Today I removed the oil fill cap on my engine while the engine was running. When the cap came off, there was a suction sound, and the engine started running rough. I put my hand over the oil fill hole, and there was definitely a vacuum there, it sucked my hand down. The engine ran rough the whole time the oil cap was off.

What could cause this? The car does burn oil, would this maybe mean the valve guides are shot and the intake stoke is causing a vacuum in the rocker arm cover area? The PCV valve was replaced not too long ago.

Anybody have any ideas as to what might be causing this? The car is a '95 SE with 155,000 miles on it. Thanks for any ideas.

Dave

Well, the PCV valve lets your engine suck out any pressure in the crankcase, so I am not surprised you had some suction there. But it sounds like maybe more than usual, and my guess is that the PCV valve could be either the wrong one or defective. I would replace it with the OEM (AC) part.

By the way it'* not primarily air pressure getting by the valve stems that creates the internal crankcase pressure. First of all, remember that down in the oil pan you have those pistons pumping up and down. And any combustion gases that get by the rings have to be relieved also.

I'll have to check the PCV valve when I get a chance to see if it seems to be working properly. Thanks.

Sounds like one of the o-rings was left out when the PCV valve was replaced.

I had the same problem with suction at the oil capon a 97 SE. The pcv valve has an o-ring on it that does not come with the replacement pcv. I got an o-ring at the hardware store and it cured the problem.

Good responses, and good things to check first. If those things are not the problem, then I'd check the "return air inlet." This is an equally important part of the positive crankcase ventilation system, and most people don't even know anything about a return air inlet. I had a situation one time (not on this particular engine design) where the return air circuit had gotten blocked off, which created severe vacuum in the crankcase.

BUT, I think you'll find your problem to be related to one of the previous responses, with one of the big clues being how the engine runs rough when you remove the cap. With all things in the PCV system operating correctly, there will not be a change in the engine smoothness when removing and replacing the cap with the engine running.

Also, the problem you have will DEFINITELY invite severe oil consumption.

Good Luck, and let us know what you find.

Well I checked the PCV valve, and it rattles when I shake it, so it'* working OK. It'* only about a year old, don't remember what brand it was, I think it might have been a Fram, or maybe an off brand. I think I bought it at Auto Zone.

Checked the large o ring in the PCV housing , and that'* in good shape. Is there more than one o ring in there?

Also, 57chevythunder mentioned a return air inlet. Where is that, what is it, and how do I check to see if it'* OK? Thanks for any info.

:? Well, this one may be pretty difficult to explain: Just a quick mention on the very basics of "positive crankcase ventilation." The PCV valve is the device that draws the fumes out of the crankcase, using engine manifold vacuum. The return air is just that: it is the fresh (filtered) atmospheric air that must be returned to the crankcase, replacing the same volume that the PCV valve is removing. (technically, the volume is a little less, due to cylinder "blow-by", but that is getting way more technical than necessary for our purposes.) As a side note, if atmospheric return air was not provided into the crankcase, then you would soon be pulling manifold vacuum in the crankcase, and introduce all sorts of bad issues.

Okay, you can see this fresh air return port as it goes through the throttle body gasket, by referring to the fourth photo posted by mkaake on June 19th at 7:02pm, on the topic of "Upper Intake Gasket" (page one of the responses). You can see that he is holding the plenum (on it'* side) such that you see the entire red silicone throttle body gasket. (note that you can see the EGR stove pipe on the right side of the phote as we view it.) Okay, the two sort of odd shaped ports adjacent to that stove pipe are the coolant ports to the throttle body base plate. Now, the perfectly round port (maybe 3/8" diameter), in about the one o'clock position of the photo as we view it, is where this "return air, or fresh air" passes from the throttle body into the plenum. The air, after entering the plenum through this port, makes a 90 degree turn, and goes a port in the bottom of the plenum, (again surrounded by red silicone gasket) into the lower intake manifold. Actually, if you refer to the second page of the same topic, and the last photo shows the port in the plenum gasket. (-at that point it is an oval shaped hole adjacent to the one round coolant port.

After it is in the lower manifold, I can't tell you it'* exact path, but it finds it'* way down into the lifter valley area. Wow, now does this sound like a real mess, or what.

Hey, I sure hope I don't "lead you down the proverbial garden path" here, and get you into someplace you don't want to go. I would hope that a good shop mechanic could help you investigate this return air path, but I'm betting that most of them are not even aware of it. Also, I certainly don't have a good feel for how likely or not that anything could be blocking those passages.

I did go out to the garage to my '99, and pulled off the air filter and box assembly, removed the throttle body inlet screen, and looked down into the throttle body to see where this passageway enters the throttle body. I found it, but again, hard to explain.

I almost wish I hadn't started trying to explain this one, and again I'm very sincere when I express my concern that I may have only made things more confusing.

Can someone else help out here ?? -maybe with some photos ?

Thanks for the explanation 57chevythunder, at least now I know more about the PCV and air intake system. Your reference to the photos was also very helpful. I replaced my upper intake 2-3 years ago because it failed, so the passages in that should be in good shape. I don't think I'll take it off again at this time to check things out though. Again, thanks for the great explanation!

-just had another thought: -isn't there two O-rings ? One large one on the twist-lock top cover assembly which also contains the MAP sensor, and a second smaller (orange) one down lower, around the PCV valve itself.

Is that one around the PCV valve missing ?? (-check for it down in the hole,,)

I still really doubt that your return air system is blocked. Have you had the plenum (upper manifold) off? or the throttle body ?