Engine bogs down under load 2K rpm max. - Page 2 - SailNet Community
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post #11 of 26 Old 11-20-2017
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Re: Engine bogs down under load 2K rpm max.

You state that the engine revs up just fine in neutral but not so when the transmission is engaged.
I suggest that you disconnect the shaft coupling behind the transmission (This is only 4 bolts) and slide the shaft and coupling aft about 1/2". Then you run the engine in neutral and in gear.
If it revs up fine in gear with the shaft disconnected then the problem will be in the shaft or cutless bearing, etc.
If the engine still boggs down, I suspect that you are looking at replacing or rebuilding the transmission.
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post #12 of 26 Old 11-20-2017
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Re: Engine bogs down under load 2K rpm max.

There might be a block in the injection system including the injectors. Make sure your fuel tank is cleaned and all ther system is checked.
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post #13 of 26 Old 11-20-2017
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Re: Engine bogs down under load 2K rpm max.

Quote:
Originally Posted by NCboatrx View Post
You state that the engine revs up just fine in neutral but not so when the transmission is engaged.
I suggest that you disconnect the shaft coupling behind the transmission (This is only 4 bolts) and slide the shaft and coupling aft about 1/2". Then you run the engine in neutral and in gear.
If it revs up fine in gear with the shaft disconnected then the problem will be in the shaft or cutless bearing, etc.
If the engine still boggs down, I suspect that you are looking at replacing or rebuilding the transmission.
.... or you will be soon replacing a head gasket. With no load on the engine the combustion chamber pressure isn't sufficiently high enough to sufficiently leak through the blown head gasket. But when under load, the much higher developed combustion chamber pressure is causing a greater leak volume through the broken gasket. If there is a such a head gasket leak, invariably it will between cylinders; and usually, the broken gasket is 'behaving' like the valve on a 'whoopee cushion'.

However, such is a much far rarer possibility than all the other possibilities offered.

Last edited by RichH; 11-20-2017 at 12:07 PM.
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post #14 of 26 Old 11-22-2017
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Re: Engine bogs down under load 2K rpm max.

Just wondering how the OP made out?

For all of the people suggesting a blocked elbow, how would that account for the excess black smoke in the exhaust?

The OP has stated that the injectors have been bench tested and are fine so they shouldn't be an issue. I would think that when re-installing the injectors and re-priming the fuel system any fuel delivery problems would be apparent. Given that the black smoke is there, fuel starvation would be contra-indicated.

One cylinder not firing seems to make sense given it can rev fine but bogs under load, so lack of compression is more likely isn't it? Excess diesel gets burned in the exhaust producing black smoke.

The diver ruled out prop and shaft issues.
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post #15 of 26 Old 11-22-2017
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Re: Engine bogs down under load 2K rpm max.

NF-
There are lots of alleged diesel mechanics but actually finding one who understands the entire engine and how to fix it, that's not easy. If you ask around for a month you're probably going to finally get a good recommendation...and find the guy is booked up for another month. So, the long list that Rich posted, and "the relentless application of logic", might be a faster fix.
It is pretty damned hard to wear out the diesel engine in a recreational sailboat. Easy to screw them up with smoke issues and other problems, but if all else fails, you might try to find a non-marine diesel shop, and see if someone would make a boat call. Long distance truckers routinely put a million miles on a diesel engine, so someone out there certainly knows how to keep them happy.
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post #16 of 26 Old 11-22-2017
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Re: Engine bogs down under load 2K rpm max.

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Originally Posted by CelticSailr View Post
For all of the people suggesting a blocked elbow, how would that account for the excess black smoke in the exhaust?
What happens with a blocked injection elbow is that the elbow is only partly to nearly fully blocked. That partial blockage when significant enough increase the (back) pressure in the combustion chambers: The back pressure doesnt allow full rpm (HP) to develop and the governor increases the fuel flow to the combustion chamber in order to meet the rpm demand of the 'throttle'; but cant'; hence, the over-fueling of the combustion chambers and 'black smoke' exiting through the already narrowed / blocked injection elbow.

The engine is now 'lugging' - the combustion pressure is now excessive, the piston connection rods and piston wrist pins are under extreme strain, and possibly even the main bearings are near to crashing into the crankshaft journals, and the possibility of blowing a head gasket is quite high.
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post #17 of 26 Old 11-22-2017
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Re: Engine bogs down under load 2K rpm max.

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Originally Posted by RichH View Post
What happens with a blocked injection elbow is that the elbow is only partly to nearly fully blocked. That partial blockage when significant enough increase the (back) pressure in the combustion chambers: The back pressure doesnt allow full rpm (HP) to develop and the governor increases the fuel flow to the combustion chamber in order to meet the rpm demand of the 'throttle'; but cant'; hence, the over-fueling of the combustion chambers and 'black smoke' exiting through the already narrowed / blocked injection elbow.

The engine is now 'lugging' - the combustion pressure is now excessive, the piston connection rods and piston wrist pins are under extreme strain, and possibly even the main bearings are near to crashing into the crankshaft journals, and the possibility of blowing a head gasket is quite high.
I've always thought of back pressure in terms of exhaust manifold and pipe lengths. Surely a misfiring cylinder can't cause excess back pressure, rather the opposite wouldn't it?
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post #18 of 26 Old 11-22-2017
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Re: Engine bogs down under load 2K rpm max.

If the exhaust gases are severely restricted from exiting - back pressure !!!!!!

I like your celtic knot.
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post #19 of 26 Old 11-22-2017
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Re: Engine bogs down under load 2K rpm max.

Quote:
Originally Posted by RichH View Post
If the exhaust gases are severely restricted from exiting - back pressure !!!!!!

I like your celtic knot.
Agreed, that I understand, I was referring to blocked injection elbow and others referring to blocks in the injection system.



The OP stated the exhaust system was new, surely the exhaust elbow would have been new as well?

Thanks
Like I indicated not that familiar with diesels but pretty competent in other engines.

How does the governor effect a rise in rail pressure? I get how it works in an over rev situation to decrease fuel pressure.

Last edited by CelticSailr; 11-22-2017 at 09:01 PM.
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post #20 of 26 Old 11-22-2017
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Re: Engine bogs down under load 2K rpm max.

Quote:
Originally Posted by RichH View Post
If the exhaust gases are severely restricted from exiting - back pressure !!!!!!

I like your celtic knot.
Thanks! It was a quick google search on my part.
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