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

[greeny]

Don't forget the tank air vent. Mine was a sintered filter on a previous boat and it blocked up with salty crud over the years. This caused a vacumn in the tank restricting ability for fuel to flow out. Engine revved fine in neutral but wouldn't rev on load when more fuel flow was required. Clean with wd 40 spray and a blast with a dinghy footpump in situ worked for me. Worth checking before you get too far in, by loosening the filler cap and testing again.

 

[Mistroma]

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.

I had this problem many years ago and thought it was fuel starvation. However I couldn't find any problems with the fuel supply. I thought about it for while and realised that an exhaust blockage would also explain the problem, but only if it was a transient issue. Fortunately, there was an easily removable short straight section leading to a stainless muffler. I found a delaminated section inside, water and gas was causing it to flip open and block the exhaust. The engine would run at tick over for ever and would develop some power before suddenly stopping. It would restart immediately and run fine at tick over again with full revs. available in neutral. I seem to remember that the problem developed fairly quickly with the engine showing power loss for a few hours before becoming much worse.

I fixed the problem by reversing the hose until I could get some new hose. OPs problem doesn't match exactly but partial delamination would probably give similar symptoms. Worth checking once fuel feed checks have been completed.

 

[No Regrets]

If it ran fine for 30 minutes after changing the filters, it's most likely to be blocked filters.

You mention crud in the bottom of the tank. Thats almost certainly the issue.

Can't think of anything else which would cause the problem from your description.....:ambivalence:

 

[steve yates]

Yup, saw a card advertising fuel tank cleaning here, he will go have a look during the week and see if there is sufficient access to do it properly.

 

[C08]

I hope you get it sorted-last thing you want on a dark windless night around Lands End is an unreliable motor!

 

[No Regrets]

I hope you get it sorted-last thing you want on a dark windless night around Lands End is an unreliable motor!

Best check the VHF batteries just in case

 

[BelleSerene]

Clutch slipping? Probably not as likely as some of the above, but if you’ve had s rope around your prop before, it can happen.

 

[lpdsn]

Another vote for fuel starvation. I had problems on and off for years. I fitted a vacuum gauge (a very worthwhile low-cost project that I can highly recommend, I wouldn't be without one now).

I eventually solved the problem by using a small, cheap electric compressor (the type for blowing up airbeds) to blow through the pipe between the tank and the primary filter. I wedged an old coke bottle full of kitchen towel on the end of the pipe to catch the gunk that came through.

I suspect the problem was more a combination of bio-diesel in copper pipes and wax dropping out of the diesel rather than bug (there were some wax deposits in the tank). If your fuel stood for four years the same might've happened in your fuel system.

The vacuum gauge means I've got baseline figures for a good system with a clean filter so I will know if the problem starts to build up again.

 

[Solent sailer]

Dont forget many systems have a small wire final filter in the banjo fitting by the fuel pump, we had a simular issue engine would start then run very rough and you couldn't rev it, after a few minutes it seemed ok. Finaly traced to crud in this inline filter, would have never looked but for a reading about someone else's derv problems.

 

[richardsboat]

I had a similar problem with my Yanmar. I first changed the filters and that improved but did not entirely fix the symptoms. I then decided to drain and cleaned out the tank. When re-fitting the tank I noticed a piece of plastic swarf (presumably from the drillings in the tank for the fittings) lodged inside the tube which sucks up the diesel. Removed the swarf, refitted everything and things ran fine from there. Buy yes if the engine runs fine on idle or low revs and struggles under load it sounds like fuel starvation. The best thing to do is go methodically through from the engine side, through filters, lines and to the tank to be sure there are no obstructions anywhere in the system.

 

[AndyL]

Check your tank breather isn't blocked.

 

[greeny]

Check your tank breather isn't blocked.

I assume he already has done - post 21.

 

[KellysEye]

>Air getting into the fuel line near on the engine

It could well be. Put kitchen roll around all joints, filter bowls and bleed screw. As air is sucked in diesel leaks out, so run the engine for at least 20 minutes at 1,000 revs then check for diesel in the kitchen roll.

 

[PaulRainbow]

>Air getting into the fuel line near on the engine

It could well be. Put kitchen roll around all joints, filter bowls and bleed screw. As air is sucked in diesel leaks out, so run the engine for at least 20 minutes at 1,000 revs then check for diesel in the kitchen roll.

Very unusual for air to get in near the engine. A leak after the fuel pump is usually that, a leak. To draw air in the leak usually has to be before the pump, where the fuel is being sucked towards the pump.

Putting kitchen roll around joints etc will obviously find some leaks, but in the case of very small leaks that allow air into the system it doesn't always work. A small leak can allow air in, as the suction is great enough to pull air in, but there might not be any fuel leaking out when the engine is stopped, as there is no pressure to force the fuel out.

A source of air getting into the fuel system that is often overlooked is the spill (return) pipes from the injectors. The rubber versions of these (often having a cloth like covering) sometimes weep a tiny amount, barely visible (the kitchen roll test works here), when the engine is stopped the fuel runs back in the system and air is drawn in through the leak. This will cause starting problems. The engine will be harder to start and when it does it will invariably run erratically and smokey, pretty much like it would after running out of fuel or having a filter change. Once the air is purged the smoke will clear and it will run fine.

However, having said all of that, it's not likely that the OPs problem is air in the fuel system. Air in the fuel doesn't cause a loss of power under load unless it is accompanied by erratic running or misfiring. It's likely to run more erratic at low engine speeds, rather than smoothly.

 

[Roger_D]

We had similar symptoms.. Thought it was air getting in. Checked everything. Turned out to be the lift pump. Good luck.

 

[steve yates]

I thought the lift pump was just to manually push fuel through the system to expel any air?
What exactly was wrong with yours?

 

[PaulRainbow]

I thought the lift pump was just to manually push fuel through the system to expel any air?
What exactly was wrong with yours?

The lift pump delivers fuel, at low pressure, to the injector pump (with the engines we are talking about). The injector pump then delivers fuel to the injectors, at high pressure, at the correct time.

Nothing to do with getting air out, the fuel is sucked from low down in the tank and doesn't have air in it.

 

[FairweatherDave]

Not related to lift pumps but I got an air leak after changing a fuel filter. It turned out I had the wrong O ring between filter and housing. Seastart bypassed the filter completely as I did not have the correct spare O ring and got me home and to the end of the season. Symptoms similar to the OP, could get it running but it died when into gear. So maybe the message is double check where you have been changing things. Good luck.

 

[Roger_D]

I thought the lift pump was just to manually push fuel through the system to expel any air?
What exactly was wrong with yours?

Apparently a faulty diaphragm, did not actually see it (In retrospect it did take ages to prime using the lift pump).....Beta say they are only good to lift fuel 0.25m.

Have now installed an in line fuel bulb pump between tank and primary filter (as helpfully noted elsewhere on this forum) which does indeed push the fuel through when pumped. This should make priming the primary filter much easier after a filter change.

 

[KellysEye]

>but in the case of very small leaks that allow air into the system it doesn't always work.

Any leak will show on the kitchen roll even a small one. The symptom of an air in the engine is it runs then the revs drop quickly and it cuts out.