Much for the followup ! It is always good when people do this, as it "closes the loop" so to speak.

And speaking for myself, I have also found this thread to be nicely educational.

Now we know for sure, that the "over-aggressive" suction on the valve side of the system was "over-powering" the ability of the fresh air side to supply return air.
(I noticed that fresh air port was pretty small, and now I know for sure that it is only large enough to work when everything else is right)

Yes, I also hope that your oil consumption will diminish.
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Old 09-01-2004, 10:29 PM   #18
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Default What size is the smaller PCV O-ring??

Apparently I am missing the smaller one.

About what size is it?? Does it fit on the end with the nipple(installs in the TOP position), or the other end(the one that installs in the DOWN position)

This definitely explains the oil consumption Ive had!!!!!

Can I get a replacement at GM parts direct, or can I go to the local NAPA???

Thanks in advance!!!

Bob
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Old 09-01-2004, 10:43 PM   #19
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Go to NAPA or a hardware store. It fits on the "down" side of the flange. I think it'* around 9/16". Remove your PCV when you're in the store and fit it if no one comes up with a size.
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Old 03-18-2016, 11:28 PM   #20
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Default 3.8 excessive engine vacuum.

I replaced the PCV valve with genuine AC Delco valve and O-ring. Had the same problem. Bad luck I guess. Replaced the PCV Valve again with one in a Dorman kit. Cured the vacuum problem. Be sure the replacement valve rattles when shaken.
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