496 has vacuum when removing oil cap?

mnm99

mnm99

Well-Known Member
Oct 2, 2015
2,447
Long Island
Boat Info
2004 340 SeaRay Sundancer
Engines
Twin 8.1 Merc
Hey guys, A friend of mine has a 496 and when he removes the oil cap he gets a large woosh of vacuum. Researching it looks like the PCV system is the problem. The PCV is built into the intake manifold. Does this sound like it would be the problem? Wondering if any of you had this problem and what was the fix. Thanks
 
Just spit balling here, as haven't worked on a boat PCV system before, This is ref to automotive type system.

Sounds like the PCV valve is stuck wide open or the hose that supplies the air into the block (usually on suction side of air filter) is plugged, not letting air in.
 
mnm99 said:
Hey guys, A friend of mine has a 496 and when he removes the oil cap he gets a large woosh of vacuum. Researching it looks like the PCV system is the problem. The PCV is built into the intake manifold. Does this sound like it would be the problem? Wondering if any of you had this problem and what was the fix. Thanks
Click to expand...

Sounds like the valve is bad. The valve should be located in a valve cover and the vacuum hose should be going to the carb base or manifold.
 
Check hoses and valves gotta be one of the two
 
PCV nipple is under the intake. Hoping it’s clogged if we take it off. There no check valves on this engine.
 
I gotta go with the first comment, posted by @Bill Curtis. In the usual form, the PCV valve is a spring-loaded check valve set so that high intake manifold vacuum (idle) will pull the valve almost closed, then, as you add throttle, the spring begins to drive it open so that there's a steady flow of air through the oil pan while underway. There has to be makeup air coming from somewhere; usually a line into a valve cover.

I had one in a straight-six acting the same way, and found the guts of the valve laying under the rocker arms once the valve cover was removed.
 
mnm99 said:
Hey guys, A friend of mine has a 496 and when he removes the oil cap he gets a large woosh of vacuum. Researching it looks like the PCV system is the problem. The PCV is built into the intake manifold. Does this sound like it would be the problem? Wondering if any of you had this problem and what was the fix. Thanks
Click to expand...
techmitch said:
Normally with the PCV valve or "nipple clogged you would have pressure at the oil cap not vacuum.
P. ositive
C. rankcase
V. entilation.
Click to expand...
Is this happening with the engine running?
 
Sounds like it would eventually blow a seal. Since it is no joke removing the manifold I would try a hack first by loosening the filler cap. If that's unacceptable I would remove the throttlebody and see what can be done there because that's where it vents to.

I've owned 2 8.1's and never had this problem or heard of it. There is talk of excessive oil usage because of this design.
 
He needs make sure air is getting sucked in or pressure out very important
 
techmitch said:
Normally with the PCV valve or "nipple clogged you would have pressure at the oil cap not vacuum.
P. ositive
C. rankcase
V. entilation.
Click to expand...

scoflaw said:
Sounds like it would eventually blow a seal. Since it is no joke removing the manifold I would try a hack first by loosening the filler cap. If that's unacceptable I would remove the throttlebody and see what can be done there because that's where it vents to.

I've owned 2 8.1's and never had this problem or heard of it. There is talk of excessive oil usage because of this design.
Click to expand...
Yes it happens when its running. When it's shut down and sits for a little it doesn't have the vacuum at the cap anymore. I went down yesterday and it's pretty bad. At a idle I can feel with my hand the vacuum pulling in the oil cap!. My friend said he installed that little muffler in the TB a while ago and not couldn't find it also. Don't know if that has anything to do with it.
Talk to me about the Throttle Body venting...Where does it vent there? There are a couple holes and one hose. I believe he took off the hose going to the TB to the rear of the manifold and there was Vacuum there also. Is that the line that is supposed to vent thru the Throttle Body?
 
First, having a vacuum on the engine is great. That says the engine is tight without excessive blowby. He has a good engine. Those of us that build high performance engines desire high internal vacuums as that is horsepower. We even add vacuum pumps to get suitable vacuum.
So, the GM 8.1 Vortec engine has an internal PCV system which is a tube integrated into the underside of the intake manifold. There are no valves or moving parts; just an orifice. That tube routes to the throttle body and the vacuum is modulated at the throttle body. If you have a vacuum then there is nothing wrong with this part of the PCV system.
However, there is a second part of the system where clean fresh air is introduced to circulate through the internal part of the engine so the gasses can be extracted and burned off through the PCV system. If that should get blocked then the possibility of a high vacuum could happen.
That little foam "muffler" is for the Idle Air Control and not the PCV.
 
ttmott said:
First, having a vacuum on the engine is great. That says the engine is tight without excessive blowby. He has a good engine. Those of us that build high performance engines desire high internal vacuums as that is horsepower. We even add vacuum pumps to get suitable vacuum.
So, the GM 8.1 Vortec engine has an internal PCV system which is a tube integrated into the underside of the intake manifold. There are no valves or moving parts; just an orifice. That tube routes to the throttle body and the vacuum is modulated at the throttle body. If you have a vacuum then there is nothing wrong with this part of the PCV system.
However, there is a second part of the system where clean fresh air is introduced to circulate through the internal part of the engine so the gasses can be extracted and burned off through the PCV system. If that should get blocked then the possibility of a high vacuum could happen.
That little foam "muffler" is for the Idle Air Control and not the PCV.
Click to expand...
Thanks. Well his other engine doesn't pull nearly as much vacuum as this one. Does that mean its bad? I have the same 8.1's in my boat and doesn't pull the vacuum like this one either. There not bad also. You mentioned..

"However, there is a second part of the system where clean fresh air is introduced to circulate through the internal part of the engine so the gasses can be extracted and burned off through the PCV system. If that should get blocked then the possibility of a high vacuum could happen."

Where is the second part of the system? How can we check it?
 
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