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Old 13-07-2020, 15:35   #16
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Re: Blow by in a diesel engine. What is the cause and how to fix.

Some times, the blowby is caused by blown head gasket, ou more precisely a beguining of slight crack in this gasket. Things will get worst with time. The best way to findout, is to remove the valve cover, and run the engine: a valve bay be leaking, or the head gasket starting to loose it. It is not a big deal.
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Old 13-07-2020, 16:07   #17
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Re: Blow by in a diesel engine. What is the cause and how to fix.

I’m trying to think of how a head gasket could cause blow by, and coming up with a blank.
Other than valve guides, there is no connection possible between the combustion chamber and the crankcase?
Oil galley maybe? But wouldn’t that burn oil like mad as it got blown in on the intake stroke?
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Old 13-07-2020, 16:12   #18
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Re: Blow by in a diesel engine. What is the cause and how to fix.

Quote:
Originally Posted by a64pilot View Post
I’m trying to think of how a head gasket could cause blow by, and coming up with a blank.
Other than valve guides, there is no connection possible between the combustion chamber and the crankcase?
Oil galley maybe? But wouldn’t that burn oil like mad as it got blown in on the intake stroke?

Totally agree, it's not the head gasket.
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Old 15-07-2020, 11:05   #19
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Re: Blow by in a diesel engine. What is the cause and how to fix.

If the head gasket is blown, and the exhaust gases will pass trough the valves pushrods, it will create an back pressure in the crankcase. It hapen long time ago when my previous boat's engine a Volvo MD17HD head gasket developped a leak and failed. In fact a blown head gasket may create many strange thing like mixing water wiith oil, blowing cooling water through the exhaust, and many other difficult to diagnostic problems.
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Old 24-12-2020, 08:43   #20
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Re: Blow by in a diesel engine. What is the cause and how to fix.

What if the problem is opposite, or just slightly more complicated?

Yesterday, while briefly operating at higher than normal RPM, we had big clouds of oil smoke out the exhaust and loss of oil out the dipstick tube. Otherwise no symptoms.

Back at the dock I checked for excessive blow-by by removing the hose from the breather vent. I found it was clear and I found almost NO PRESSURE on the breather vent outlet even at 3000RPM.

Should my Yanmar 3JH with 6500 hours have some amount of blowby, especially considering the symptoms exhibited yesterday?
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Old 24-12-2020, 08:53   #21
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Re: Blow by in a diesel engine. What is the cause and how to fix.

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Originally Posted by wingssail View Post
What if the problem is opposite, or just slightly more complicated?

Yesterday, while briefly operating at higher than normal RPM, we had big clouds of oil smoke out the exhaust and loss of oil out the dipstick tube. Otherwise no symptoms.

Back at the dock I checked for excessive blow-by by removing the hose from the breather vent. I found it was clear and I found almost NO PRESSURE on the breather vent outlet even at 3000RPM.

Should my Yanmar 3JH with 6500 hours have some amount of blowby, especially considering the symptoms exhibited yesterday?
That sounds like a clogged breather baffle in the valve cover. There is a screen under the valve cover that stops gack from clogging up the breather hose from valve cover to air cleaner. Typically if your dipstick pukes only at high rpm it's something in the crankcase vent system.

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Old 24-12-2020, 09:52   #22
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Re: Blow by in a diesel engine. What is the cause and how to fix.

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Originally Posted by a64pilot View Post
I’m trying to think of how a head gasket could cause blow by, and coming up with a blank.
Other than valve guides, there is no connection possible between the combustion chamber and the crankcase?
Oil galley maybe? But wouldn’t that burn oil like mad as it got blown in on the intake stroke?
A leaking head gasket could easily pressurize a pushrod cavity or a cam chain cavity along side of or between cylinders, thereby causing excessive base pressure pressure and leakage out of the dipstick or crankcase breather.
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Old 24-12-2020, 10:11   #23
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Re: Blow by in a diesel engine. What is the cause and how to fix.

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Originally Posted by wingssail View Post
What if the problem is opposite, or just slightly more complicated?

Yesterday, while briefly operating at higher than normal RPM, we had big clouds of oil smoke out the exhaust and loss of oil out the dipstick tube. Otherwise no symptoms.

Back at the dock I checked for excessive blow-by by removing the hose from the breather vent. I found it was clear and I found almost NO PRESSURE on the breather vent outlet even at 3000RPM.

Should my Yanmar 3JH with 6500 hours have some amount of blowby, especially considering the symptoms exhibited yesterday?
Yes, at 6500 hours there will be a degree of wear, and there will be some blow by.

When you ran the 3000 rpm test at the dock was the engine in neutral or in gear? If in neutral, there is almost no fuel being used to spin the engine up to that rpm, therefore very little combustion pressure to cause blow by. If the engine was in gear it would have required much more fuel to reach 3000 rpm, ( hence combustion pressure) and consequently more blow by.

I like to check blow-by with the oil fill cap removed instead of pulling off the breather tube. That eliminates the possibility of a restricted breather system.
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Old 24-12-2020, 11:37   #24
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Re: Blow by in a diesel engine. What is the cause and how to fix.

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Originally Posted by Cliffhanger View Post
One of the engines on my cat has developed a problem. (Volvo Penta MD2020.) Lots of fumes coming out of the filler cap and/or dipstick when open. But otherwise the engine runs perfectly. No smoke out of the exhaust, no lack of power, idles and runs smoothly. It has done about 1500 hrs and has overheated a couple of times - a problem that is now fixed. So, I'm not really sure what is wrong. I am guessing blow-by and perhaps carbon build-up on the compression rings but not sure.

So, any ideas what the problem is and what is the best way to fix it?
Take off the valve cover and clean out the screen and passages that is for the crank case vent system. The crank vents back into the intake manifold via the valve cover. The screen is in the center of the valve cover.

Best way to test rings (for blowby) is to do a cylinder leak down test. Most people have no idea how to do this and it requires an air source and a leak down tester that will work on your engine.

Best way to measure crank case pressure is with a manometer on the oil filler cap or on the breather hose, breather hose being the most common place to tap the system. The MD 2020 has the breather hose plugged off and vents crankcase pressure internally through the valve cover.

Yes a head gasket can cause excessive blowby but it's rare. More often the cooling system is effected but if you loose the compression ring seal on the head gasket and compression goes into an oil return or pushrod hole you will have ton of blowby. Very hard to imagine this on a MD2020.
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