Whale blow pressure release from oil cap!

[Rodek]

Had a recent engine rebuild. When I remove the oil cap to check the oil, there's a significant release of pressure (sounds like a truck's air brakes). The car didn't run the best prior to rebuild but it never had this symptom before. Is this normal? If not.... what could be the issue?

[Tea Tray]

Sounds like a plumbing issue when the motor went back in.
The scavenge side of the oil pump moves oil and a lot of air to the tank. The large hose that goes from the oil tank to the triangle-of-death hose fitting provides makeup air to the crankcase.

If you have a pressurized oil tank then something is plugged or plumbed incorrectly.
Disclaimer: I’m not familiar w/ differences between SC and Carrera.

[Uwon]

+1
I would not run the moor until investigated further.
M2C
Johan

[stownsen914]

The tank should be vented to atmosphere (or to the airbox). Sounds like that's either plugged or mis-plumbed.

[boyt911sc]

Originally Posted by stownsen914

The tank should be vented to atmosphere (or to the airbox). Sounds like that's either plugged or mis-plumbed.

The SC’s oil tanks are never vented to atmosphere or hooked up to the airbox. Go double check your set-up. The restrictor inside the vacuum line going to the oil tank could be missing. Something is not right.

Tony

[stownsen914]

The oil tank has to be vented somewhere, no? My bad if it's not atmosphere or the airbox. My point is to check the vent.

[Tom_in_NH]

On my 83 SC, the upper port of the oil tank filler neck was routed, by the factory, to the CIS sensor plate/throttle body boot. Picture below shows the 90 degree fitting (circled in red) that hose was connected to.

[Rodek]

Thanks for the info so far! I checked and the restrictor is in the oil tank hose... mounted in the hose by the oil tank. There's a hose connected to the back of the breather. I'm not certain if the flame restrictor is mounted in that 90 degree fitting or not. Just for the sake of it, if it is...what else could be causing the issue? Maybe something airbox related?

[stownsen914]

Confirmed that the hose (and where it attaches to the tank and airbox) is unobstructed and not plugged? Especially when service has been done, sometimes people will put a plug or stuff a rag in openings, and forget to remove them. Make sure air can actually pass through that hose.

[john walker's workshop]

Might want to do a leakdown.

[acme911]

Take it back to the shop that did the work.

[porschyard]

There should be vacuum present at the oil fill tube when the cap is removed with the engine running. That is the reason the idle drops and the fuel mixture goes lean when you remove the cap on a closed crankcase ventilation system because you are introducing unmetered air (vacuum leak) If there is actually positive pressure release when you remove the cap then i would check the vent hose from the top of the oil fill tube to the CIS air boot for blockage.

Richard

[boyt911sc]

Originally Posted by john walker's workshop

Might want to do a leakdown.

Excessive blow by gas passing through the cylinders could be the culprit(?). A leakdown test as suggested above could help find the answer.

Tony

[Trackrash]

Excessive pressure in the crankcase can cause rings to not seat. AHIK

[Rodek]

Ok. Starting from the easiest to more difficult what I've gathered is I need to check the vent hose that goes from the top of the oil fill tube to the back of the air box for a possible blockage. Out of curiosity, could there be a possible blockage somewhere else or is there an oil tank pressure relief valve that could be faulty? Next, if those areas check out I should conduct a leak down test.

Here's some additional info if this helps:

The car starts and idles fine. The oil pressure gauge doesn't register abnormally high pressure as compare to pre-engine rebuild pressure. Slight oil leak appears to be near right lower valve cover. (We have ordered turbo valve covers to replace the stock SC covers).

[Tea Tray]

I’m pretty confident your pressurized oil tank issue is due to a rag left in the return air hose that goes from the oil tank to the triangle of death crankcase “vent” hose fitting.

Let’s say you left your car sitting for a couple weeks and oil gradually leaked through the pressure side pump and left a quart of oil in the sump.
You start the engine and the scavenge pump does it’s job by pumping oil back upstairs at about 1.7 times the volume of what the pressure pump draws from the tank.
In a short bit of time, the scavenge pump goes from an oil pump to an air/oil pump. That’s the nature of dry sump engines.
The blocked air return hose restricts air flow resulting in the oil tank becoming pressurized.

[Rodek]

Originally Posted by Tea Tray

I’m pretty confident your pressurized oil tank issue is due to a rag left in the return air hose that goes from the oil tank to the triangle of death crankcase “vent” hose fitting.

Let’s say you left your car sitting for a couple weeks and oil gradually leaked through the pressure side pump and left a quart of oil in the sump.
You start the engine and the scavenge pump does it’s job by pumping oil back upstairs at about 1.7 times the volume of what the pressure pump draws from the tank.
In a short bit of time, the scavenge pump goes from an oil pump to an air/oil pump. That’s the nature of dry sump engines.
The blocked air return hose restricts air flow resulting in the oil tank becoming pressurized.

Thank you for the excellent drawing and explanation. This may sound a bit gross....So, I took the breather hose off at the oil tank. I can see the brass restrictor in there. It's clear. I cleaned the hose off and tried to physically blow air through the hose with no luck. I then tried to suck air through the hose and no so luck again. If the hose was NOT restricted by potential offending rag, would I be able to blow air through it?

[Tea Tray]

Yes, you should be able to blow through the restrictor equipped hose. The cover/fitting adapter on the engine is straight through.

[porschyard]

Originally Posted by Tea Tray

I’m pretty confident your pressurized oil tank issue is due to a rag left in the return air hose that goes from the oil tank to the triangle of death crankcase “vent” hose fitting.

Let’s say you left your car sitting for a couple weeks and oil gradually leaked through the pressure side pump and left a quart of oil in the sump.
You start the engine and the scavenge pump does it’s job by pumping oil back upstairs at about 1.7 times the volume of what the pressure pump draws from the tank.
In a short bit of time, the scavenge pump goes from an oil pump to an air/oil pump. That’s the nature of dry sump engines.
The blocked air return hose restricts air flow resulting in the oil tank becoming pressurized.

Nice diagram but your air flow direction is not correct and you left off the breather hose from the top of the oil tank to the cis intake boot. Crankcase air flows from the case vent port to the oil tank in the opposite direction that you depict in your diagram and the vented air(blow by) is fed back in to the intake on closed loop systems via the hose from the top of the oil fill pipe to the cis boot (which is missing in your diagram) and burned with the intake air.

As i mentioned earlier IMO the breather hose from tank "vent" to cis boot is blocked

Richard

[stownsen914]

Sounds like you've got a vent problem. You're onto something.