Crank Case Pressure?

I have a remanned 7.4 MPI long block. It was originally a 1999 Mercruiser replaced by a Jasper long block. Last weekend I did an oil change at the Jasper mandated 10 hour mark. I noticed during the process that the oil was down a quart or two. I can't say for sure how much was originally put in by the shop. I also noticed that when running at idle there was a noticeable vacuum on the oil cap. It seemed excessive to me as there was a pretty good pull on the cap making it hard to remove at idle. Anybody have any thoughts?

Re: Crank Case Pressure?

Not sure what configuation you have, but there should be no vacuum or pressure in your crankcase. Example, PCV on one valve cover, open breather on the other. Call the guy you bought it from. My experience of big blocks, and I have had several. yes they do use oil.

Re: Crank Case Pressure?

buckeyebus said:

I also noticed that when running at idle there was a noticeable vacuum on the oil cap. It seemed excessive to me as there was a pretty good pull on the cap making it hard to remove at idle. Anybody have any thoughts?

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Yep, sounds like someone did not replace/install the breather tub going to the flame arrestor and/or no breather tube and possibly bad or no PCV valve installed in the other valve cover where the vacuum hose/line is attached.

Bottom line, on one valve cover you should have an open tube going up to the flame arrestor and on the other valve cover you should have a PCV valve with a tube/hose going to a manifold vacuum source.

If no PCV, then you should have at least the one vent tube

And yes, a malfunctioning or improper PCV system can "suck" oil from an engine

Re: Crank Case Pressure?

I noticed during the process that the oil was down a quart or two. I can't say for sure how much was originally put in by the shop.

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Ayuh,.. Don't you Check it,..?? Every time you fire it,..?? Pre-trip inspection 'n all,..??

Re: Crank Case Pressure?

It has the breather tube and PCV valve. The PCV valve sounds free when I remove it and shake it. Someone mentioned that the intake gasket for a carbed motor is different then the one for MPI and that if the carbed one was used on reassembly it could cause the negative pressure in the crank case. I haven't gotten a hold af the mechanic yet. I'm not sure how helpful he'll be as it was replaced on the previous owner's dime.

The motor otherwise runs fine and I only noticed the vacuum by dumb luck.

While I try to check everything fairly often I don't do it every trip. Thanks for the responses so far.

Re: Crank Case Pressure?

Fairly often I don't think is going to be enough if you want your motor to last. A quart or two is quite a bit. It's normal for a new motor to use a little bit of oil during the break in period, but you don't want to let it run dry. Might want to check it more often.

Re: Crank Case Pressure?

OF COURSE there's vacuum on an engine with a PCV valve. That's how the system works..draws air in a valve cover, thru the PCV valve, into the intake manifold.

In your case, the vacuum also indicates you have good engine sealing...acceptable blowby past the rings.

BTW, oil dipsticks don't wear out from usage......hint hint.

Re: Crank Case Pressure?

Aloysius said:

OF COURSE there's vacuum on an engine with a PCV valve. That's how the system works..draws air in a valve cover, thru the PCV valve, into the intake manifold.

In your case, the vacuum also indicates you have good engine sealing...acceptable blowby past the rings.

BTW, oil dipsticks don't wear out from usage......hint hint.

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Aaaaa, PCV doesn't stand for Positive Crankcase Vacume... Sure the valve and vacuum source are there to "pull" from the crankcase but not to create an internal "vacuum environment." The "valve" actually prevents that as does the other breather tube...

Buckey,
Check to ensure the "breather tube" going to the spark arrestor is clear - as in you should be able to blow air through into the valve cover etc.

If the breather tube is clear, all of this "vacuum" you have (for whatever reason), should be very noticeable through the breather tube. IE - the breather tube should be venting the vacuum vise having a hard to remove oil cap etc..

With engine running/idling pull the PCV valve out of the valve cover and check it's operation. And if it is the valve causing all the vacuum your should be able to tell when you remove it.. Or you will find that once removed the valve grommet/hole in the valve cover will be sucking air as well.

This should not be too hard to figure out. And the possible intake gasket mix up could be valid.

Good luck,

Cheers

Re: Crank Case Pressure?

I won't be back to the boat until Friday morning. I'll check everyone's suggestions then and report back. Thanks for all the helpful responses.

KJ, One thing I did notice is that if I pull the PCV valve whil the engine is running it does cause the idle to jump a few hundred RPM's like an intake leak would?

Re: Crank Case Pressure?

buckeyebus said:

I won't be back to the boat until Friday morning. I'll check everyone's suggestions then and report back. Thanks for all the helpful responses.

KJ, One thing I did notice is that if I pull the PCV valve while the engine is running it does cause the idle to jump a few hundred RPM's like an intake leak would?

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Hmmm,

With a breather tube - IE open crankcase, I wouldn't think you would get a bump in rpm by removing the PCV valve from the valve cover. And I am assuming you mean you pulled the valve out of the grommet yet left the vacuum line attached to the valve correct?

When back at the boat, with engine idling - filler cap etc all on, remove the PCV valve from the valve cover (leave the PCV valve attached to the vacuum hose) and then plug the grommet/hole with your thumb. If you have suction at the grommet (and engine rpm drops to normal), your "vacuum source/leak" is definitely somewhere else - IE intake manifold gasket or other install issue etc.

Also check the breather tube for proper "open" air flow to/into the other valve cover.


Good luck

Cheers

Re: Crank Case Pressure?

Will do. Thanks!

Re: Crank Case Pressure?

PCV is Positive Crankcase Ventilation..uses manifold vacuum to draw air thru the engine. If there isn't a vacuum at idle, you gotta problem. Frankly, that's how you test the system..should be drawing slight vacuum on the crankcase.

The PCV valve has a metered orifice to only draw a certain amount of air through the engine.

This "unmetered air" affects idle speed and mixture. Plug the PCV valve and the engine won't run without readjusting stuff. If you remove the PCV valve from the valve cover and the engine speeds up, it says the idle mixture is a little rich, which is fine.