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Blow-by but good compression?

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Syzygy

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Joined: Sat Nov 20, 2004 2:38 pm

Location: Newcastle

Post Tue Sep 01, 2015 8:02 pm

Blow-by but good compression?

A little bit baffled by what's up with my 5L hilux motor.

It's been running a turbo for around 5 years with no dramas. This year I've noticed that the rocker cover vent hose has been venting a hazy oily smoke that wasn't there previously. The gas seems to pulse out of the vent hose at around 1 litre per second at idle. It's quite a flow. If blocked the pressure will build.

I did a compression test on it today expecting to find a dodgy cylinder, but results of the compression test look to be reasonable.
Cylinders from the front to back returned 570psi, 560, 540 and 550.

Is there enough variation here to be concerned?

I guess the gas has to be coming from the cylinders... But why did the compression test not show an issue?
1986 2L-T Bundera
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DamTriton

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Post Wed Sep 02, 2015 6:56 am

Re: Blow-by but good compression?

Syzygy wrote:A little bit baffled by what's up with my 5L hilux motor.

It's been running a turbo for around 5 years with no dramas. This year I've noticed that the rocker cover vent hose has been venting a hazy oily smoke that wasn't there previously. The gas seems to pulse out of the vent hose at around 1 litre per second at idle. It's quite a flow. If blocked the pressure will build.

I did a compression test on it today expecting to find a dodgy cylinder, but results of the compression test look to be reasonable.
Cylinders from the front to back returned 570psi, 560, 540 and 550.

Is there enough variation here to be concerned?

I guess the gas has to be coming from the cylinders... But why did the compression test not show an issue?


My guess would be the compression test was done under atmospheric conditions. When in use the engine is operating under increased manifold pressure from the turbo = increased cylinder pressure. I would say your 540 psi cyl may be the culprit.
George Carlin, an American Comedian said; "Think of how stupid the average person is, and realise that half of them are stupider than that".
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GUtripper

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Location: Melbourne

Post Wed Sep 02, 2015 11:35 am

Re: Blow-by but good compression?

DamTriton wrote:
Syzygy wrote:A little bit baffled by what's up with my 5L hilux motor.

It's been running a turbo for around 5 years with no dramas. This year I've noticed that the rocker cover vent hose has been venting a hazy oily smoke that wasn't there previously. The gas seems to pulse out of the vent hose at around 1 litre per second at idle. It's quite a flow. If blocked the pressure will build.

I did a compression test on it today expecting to find a dodgy cylinder, but results of the compression test look to be reasonable.
Cylinders from the front to back returned 570psi, 560, 540 and 550.

Is there enough variation here to be concerned?

I guess the gas has to be coming from the cylinders... But why did the compression test not show an issue?


My guess would be the compression test was done under atmospheric conditions. When in use the engine is operating under increased manifold pressure from the turbo = increased cylinder pressure. I would say your 540 psi cyl may be the culprit.


Well yeah, but.....
Its still blowing at idle, ie under no boost.

Pop the dipstick out, does the venting increase or decrease at all?
Is there a single vent hose out of the rocker cover, or two? Is there a PCV installed upside down perhaps?
Hows the engine temp? Is coolant disappearing?

Normally a cracked ring and lower compression would still be evident at idle.
My friends aren't holding me back..... I'm pulling 'em forward.
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mkpatrol

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Joined: Tue Jan 13, 2004 12:09 pm

Post Wed Sep 02, 2015 1:48 pm

Re: Blow-by but good compression?

Every engine has a certain amount of blowby, some more than others but they can still be OK. By design the rings cant hold it all in hence the positive crankcase venting system.

How much oil is it using?
Is it coating the inlet tube to the point where it is dripping oil?

If its negligible to those two questions I wouldn't worry about it.

General rule of thumb for a compression test variation is 10%, your readings are well within that so I wouldn't stress.

My Pathfinder has had blowby since new, coats the inlet wall but not excessively and uses fark all oil. Now has 150k on the clock with no issues.
Don't ask me, ask them. I'm just runnin for my life myself.
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No they ain't, I'm just in front...............
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DamTriton

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Location: Drownsville if not Brownsville

Post Wed Sep 02, 2015 2:02 pm

Re: Blow-by but good compression?

GUtripper wrote:
DamTriton wrote:
Syzygy wrote:A little bit baffled by what's up with my 5L hilux motor.

It's been running a turbo for around 5 years with no dramas. This year I've noticed that the rocker cover vent hose has been venting a hazy oily smoke that wasn't there previously. The gas seems to pulse out of the vent hose at around 1 litre per second at idle. It's quite a flow. If blocked the pressure will build.

I did a compression test on it today expecting to find a dodgy cylinder, but results of the compression test look to be reasonable.
Cylinders from the front to back returned 570psi, 560, 540 and 550.

Is there enough variation here to be concerned?

I guess the gas has to be coming from the cylinders... But why did the compression test not show an issue?


My guess would be the compression test was done under atmospheric conditions. When in use the engine is operating under increased manifold pressure from the turbo = increased cylinder pressure. I would say your 540 psi cyl may be the culprit.


Well yeah, but.....
Its still blowing at idle, ie under no boost.

Pop the dipstick out, does the venting increase or decrease at all?
Is there a single vent hose out of the rocker cover, or two? Is there a PCV installed upside down perhaps?
Hows the engine temp? Is coolant disappearing?

Normally a cracked ring and lower compression would still be evident at idle.


There's a difference between a cylinder under compression only (compression test) versus compression and ignition.
George Carlin, an American Comedian said; "Think of how stupid the average person is, and realise that half of them are stupider than that".
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Syzygy

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Posts: 96

Joined: Sat Nov 20, 2004 2:38 pm

Location: Newcastle

Post Wed Sep 02, 2015 10:03 pm

Re: Blow-by but good compression?

To answer the questions, just a single vent hose to atmosphere. No PVC etc. the oil use seems in check. I'll have a look with the dip stick out tomorrow but it's a relatively small diameter opening. Can't see it effecting things greatly.

It does get a very small amount of oil in the coolant suggesting a small head issue. Has done this for a while. No temp or other issues though. Doesn't seem to lose coolant.

After sleeping on it, the gas has to be coming from blowby. Even though it's evident at idle the compression test did not show a major issue. One concern is that all of my results are well above new figures suggesting a faulty gauge. There is no way to know how far out the gauge is so I may look for another.

The car runs and drives well, sounds fine and doesn't use much oil so it's not likely to be an immediately serious problem. I'd just like to get on top if things early if it's an issue that is going to cause serious problems down the track.
1986 2L-T Bundera
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mkpatrol

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Post Thu Sep 03, 2015 8:40 am

Re: Blow-by but good compression?

I wouldn't immediately discount the gauge. The values may be up because the oil rings may be worn (and potentially causing the blowby). Did you do a wet test? You didn't say originally that these reading were over specification. How far over are they?

One of the best ways to check a diesel engine for cylinder leakage is to do a cylinder leakage test with a cylinder leakage test gauge (obviously :P). This will show you exactly where the cylinder is leaking from and by how much. IE, if its a valve you will hear the air leaking either in or our of the manifolds. If its the crankcase then you will hear it in the PCV or the oil cap. If it is the crank case then you will get a gauge of the percentage of leakage past the rings. That will give you the most accurate idea of where you are at.

Oil in the coolant wont produce blowby, blowing a head gasket between the cylinder and an oil port will but then you will be losing compression hand over foot and have a miss. There will be so many other symptoms going on.

Having said all of that.......I still don't think there is an issue if its not using oil.
Don't ask me, ask them. I'm just runnin for my life myself.
Well they are all following you...
No they ain't, I'm just in front...............
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love ke70

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Joined: Fri Jun 29, 2007 7:57 pm

Location: yatala, halfway between brisbane and GC

Post Wed Oct 07, 2015 9:03 pm

Re: Blow-by but good compression?

I would say, oil in coolant, and blow by, and boost for years = motor on the way

Mine first time was a cracked piston

Then I had issues loosing coolant for no reason and ended up being a liner low of the top of the bore (Thanks UMR engines) Under both of these conditions nothing was apparent upon standard testing procedures...
97 GQ patrol coilcab. TD42, safari turbo kit with fiddled turbo, D-GAS kit. dyno results to come...
4inch lift, king springs, efs and procomp shocks
315/70R16 cooper ST's
found fuel economy...

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