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

[PaulRainbow]

>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.

Still not got the hang of the quoting i see

Small air leaks in the suction side of the fuel system will not necessarily leak diesel, therefore the paper towel method you describe will be less than helpful.

Symptoms of air in the fuel system will vary, anything from plain non-starting to a minor misfire. Depends on a number of factors, where the leak is, how bad it is, the engine, engine speed, engine load etc.

None of the symptoms of air in the fuel match the description given by the OP.

 

[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?

If you want to quickly narrow down the problem, do what i said earlier.

Take the pipe off of the filter outlet and put it in a can of diesel, if it runs ok the problem is the filter or before the filter, rules out the lift pump and anything after the filter. If it doesn't run ok, the problem is after the fuel filter. Could be the lift pump, does it have a gauze filter in it ? Is there a secondary filter in the engine ?

If it was running ok above, reconnect the filter and take off the inlet pipe, put this in a can of diesel. If it runs ok now, it's the filter, but if the filter is badly contaminated you may also have an issue with dirt in the fuel tank. If it doesn't run ok, the filter isn't the problem, you need to look before the filter, most likely dirt in the tank or a blocked gauze filter in the tank (if one is fitted).

Running the engine from a can will empty the can into the tank via the return pipes, if this is happening too quickly divert the return to the can.

If you do the above, and carry some cans of diesel onboard, you will be have the pipes ready to be able to quickly bypass the tank and primary filter should the problem re-occur on the rest of the journey home.

 

[pandos]

I had this problem in the past, it turned out to be the tank breather was looped high up in a cabinet, presumably to prevent water getting in when heeled,

This loop had fallen over and was compressed at the fold, as fuel was used a vacuum formed in the tank over time so that eventually fuel starvation set in, dipping the tank through the internal fill point of course released the vacuum so even checking that I had fuel or topping up the tank when I was using 20l jerrycans was sufficient to keep things going for long periods the problem only manifested when the tank was half filled so the filler was not opened for long periods....

BTW i previously had fitted a facet pump for bleeding purposes (hand lift pump impossible to get at with a hot engine) but this got me home when I was having the problems, prior to my discovering their cause/solution.

 

[lpdsn]

If you want to quickly narrow down the problem, do what i said earlier.

Take the pipe off of the filter outlet and put it in a can of diesel, if it runs ok the problem is the filter or before the filter, rules out the lift pump and anything after the filter. If it doesn't run ok, the problem is after the fuel filter.

+1

Also a handy thing to practice in case you lose the engine and want to rig up something for the last few hundred yards after sailing up to your chosen marina or harbour.

 

[Dull Spark]

An oily rag sucked into the air filter housing gave me similar symptoms some years ago. Full speed with no load. Lower speed under load. BUT.... lots of black smoke as well. Good luck.

 

[Kinsale373]

I had a similar issue while trying to beat the current coming around from the east into into Cherbourg two seasons ago. The revs suddenly dropped from 3000 to 1000. I managed to coax her back to 1500 revs using the throttle. A mechanic found that the issue was a slightly misplaced o ring on the CAV type water seperator/ filter. I was sceptical at the time but since i've been very careful when re fiting these filters and have not seen the problem since. So it was a small amount of air being sucked in when the system was under most pressure.
Kinsale 373

 

[steve yates]

Based on the fact that I know the tank is rusting, and had been run right down to a few litres previously and started doing this before, I would assume that it is crud. It was a brand new primary filter when it did this on the way to milfordhaven, but ran fine for 20 miles or so. Then was under sail, the engine went back on when headed by the wind and she was being bounced around a lot off skomer.
When in port, the new filter had some big lumps of black crud. So I added 10l of fuel to the tank, and changed the primary and fuel filter. I had the prop checked by a local diver just in case.
Reading all the replies, it looks like I should check the o-rings and replace the filter again, and check the gauze strainer on the tank exit, and blow through the fuel line to the filter. Then check the exhaust for delamination too just in case.
I have a bigger cav style filter and waterseparator on board, so will get that installed before I leave port again. I will also try and get the tank cleaned.
Of course, the issue is testing it in the real world, I need to motor for at least an hour, preferably in bumby conditions, and get back into port if it doesn't work , I like the idea of an alternative feed direct from a Jerry an, but how do you stop it spilling of the boat is pitching all over the place? Could you use an outboard remote fuel tank?
And can you t a fuel line into your system with an on/off tap for emergency backup? So you just open a can and drop the end of the hose in? Rather than faffing about with spanners and spilling fuel when it's bumpy?

 

[seaangler23]

Some engines have mesh filters in so,e stupid places, my buhk has one in the lift pump the previous owner removed and ive seen one in a union that was all gunged up, from the outside it looked like any other so never thought to look

 

[PaulRainbow]

I like the idea of an alternative feed direct from a Jerry an, but how do you stop it spilling of the boat is pitching all over the place? Could you use an outboard remote fuel tank?

You can if it's got provision for the return, but i doubt that would be the case. You could get a 22l plastic fuel can and drill a couple of holes in it for the two pipes. Stick a fuel pickup pipe in, going almost to the bottom of the can and a return pipe, just going into the top of the can. Stick them in with Sikaflex or even silicone.

And can you t a fuel line into your system with an on/off tap for emergency backup? So you just open a can and drop the end of the hose in? Rather than faffing about with spanners and spilling fuel when it's bumpy?

You can tee the pickup just after the primary filter and the return wherever is the most convenient. You will need to change over to the emergency can, by which i mean closing the original pipes off. You can fit a pair of three way valves or some tees and quarter turn taps.

 

[lpdsn]

Of course, the issue is testing it in the real world, I need to motor for at least an hour, preferably in bumby conditions, and get back into port if it doesn't work , I like the idea of an alternative feed direct from a Jerry an, but how do you stop it spilling of the boat is pitching all over the place? Could you use an outboard remote fuel tank?

I've probably said it higher up, but I'd really very strongly recommend fitting a vacuum guage between primary filter and lift pump. It's the difference between being blind and being able to see. Then you wouldn't have to go out and bounce around until the engine stops, you could see the problem starting to build up or not and could abort the test before it becomes problematic.

In terms of splashing from the jerry can I've never had a problem. I've a boat the sails better than she motors so when I've used the technique I've tried to sail as far as I could without getting into a confined space then used the engine. However I did use it once when the wind dropped suddenly from F6 to F0 and the engine dies ten minutes after starting. No splashing, but there was a crew member down below to hold it all in place and to monitor the level.

The real risk is that most of the fuel that goes through the lift pump ends up going back down the return pipe, so the jerry can will empty far, far quicker than you expect. First time I did it the engine died when I was on the pontoon tying the boat up - that was close!

 

[lpdsn]

Stick them in with Sikaflex or even silicone.

Is that a good idea? I've heard that diesel can slowly dissolve Sikaflex causing blockage of filters. I certainly been told a tale by one owner who had a very near miss after his filter clogged with tiny balls of Sikaflex after a 'professional' bit of work on the tank.

 

[PaulRainbow]

The real risk is that most of the fuel that goes through the lift pump ends up going back down the return pipe, so the jerry can will empty far, far quicker than you expect. First time I did it the engine died when I was on the pontoon tying the boat up - that was close!

Not so much a risk as a certainty, which is why i said route the return back to the can

Is that a good idea? I've heard that diesel can slowly dissolve Sikaflex causing blockage of filters. I certainly been told a tale by one owner who had a very near miss after his filter clogged with tiny balls of Sikaflex after a 'professional' bit of work on the tank.

It's only to hold the pipes in the can, so it shouldn't be exposed to diesel, as such. It's also only a temp job as a backup in case of further blockages while he gets the boat home.

 

[dustynet]

Hi Steve,

I know this thread has been dead for over 4 years now but after out chat, and following your tread history (along with others journeys as new owners of used boats) I find these really interesting, particularly when the final solution has been discovered. As you can probably guess, im still reading through, so apologies if this is addressed in a more recent thread. Assuming thats not the case, can you recall what the root cause was?

Cheers

 

[dustynet]

As suspected, Ive read the next thread which answers the above question

 

[HissyFit]

... set up a bypass using some hoes and a jerry can.

What kind of hoes do you have in mind? The garden variety, or do we need to solicit help from the red light district? Or perhaps you mean 'hose'?

Sorry, couldn't help myself, since nobody else seems to have spotted it.