What would cause a deisel to lose power under load, but run fine in neutral

[steve yates]

Yanmar ygm30. It failed on me off skomer, which i put diwn to low fuel level and crud from bottom of tank.
10l of fuel added and new filters, it ran for 30 mins then started fading. Wasnt reaching full revs, now tied up in milford dock and gets full revs easily in neutral?Any ideas?

 

[Normalfornorfolk]

Sounds like fuel starvation to me. Enough fuel getting through to spin the engine over but not enough to sustain a load. Blow fuel lines through from filter to tank, check free fuel flow at filter. Recheck filter for cleanliness and ensure filter inlet drillings are clear. Sounds as though your tank may need a thorough clean out before you go any further too.

 

[philwebb]

You could have something on your prop! Otherwise did it show any signs of overheating? When you changed the primary filter was it full of gunk? Sometimes if you stir up debris from the bottom of the tank by going through rough water you might have to change your filter more than once.

 

[No Regrets]

Remember your engine won't draw anywhere near as much fuel in Neutral as under load!

As the post above suggests, probably fuel starvation. The tanks should be checked, cleaned and new filters fitted once run for a while. It sounds like the new filters are blocked, as the old ones didn't or couldn't accept the entire amount of crud in the tanks.

With regard to the crud in the tanks, sounds like The Bug. Once cleaned, the remaining fuel needs some Marine 16 (before replacing filters!!) adding to kill off, and then future doses to prevent. This is very very common... :ambivalence:

 

[jamie N]

Air getting into the fuel line near on the engine?

 

[Poignard]

The OP says it ran for 30 minutes before fading after he changed the filters and added clean fuel. If during that 30 minutes it was under load then the engine would seem to be ok and the problem lies with the fuel supply to the engine. Easy to prove by running the engine, under load, with clean fuel supplied direct from a container instead of the tank.

 

[johnlilley]

Fuel starvation is, as others say, possible however it is often overlooked that the exhaust can become blocked. This is often impossible to notice externally but the flexible pipe can delaminate internally and the layers act as a valve. Usually caused by overheating when the inlet water seacock is partially blocked with a short time of no exhaust water. The engine revs under load for a few seconds then drops down not quite stopping.

 

[alant]

Yanmar ygm30. It failed on me off skomer, which i put diwn to low fuel level and crud from bottom of tank.
10l of fuel added and new filters, it ran for 30 mins then started fading. Wasnt reaching full revs, now tied up in milford dock and gets full revs easily in neutral?Any ideas?

Have you checked the prop?

 

[savageseadog]

If there's loose material in the tank or pipework of any kind it will run OK on light load but starve the engine on heavy load. Look for paint flakes, rust, sand or bug goo.

 

[Just_sayin']

Have you checked the prop?

Have you bothered to read the thread ??? !!! ??

You could have something on your prop!

 

[steve yates]

Have you bothered to read the thread ??? !!! ??

Lol, the prop has been checked by a diver yesterday, just to eliminate any other potential issues before I left.

It occurs to me now that after removing the old filter, I should probably have opened the fuel cock and ran fuel through the pipe till it was coming out clean, to get rid of any crud in the line! Is that normal practise?

I just made it into Milford dock, with the revs dropping right down. In neutral straight away they were fine, and it ran ok again moving out of the dock and into the marina, after a short rest.
Could we'll be a chunk of crud in the line?

There is no diesel bug, we have checked in Scotland, but there is muck and rust at the bottom of the tank.
It's one of the scheduled jobs once in Essex. Take out the old original mild steel tank from 1974 and replace with a plastic one.

I'll try that tomorrow.

 

[steveeasy]

Is there a filter you have missed?. have the filters you have changed got new seals fitted. have the bleed screws got sealing washers fitted. all could draw air in to the system. perhaps fit a portable fuel tank to eliminate old crud.

Steveeasy

 

[alan]

My friend with a 3YM30 has had this problem over a couple of years. One time when I was with him, we had to get a tow into Rome as we were convinced that we had something on the prop! Next day a diver went down (brave man to dive in the River Tiber) and could find nothing wrong. This has happened three or four times since; the engine begins to judder and almost stall; then in neutral you can achieve maximum revs. Put it into gear and it is OK at tick-over but judders when you increase revs. After leaving the boat for some hours (e.g. overnight) all returns to normal.

To cut a long story short, I am convinced that he has gunk in his tank and this blocks the gauze filter on the end of the fuel pick-up in his tank. I have now persuaded him to get a mechanic to remove his tank, and have it, drained, steam cleaned and then clean fuel put in!!

I too was surprised that you could achieve max revs in neutral but as soon as there was any sort of load applied, the boat would try and shake itself to pieces!!!

Sounds to me that you should look at your fuel tank and make sure there is no gunk in the bottom - this may be harder to do than you think if your boat has a tank with no inspection hatch.

Good luck.

Alan.

 

[alant]

Have you bothered to read the thread ??? !!! ??

Why, the OP didn't mention checking it.
Had simiilar problem recently about 500 miles from Azores, couldn't achieve normal revs & when finally in Horta, couldn't even see prop tips because of huge mop of polyprop around it.

 

[steve yates]

The gauze filter at the start of the fuel line! That could well be a very good point, when we drained and tried to clean the tank as best we could originally, it was well clogged up!

 

[Poignard]

The gauze filter at the start of the fuel line! That could well be a very good point, when we drained and tried to clean the tank as best we could originally, it was well clogged up!

I've had the same symptoms with rust flakes in an outboard motor tank. Idled ok but open the throttle and it died.

 

[ShinyShoe]

There is no diesel bug, we have checked in Scotland,

Steve - you know you aren't in Scotland anymore...

 

[Just_sayin']

Why, the OP didn't mention checking it.
.

But post #3 asked the same question as you hence my question.

Following your logic there should be countless replies suggesting checking the prop.

I wonder if the OP has checked the prop.

 

[steve yates]

Steve - you know you aren't in Scotland anymore...

True, but if it didn't form when sitting idle for 4 years, with water in the diesel, I wouldn't expect it to appear now having been used and a reasonable supply of fresh diesel going in ( with biocide)
But, good point, just because I wouldn't expect it, doesn't mean anything.

 

[PaulRainbow]

As several others have said, most likely fuel starvation. Unlikely to be drawing air, or you'd get a misfire, rather than power loss.As parsifal suggested earlier, set up a bypass using some hoes and a jerry can. Connect it after the primary filter, if that works, try it before the primary filter. That will tell you if the filter is blocked or the tank filter is blocked (assuming it has one)

Don't run it too long or the return system will empty the can into the tank. If you route the return to the can it'll run for as long as 20 litres of fuel lasts. If the tank is full of shit some cans of fuel might prove to be a more reliable way of getting the boat back to Essex.