Engine problem

[clonlara]

Motoring home today into a 40-45 knt wind after an abandoned race the revs on the volvo 2020 on our Elan 31 started to rise and fall alarmingly. With the throttle set at 2000 revs the engine would die down to 500 revs and then rise again. This happened constantly. Any ideas as to what may have caused this. Given the weather conditions it was not a situation I would like to repeat.

 

[fisherman]

It'll be air.

 

[Sandy]

How much fuel have you got in the tank?

 

[theoldsalt]

When this happened on my Thornycroft engine it was caused by a failing lift pump that could not keep up with the demands of the injector pump.

 

[clonlara]

Plenty of fuel in the tank.This started last year but someone suggested fitting the priming bulb from an outboard tank and this seemed to work but today this was fully depressed so was useless. Could be air or fuel starvation. The engine never stopped but it was fairly nerve wracking all the same.

 

[john_q]

When this happened on my Thornycroft engine it was caused by a failing lift pump that could not keep up with the demands of the injector pump.

+1 weak fuel pump

A clue

I had first changed the RACOR 500 primary filter and topped it up with diesel, when I checked again after a few hours the diesel no longer upto the top of the filter

 

[robmcg]

Blocked fuel filter. Conditions have stirred up all the crud in the tank and its now residing in your primary fuel filter.

 

[NickRobinson]

Blocked fuel filter. Conditions have stirred up all the crud in the tank and its now residing in your primary fuel filter.

Blockage- upstream from the hand pump- the depressed fuel hand pump is the clincher. On my camper it was debris in the tank pick up pipe.

N

 

[Davegriff]

Blocked tank breather.

 

[rogerstown]

Blocked fuel filter. Conditions have stirred up all the crud in the tank and its now residing in your primary fuel filter.

That has been my experience in the past. Change the filters and problem solved, until the next time engine under pressure in heavy weather.

 

[duncan99210]

Blocked fuel filter. Conditions have stirred up all the crud in the tank and its now residing in your primary fuel filter.

+1
The collapsed fuel pump bulb shows that there's no fuel getting through the system to the engine, somethings blocking the pipe/filter. Replace the filters, that might do it but you'll probably have to clean the tank as well to get rid of all the crud. Heavy seas have stirred up the crud in the bottom of the tank, ha pence the problem today.

 

[superheat6k]

I would place money on a slug of muck on the fuel pickup, stirred up by the rough motion.

I would disconnect the priming bulb, then turn it around to discharge into the tank, with a temporary hose into a jug of clean diesel.

I would then suck out the entire tank via the filler through a strainer to remove as much of the muck layer as possible. If you have a tank access hole then cleaning through that would be ideal.

 

[LittleSister]

Blocked fuel filter. Conditions have stirred up all the crud in the tank and its now residing in your primary fuel filter.

+1

Change your primary filter. That will probably cure it. You may or may not have further crud in your pipes so take another filter when you next go out in case.

If it keeps happening (probably won't, in my experience) then remove inlet pipe to filter and drain tank though this (to wash through crud in the pipes). Don't drain it through a filter - you want the flow as fast as possible. However, do filter what you've collected before (or as) you return it to the tank.

In the unlikely event that any problem continues, then empty tank and clean crud out of the bottom of it.

If you still have a problem after that look for air leaks.

 

[charles_reed]

Plenty of fuel in the tank.This started last year but someone suggested fitting the priming bulb from an outboard tank and this seemed to work but today this was fully depressed so was useless. Could be air or fuel starvation. The engine never stopped but it was fairly nerve wracking all the same.

Suggest you've probably got a partial blockage in the fuel pipe. Put a high-pressure air line on it from lift pump to tank.

The final solution, which I had to undertake, was to have the tank out and steam-clean it. There was about 5 litres of grunge at the bottom - just cleaning the fuel was not an option.

 

[CAPTAIN FANTASTIC]

+1; I would start by changing the fuel filter, followed by checking the pipe from the tank to the injectors to ensure free flow of fuel. If there is no fuel restriction, I would suspect the fuel pump.

 

[Mandarin331]

+2. Change the final filter, remove and clean out the water separator. It may be something more complex but given the bumpy ride this sounds the most likely and simplest cause.

 

[Erwin Swart]

I had two years ago same problem but then in very calm wheather. After my first notification this problem was happening randomly.
I replaced and cleaned all fuel filters, checked for air but couldnt find a solution but it still happened. Generated an external fueltank and connected to engine and the revs were still going up and down.

after many hours i hired an engineer and they noticed a faulty compress test. Two out of 3 cylinders werent correct and this didnt gave me any starting problems at all.
decided to take open the engine and found dirty pistons which caused the issues.

so my advice is to replace filters and check for air, generate an external fuel tank and see what happens

hope you have more luck than i had in the past

 

[KellysEye]

While changing the filter it would be worth putting two filters in parallel with on/off switches on the fuel out side. That's what I did and all mobos have.

 

[clonlara]

Thanks for all the advice so far. the primary filter only has a couple of hours on it and seems spotless as does the separator. This is a boat which is used exclusively for racing and the engine only has 250 hrs all in fresh water. The engine is very well minded and serviced annually regardless of hours.

 

[fisherman]

The collapsed bulb, as said above, indicates a blockage between it and the tank, but might also be that the depression, or lack of pressure in the fuel coming to it results in it collapsing and maybe the collapse obstructs the valves in it.