Insane crankcase vacuum

HELP! I have a 2001 GMC Express van 3500 series 8.1 litre vortec. We need this van. We have a disabled son and work with youth ministries.

Some time ago (14 months) while the vehicle was running I noticed a slight vacuum when I removed the oil filler cap. I was told at the time this crank case vacuum was part of the engine design for these vehicles. Several days ago my van began to show an engine fault light and was idling high. At that time I once again removed the oil filler cap but this time the vacuum was tremendous! It makes my Hoover central vac look like a play toy! I have no idea if this crank case vacuum increase was slow or instantaneous. I also do not know if it is related to my high idle. My first concern is this crank case vacuum, I can have the faults diagnosed. I have been told by the dealer my engine has no P.C.V. valve. What are your thoughts? What is causing this insane crank case vacuum?

P.S. my van is not a Savana, the drop downs made me choose that

Crank cases don’t develop vacuum, they develop pressure from blowby. The fumes then waft up the oil return passages and are drawn back into the engine from under the valvecover via a tube and an orafice in the throttle body assembly. That level of vacuum under the valvecover makes me wonder of the oil return passages are getting plugged up.

thank you for your reply and correction. I assumed the fill tube simply dumped into the crankcase because my oil filler cap is mounted in the front of the engine housing. When I removed the engine cowling I can see no hoses or caps on the valve covers (however there is a lot of stuff crammed in there and I may have missed something)

Thank you for your reply and correction. I assumed the fill tube simply dumped into the crankcase because my oil filler cap is mounted in the front of the engine housing. When I removed the engine cowling I can see no hoses or caps on the valve covers (however there is a lot of stuff crammed in there and I may have missed something). One mechanic felt I possibly have a cracked at the bottom of my intake manifold. He suggested this possibly might draw air up from the crankcase. Following your line of thought I can tell you my oil pressure gauge went from good to great when I removed the oil filler cap and allowed the vacuum pressure to neutralize. (it was something I observed) when the air was unrestricted and allowed to enter the oil filler cap my oil pressure increased about 40%. However the volume of air going into the filling tube at that time was crazy. I am wondering if the oil pump could actually displace that much volume.
Again I welcome your insights and feed back
Sincerely, Winton

Okay, so what are the operational symptoms and what are the five digit OBDII codes?

For the high crankcase vacuum, check for a hose from one one of the valve covers that goes to the intake air inlet tube, somewhere between the air filter and the throttle. Make sure that this hose isn’t collapsed or clogged.

The oil pump neither adds pressure to the crankcase or rakes pressure away from your crankcase. The volume of oil remains the same, it simply gets circulated. Having said that, an extremely high vacuum in the crankcase could make it difficult for the pump to remove oil via the pickup tube, so your oil pressure spike when you removed the fill cap makes sense.

Check the breather hose. It should prevent excessive vacuum fro building in the PCV system. I’ve attached a link to an explanation of the system.

Has the PCV valve and system been checked? Is the 8.1 litre really the right engine size?

Hard to check something that isn’t there. (Not all engines have PCV valves.)

This issue that tardis brings up is a very important one. We as advice giving members must “bone up” a bit on the different types of crankcase ventilation systems being used today. Myself, I have no idea what technique is used in this engine but I will take a look, hope I don’t reach the limit of my source for model specific technical information.

Thanks everyone for the thoughts and advice. I will confirm that YES I do have a strong vaccum in my crankcase. Yesterday I pulled the dipstick while the engine was running, I could both hear and feel the suction through the dipstick tube! I have confimed the engine size and type.
Two days ago I spoke with a fellow who is renowned for mechanical knowledge. Without hesitation he stated that my engine was designed to have crankcase vacuum, although mine is excessively excessive. Today I will be looking for a collapsed hose near my intake. Will update everyone soon. Once again a big thanks to everyone

I have to ask, did “Mr Renowned” tell you what was wrong with your engine? or were you limited to one question? (I know how these guys can get). What is the idea behind “a collapsed hose near the intake” being the culprit? Can you describe the crankcase ventilation technique used on your vehicle (along with the components used?)

If the air filter is badly clogged vacuum could accumulate at the crankcase vent tube?

You raise a good point with “Mr. Renowned”, and after the fact I wish I had asked him more details. Unfortunately at the moment we were very pressed for time. However by nature I like understanding or comprehending things rather than just cookie cutter answers. I can say this gentleman I asked is not loud or proud. He does have a very meek disposition and does not come across as a know it all.
As for my air filter, coincidently I replaced it shortly before I discovered this problem, I am going to do a couple simple test shortly and see if anything develops.

i see nobody understands what you are saying, i do
i have a 98 2500 with a 5.7 doing the same thing thats why i ended up here.
this is way above normal vacuum. the pcv valve on engines draws tha fumes out.
this vacuum is drawing into the crankcase extremely hard it will pull the oil cap to the tube from an inch away that is not from a stuck pcv.
i have not found it yet but it is like the intake is pulling throu the crankcase i thinking it may be a cracked intake or a bad intake gasket. i was hoping to have some simpler ideas come up but i can see nobody seems to underdstand how strong this vacuum is.
and mine also idles very hi i am running about 15 to 1800 and then dropps when the oil cover is removed.
i was reading bad mass air low sensor and bad 02 i changed tha mass air flow because i had one here. all four codes went away the vacuum and high idle still there. and the engine is a little rough (small vibration) like a plug missfire but they are all running fine.
but i do know what you are saying this is excessive i have all chevy’s my 305 does not act like this at all.
i will keep checking.

this what i found somewhere else. i hoping for an easier answer but this is what i was expecting to find out. i will look for any other ideas

The PCV valve e could have been the problem but since you already changed it is not likely, other than that the intake gaskets are about the only thing that would cause that problem, leaking on the inside of the engine.

Expert: zzdaryl
Pos. Feedback: 99.6 %
Accepts: 2505
Answered: 8/29/2009
Chevy Technician 26 Years of working in a multi model garage

Intake manifold gasket failures have causeg this in numerous “V” engines from nearly every manufacturer over the years…burning oil and stinky engine oil can result as well.

Hi Cookyz,
it’s been months since your last posting. and unfortunately my hi idle has not been resolved. Between a full life and low bank account this problem still exists. While I have had a lot of postings, yours was the only one that hit the bull?s-eye. Can you tell me if you ever found a definitive answer to your high idle problem. I would love to finally get my engine running properly.
Signed Winton. P.S. here is our email address if that proves to be more helpful

Happy to see the intake manifold gasket mentioned. Old Pontiac V-8 engines didn’t have that problem because you could see under the intake manifold. Most others could have a problem. Chevy could have a leak.

I have this issue as well and believe the dealer sold it to me this way as a brand new van. There is pretty much no vacuum in my intake manifold but a ton of vacuum under the oil cap. My diagnostic system isnt picking it up either though hg drops to zero. This can not be a intake manifold gasket because the vacuum would pull from the ambient air not crankcase. I don’t think it’s valve cover gasket either because the same ambient reason. This has to do with a bad piston rings or something in the piston area or internal. Getting a mechanic to to go that far is very hard even under warranty. Some of these answer are ignorance and alot of mechanics like that game. I have had my pcv valves tampered with and the pcv internal valves were removed to allow vacuum into the intake from crankcase.

I can see no way for rings or pistons to generate a vacuum in the crankcase. Please explain.

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