Boost coming up through oil filler cap??

[OleH]

I pressure tested the piping from the turbo all the to the intake, to check for boost leaks. At approximately 1,5 bar i heard that the pressure came up through the engine oil filler cap.

Is this normal?

 

[Slunch]

There may be a simple explanation, the finer points of the breather system are lost on me, but I'd consider doing a cylinder compression test.

 

[OleH]

There may be a simple explanation, the finer points of the breather system are lost on me, but I'd consider doing a cylinder compression test.

My thoughts exactly, but I did a compression test last August with OK results.

 

[Slunch]

I'm speculating and others may put me right. The air could be pushing past the valve stem seals on open valves. I'd guess these don't usually see a lot of pressure. Air may be creeping past the rings or through the ring gaps and you're just pumping the system up. However I would expect it would get out through the breather system rather than your oil cap. Have you modified the breather system? There is a valve just next to the oil cap, I replaced this once cause it got jammed, not sure it's essential as the head also connects to the oil condenser at the front and intake pipe/ catch can if you have added one. If you're testing for boost leaks it's not much of a faff to do a compression test while you're at it. For peace of mind.

 

[PobodY]

I pressure tested the piping from the turbo all the to the intake, to check for boost leaks. At approximately 1,5 bar i heard that the pressure came up through the engine oil filler cap.

Is this normal?

How/where did you test it?

Obviously the pipework before the turbo (but after the AFM) is connected to the rocker cover, so if you pressurise that it will get to the oil filler.

If you have the PCV fitted still (that little valve near the filler cap), I suppose there's a possibility that it's letting air back past it and in to the rocker cover. - I thought that was after the throttle bodies, so they'd need to be leaking (although that's not unlikely) in addition to the PCV not functioning (since it's supposed to pull the crank case emissions out under manifold vacuum).

I'm not a mechanic, but if air was leaking past the valves wouldn't that mean you have no compression?

 

[OleH]

How/where did you test it?

Obviously the pipework before the turbo (but after the AFM) is connected to the rocker cover, so if you pressurise that it will get to the oil filler.

If you have the PCV fitted still (that little valve near the filler cap), I suppose there's a possibility that it's letting air back past it and in to the rocker cover. - I thought that was after the throttle bodies, so they'd need to be leaking (although that's not unlikely) in addition to the PCV not functioning (since it's supposed to pull the crank case emissions out under manifold vacuum).

I'm not a mechanic, but if air was leaking past the valves wouldn't that mean you have no compression?

This is how I did it:

1. Warmed up the engine to normal temperature
2. When this was achieved, I removed the fuse for the fuel pump to remove the fuel pressure
3. Removed the spark plugs
4. Removed the coil spark plug wire (in lack of a better description)
5. Inserted the pressure gauge and hose
6. Fully opened the throttle
7. Let the starter turn 4-5times
8. Read the pressure

 

[Slunch]

I think Pobody meant how did you do the boost leak test. If you pressurised from the air intake on a standard set up you may be pressuring the rocker cover, crankcase and breather system, the only way out would be through the oil filler. If pressurising post turbo it may indicate a obstructed breather system. Bad news as you may see unacceptable crank case pressure when driving,

 

[PobodY]

^ Yes, that's what I meant.

I'd guess the simple answer is that the PCV is knackered.

 

[Slunch]

Display excessive ignorance what is a pcv?

 

[OleH]

I think Pobody meant how did you do the boost leak test. If you pressurised from the air intake on a standard set up you may be pressuring the rocker cover, crankcase and breather system, the only way out would be through the oil filler. If pressurising post turbo it may indicate a obstructed breather system. Bad news as you may see unacceptable crank case pressure when driving,

Sorry, I misread his post :doh:

This is how I check for vacuum leaks:
1. Remove the air filter pipe from the inlet on the turbo
2. Insert a plug with a tire valve, in to the turbo inlet
3. Pressurise the system with an air compressor
4. Listen for leaks

Hope this made any sense ;-)

 

[OleH]

^ Yes, that's what I meant.

I'd guess the simple answer is that the PCV is knackered.

I'll have a look at the PCV tomorrow, thanks

 

[PobodY]

Display excessive ignorance what is a pcv?

Positive Crankcase Ventilation. - It's essentially a one-way valve that's supposed to allow crankcase vapour to be pulled through the throttle bodies under vacuum (and prevent pressurising of the crankcase under boost).

Technically the system comprises of two parts: the valve that allows vapour to be pulled out; and the breather than allows air to be pulled in to replace it.
So really I should have said "PCV valve"

 

[Slunch]

Ok. Thanks Pobody, so what happens to blow by gases when the engines not running on vacuum? Out through the breather system and recirculated through the boost charge to gunge up the intercooler and plenum etc? Guess that's why a catch can is a good idea, along with the fact that blow by has lower oxygen content- probably a bit warm as well.

I would have thought that the biggest issue with a stuffed PCV valve is that some metered air get dumped to atmosphere/ intake, through the breather, and you run a bit rich. Easy solution, if you're that fussed about having a bit of extra fuel available when jamming 1.5 bar down your engines neck- plug it up. Saves all that gunge getting into the throttles and IAC valve as well. Does anyone plug the PCV check valve?

You're probably correct that what OleH is hearing is air bleeding through the PCV check valve, but that's not what he seems to be saying. He seems to be saying that he can hear air forcing out around his oil filler cap. If that is the case he's pressurised his rocker cover and, presumably his crank case, which would indicate that his breather system is not open to atmosphere. That could be a serious problem when he starts driving it- cue blown oil seals.

While we're on it what's a leak down test, why do you do one at TDC, why are valve stem seals designed to leak, and, hopefully, where does the leaking air on a leak down test go?

I'm no mechanic either.

 

[OleH]

Ok. Thanks Pobody, so what happens to blow by gases when the engines not running on vacuum? Out through the breather system and recirculated through the boost charge to gunge up the intercooler and plenum etc? Guess that's why a catch can is a good idea, along with the fact that blow by has lower oxygen content- probably a bit warm as well.

I would have thought that the biggest issue with a stuffed PCV valve is that some metered air get dumped to atmosphere/ intake, through the breather, and you run a bit rich. Easy solution, if you're that fussed about having a bit of extra fuel available when jamming 1.5 bar down your engines neck- plug it up. Saves all that gunge getting into the throttles and IAC valve as well. Does anyone plug the PCV check valve?

You're probably correct that what OleH is hearing is air bleeding through the PCV check valve, but that's not what he seems to be saying. He seems to be saying that he can hear air forcing out around his oil filler cap. If that is the case he's pressurised his rocker cover and, presumably his crank case, which would indicate that his breather system is not open to atmosphere. That could be a serious problem when he starts driving it- cue blown oil seals.

While we're on it what's a leak down test, why do you do one at TDC, why are valve stem seals designed to leak, and, hopefully, where does the leaking air on a leak down test go?

I'm no mechanic either.

You are right, I first noticed it by seeing small bubbles around the oil filler cap because there was a little bit of water around there. Then I opened the oil filler cap and I could hear the air coming out the oil filler hole.

As mentioned I will check the PCV valve as soon as I get time. I have also been thinking about fitting an oil catch can :-D

 

[Fast Guy]

While we're on it what's a leak down test, why do you do one at TDC, why are valve stem seals designed to leak, and, hopefully, where does the leaking air on a leak down test go?

I'm no mechanic either.

You do it at TDC on the compression stroke so that both inlet and exhaust valves are closed then in theory the cylinder will pressurise if it's not leaking too much. The leaking air goes where ever the leak is. ie Out the inlet, out the exhaust, into the cooling system or into the crankcase.

 

[Slunch]

Thanks Fast Guy, would I be right in saying the piston rings are always going to leak, it's just a matter of how much. And if the cylinder is compressed through the open inlet valves, spark plug in, you'll get air passing the valve stem seals?

 

[Fast Guy]

A leak down test removes the spark plug and pressurises the cylinder with both valves closed.

I'd imagine the rings would leak but there should be limits (I don't know what they are though) as to how much over how long. Valve stem seals shouldn't be seeing air with a leak down test - they will with a home made inlet system pressurisation test.

 

[Slunch]

Thanks again, I understand that about Leak down tests. I'm just trying to figure out why OleH is pushing air around his oil filler cap. If it was just the PCV valve leaking, which is around an 8mm inlet, with an obstruction in it, then I would of thought the 10mm ish breather system on the rocker would bleed it away without a noticeable increase in pressure- unless it was pretty badly blocked. If air is pouring in round the valve stems, and creeping past the piston rings as well then that may explain it. Valve stem seals are just a push on fitment aren't they?

What are your thoughts about plugging the PCV valve? I've almost talked myself into removing mine.

 

[Auswald]

I'm new

Sorry, I misread his post :doh:

This is how I check for vacuum leaks:
1. Remove the air filter pipe from the inlet on the turbo
2. Insert a plug with a tire valve, in to the turbo inlet
3. Pressurise the system with an air compressor
4. Listen for leaks

Hope this made any sense ;-)

I'm new to this site, so hi! My background is BMW master tech
and I have my own garage and I'm an Aussie
m
can I ask what the problem was for you to do the vacuum leak test?
I would be more inclined to do a smoke test for that?

 

[OleH]

I'm new to this site, so hi! My background is BMW master tech
and I have my own garage and I'm an Aussie
m
can I ask what the problem was for you to do the vacuum leak test?
I would be more inclined to do a smoke test for that?

No issues really, engine is running OK, but when I pressurised the system the air is coming up the hole for filling oil on the engine. I'm just wondering if this is normal :?