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Old 13-07-2020, 03:57   #1
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Blow by in a diesel engine. What is the cause and how to fix.

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?
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Old 13-07-2020, 04:27   #2
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Re: Blow by in a diesel engine. What is the cause and how to fix.

It depends on what you mean by "lots". Mine has a little pressure if you hold your hand over the open filler cap, but my mechanic firmly suggested doing nothing about it -- could be many years of faithful service ahead before it starts getting bad enough to need work. Since he talked me out of some significant business I figured he was telling me the truth.
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Old 13-07-2020, 04:36   #3
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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 Tillsbury View Post
It depends on what you mean by "lots".
Thanks for the reply.
So, it looks like it is puffing smoke out the filler cap. I know a little blow-by is normal but the question is "how much is too much" and can it be improved by those fuel and oil treatments.
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Old 13-07-2020, 05:23   #4
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Re: Blow by in a diesel engine. What is the cause and how to fix.

Is the blow by mostly when the motor is cold/not fully warmed up? If it mostly goes away when warmed up, you are starting to get more slop in the cylinders/pistons.
Could be carbon build up causing the rings to not seat well or possibly other damage from the over heating issue you had previously.
If you suspect carbon/cylinder glaze, could run a "decarbonizer" in the fuel to see if this helps. This would be the least invasive method to start with.
Would do a compression test to see which cylinders are low and have a baseline/measurable way to determine if the decarbonizer is doing anything.
If the blow by is fairly bad, would also suspect the crankcase oil needs to be changed. If it's sludgy, then may need to do a flush out from the pan side also.
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Old 13-07-2020, 05:24   #5
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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
and can it be improved by those fuel and oil treatments.
Sadly no.

I suspect the overheating issue may have caused the problem to either the piston or rings. You could try a compression test to see what the results are. But without stripping it all down I think your stuck. Can you live with it?

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Old 13-07-2020, 05:39   #6
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Re: Blow by in a diesel engine. What is the cause and how to fix.

First let’s clean the crankcase vent just to be sure it’s not partially clogged. All engines have blow by of course, just normally it goes out the vent unless it’s too much for the vent to handle, or the vent gets clogged.
High cylinder temps can glaze a cylinder wall, it’s possible that a high detergent additive in the oil could help, like sea foam, if it’s glazed it could help clean it off.

This is more of the won’t hurt is easy to do and isn’t expensive, so why not try than it’s a sure it will fix it kind of thing.

It’s also possible that as long as your not babying the engine that the rings will reseat.
The higher the pressures are in the cylinder, the more pressure against the cylinder walls from the rings, higher pressure of course helps them “wear in” or seat.
Glazing is harmful in that it elevates oil temp, dirties the oil up faster and increases ring wear, but it’s pretty much something you have to live with as the fix is so close to an overhaul, might as well overhaul.

If the oil in this engine turns black faster and blacker in total than the other engine, then that supports high blow by exists.
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Old 13-07-2020, 05:59   #7
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Re: Blow by in a diesel engine. What is the cause and how to fix.

As mentioned by Tillsbury, how much is a lot? Usually “puffing” is a sign of a slight piston scuff/seizure, and your history of overheat points in that direction.

Sometimes an overheated piston scuffs slightly in the area of the wrist pin boss, (as that’s where the most piston material is and thus the expansion is greatest), and this scuff drags some aluminum up over the piston rings, locking them in place. Once the rings can’t move they allow blow-by to occur and this blow-by pressure is what you see at the filler cap. It could be minor, or can continue to develop and grow worse.

It’s also possible that the overheat slightly warped the cylinder head, allow combustion pressure to leak past the head gasket into the crankcase and that’s what is causing the puffing.

I would suggest having a compression test done. This can help tell which cylinder has the problem.

The piston in the attached photo came from an engine that was “puffing” heavily.
It has suffered a “four corner scuff”, where the piston expanded too much and scuffed on all four “corners”, about 45 degrees off center from the wrist pin. The aluminum was drug up over the rings and stuck the rings in their grooves. With the rings stuck, blow by came down the sides of the piston and the engine was puffing heavily at the oil fill cap. The blow-by burned the oil off the cylinder wall and allowed further damage on the sides of the piston above the ring area.

The cause is uncertain, but it is known that the engine was almost new, under 10 hours, and the boat was being pushed hard to make a delivery deadline. It probably overheated, and the lack of break-in didn’t help matters.
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Old 13-07-2020, 06:03   #8
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Re: Blow by in a diesel engine. What is the cause and how to fix.

First things first.

https://www.cruisersforum.com/forums...le-154766.html
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Old 13-07-2020, 06:10   #9
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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 DougR View Post
As mentioned by Tillsbury, how much is a lot? Usually “puffing” is a sign of a slight piston scuff/seizure, and your history of overheat points in that direction.

Sometimes an overheated piston scuffs slightly in the area of the wrist pin boss, (as that’s where the most piston material is and thus the expansion is greatest), and this scuff drags some aluminum up over the piston rings, locking them in place. Once the rings can’t move they allow blow-by to occur and this blow-by pressure is what you see at the filler cap. It could be minor, or can continue to develop and grow worse.

It’s also possible that the overheat slightly warped the cylinder head, allow combustion pressure to leak past the head gasket into the crankcase and that’s what is causing the puffing.

I would suggest having a compression test done. This can help tell which cylinder has the problem.

The piston in the attached photo came from an engine that was “puffing” heavily.
It has suffered a “four corner scuff”, where the piston expanded too much and scuffed on all four “corners”, about 45 degrees off center from the wrist pin. The aluminum was drug up over the rings and stuck the rings in their grooves. With the rings stuck, blow by came down the sides of the piston and the engine was puffing heavily at the oil fill cap. The blow-by burned the oil off the cylinder wall and allowed further damage on the sides of the piston above the ring area.

The cause is uncertain, but it is known that the engine was almost new, under 10 hours, and the boat was being pushed hard to make a delivery deadline. It probably overheated, and the lack of break-in didn’t help matters.
You overheat / melt a piston from combustion chamber temps too high. That doesn’t correlate to what people call an overheated engine though, without a pyrometer you don’t know exhaust temps, just water temp and if the cooling system is in good shape the water won’t overheat.

Gas motors usually cook a piston like that from being too lean, Diesels just the opposite, the do so from being over fueled.

I’ve melted so many two stoke pistons that I lost count, the aluminum would smear over the rings, loss of compression, engine dies.
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Old 13-07-2020, 06:50   #10
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Re: Blow by in a diesel engine. What is the cause and how to fix.

a64 has it right. BTW - I used to work in a shop port mapping 2-strokes - fun times doing the carb jetting, start fat and lean down without getting a heat sieze.
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Old 13-07-2020, 11:54   #11
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Re: Blow by in a diesel engine. What is the cause and how to fix.

Do you give your engines a regular hard power run every few hours of at at least 80% power for five minutes? If you persistently run your engine(s) at low power, you will glase the bores and you will get blow by. It is more of a problem these days because owners insist having or putting larger engines in their boats and then because they don't need to run the engines hard to achieve a good cruising speed, they don't and glasing is the result. Never used to be a problem when boats had smaller engines. My 1980s built Tradewind 33 is an 11 tonnes boat and still has the original Bukh 20 hp engine but a lot of lighter 33 foot boats are sold with 30 or even 40 hp engines these days. It will give you more power to punch through short seas but when cruising under power in light seas, people forget to run the engine hard every few hours. Have al look in you engine handbook, it will be an advisory in there. It certainly is in Yanmar Manuals.
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Old 13-07-2020, 12:42   #12
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Re: Blow by in a diesel engine. What is the cause and how to fix.

If it was my engine, I would try some of the “overhaul in a can” products from the auto parts store. I found that adding a can of Restore at every oil change to my Kubota based 3-cyl Northern Lights generator cut oil consumption to near zero. I was adding a pint of oil every 5 or 6 hours and it dropped to a pint or two every 100 hours.

What do you have to lose?
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Old 13-07-2020, 13:29   #13
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Re: Blow by in a diesel engine. What is the cause and how to fix.

I personally would remove the valve cover. Check the small hole in the passage from valve cover to intake side which is the engine vent system. Spray carb cleaner works well for this. See at 11:29 on following Youtube video. May as well check your valve clearances while there.





Unvented crankcase pressure will either quickly wear out your crank seals or blow them out of the block. So while cleaning the vent system may not be your whole issue it will help prevent any more damage if you have blowby. If you still have pressure (if you have a manometer it should be near zero at idle and no more than about .25PSI at 2000RPM ) you could always try products like Seafoam "engine treatment" or Rislone "ringfree". Or you could remove the plug from the valve cover and run a separate engine vent with a hose looped above the top of the engine to some kind of catchment like an empty gallon jug. I had a guy that was extremely frugal, had massive blowby and would save the vented oil, filter it through a coffee filter and dump it back in the engine. Worked for about 5 years.



If it is indeed blowby and one of the engine treatments and running at full load doesn't do it you might be looking at a rebuild possibly with sleeves (read $$$$). Hard to say without looking at it though.
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Old 13-07-2020, 14:28   #14
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Re: Blow by in a diesel engine. What is the cause and how to fix.

If it has overheated, the piston rings may have lost their tension. If this has happened the engine needs to be stripped and the rings replaced.

If you have been using this engine and its alternator as the charger for the battery bank, running it for long hours at light load, you may have glazed up the bore. This needs catalyst in the fuel and running the engine under load.

If the cylinder head gasket has a leak into the oil galleries this too will cause blow by--but that has a characteristic sound and the engine will be difficult to start--so it is less likely but still a possibility. Blown gaskets quickly get worse as a rule.
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Old 13-07-2020, 15:24   #15
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Re: Blow by in a diesel engine. What is the cause and how to fix.

Once an engine is broken in, glazing in the manner you guys describe is exceedingly rare.
US Army conducted a study with support from most of the major engine manufacturers at the time to determine the extent of the damage from excessive high idling, meaning what it sounds like and engine under little to no load, because the Army does that extensively, with every engine being run for about an hour or so every week during motor pool time, and never being out under load.
The results of that study are out there, and I’ve found the once or twice, but the results were that except for stacking or Diesel slobbering, there was essentially no effect.
You can sometimes read a study and determine what they study was trying to find, I feel that the study was made to prove that excessive running at light loads was harmful and to drive the Army towards loading the engines up, and that’s not what was found.
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