My son has a 1995 SL1 with 113,000 miles. It runs great but does have a small amount of white smoke [only when under acceleration] and uses about a quart of oil every 1,000 miles. It was using even more before someone suggested that we change to a PCV with a fixed diameter hole rather than the OEM one. The compression is 185 -190 across all cylinders.

The other day he came by the house and said it was a little low when he had checked the oil that morning so I figured I would top it off for him. The Engine was idling at the time and I was surprised to see that I couldn't add oil with the engine running. The air coming out of the oil cap opening was so strong that it just blew the oil right back up and all over me and the engine compartment. The baffles beneath the opening are intact or it would probably have been much worse.

No problem on turning the engine off before adding oil of course but it did start me to thinking about what one should expect in terms of pressure or vacuum there.

My daughter has a Volvo and the entire PCV system was recently replaced. Before it was replaced there was positive pressure at the oil opening and after replacement there was a slight vacuum. I know they are very different engines with very different PCV implementations but I would have thought that there being either pressure or vacuum inside the engine compartment would be essentially the same.

My question is: "What should one see at the oil cap -- pressure or vacuum? ...and how much of either?"

Anyone?

 

It should be a slight vacuum. Who ever told you to replace the PCV with a fixed diameter hole gave you bad advice. You should go back to the PCV valve and see if you get the vacuum that you should see.

If you still get positive pressure, then you have a lot of blow by. A compression test does not rule out bad rings. In fact, I think your compression indicates a lot of carbon build up inside the combustion chambers.

Instead of checking for pressure at the oil fill cap, instead check at the fresh air hose. It goes between the valve cover to the duct between the air filter and the throttle body. Unplug the hose and check for a slight vacuum there on the valve cover side.

 

Take the time and go to the new member area and give yourself a proper introduction. While your at it open the USER CP go to signature and add your first name, location, and car info.

 

@Keith: Thanks. I'll change the PCV and test for a vacuum at the fresh air hose. Do you know of any effective additives for clearing out carbon buildup in the combustion chamber?

Btw, the suggestion for the fixed replacement PCV is all over the santurnfans forum.

@sw2cam: Looking at my post, it already has first name and car. I'll add the location.

 

The PCV valve is a good suggestion. Put it back to OEM configuration. Screwing around like that can back you into other problems.

As to vacuum or pressure at the add oil fill is open for question with the variable being when engine and how old and how many miles on it. I would not use that as a diagnostic.

If the pressure, which is essentially cause by leakage past the rings is gonna very for a lot of reasons I would not even hazard a guess as what you would find. Why do I say that? 50 years of shade tree mechanicing and diagnosing and repairing my motors or building them, what goes on there is not a valid diagnostic
There may be an indicator of something going on but I would not be worried about any of the 7 cars I owned if I could not add oil with the engine running, which is why I always shut the engines off to add oil.

Is it the rings? Could be. But 180 - 190 #s us an indication there is no problem.
What do I do to diagnose ring problems? The old stand by is to run a compression check and record the value and then squirt some motor oil through a spark plug hole and re-measure. If the rings are good than you will pretty much repeat the test with pretty much the same readings.
If the rings are bad? Usually indicated by a low compression number which you do not have! The new reading with the oil added through the spark plug hole will read higher.
How much? Dunno and don't care, higher means ring problems providing original reading low and yours arn't.
I don't buy carbon build up one teeny weeny bit. From the numbers posted it sounds good to me.
As to motor oil used, the heaviest you can find and run in it. A 10W30 or a 10W40 is better than adequate at 113k miles. your mean winter temperature is the key to the weight of oil used. if it gets real cold for long periods of time a 10W30 and then change back to a 10W40 in the spring.
The lowest winter temperature I will see where I live is generally about 40 degrees, so I use a 20W50 year around
I personally would never use a 5W20 ever.
Mostly because I would not live in an area that is below zero most of the year.