Deisel engine - dies at high revs

Modulation

Modulation

Well-Known Member
Joined
30 May 2001
Messages
495
Location
finder
Visit site
My boat is fitted with a Yanmar 2GM20 - newly installed 3 years ago.
It's been problem free since the beginning but over the last month has developed a tendency to slowly die away when I try to open the throttle. I can get boat speed up to 4.4 knots whereas before I could easily run at 6 knots for as long as I felt - which, to be fair was never more than half an hour or so, unless stemming the tide. If I try to go faster than 4.4 knots, even in flat water, I can hear the engine surge for a minute or so before slowly losing revs - often quite sharply.
I know it sounds like a fuel supply problem, but I cleaned out all the supply lines and renewed the filters last year as part of normal maintenance.
Any ideas?

Brendan /forums/images/graemlins/confused.gif
 
When did you last clean your tank out, could be partially blocked there. Also this may sound stupid but is there a good supply of clean air to the engine and is the airfilter clean.
 
Sounds like a fuel flow problem. If the air flow is restricted you would expect it to make black smoke.

Check the tank breather and that the tank outlet valve is fully open and not faulty.
 
change the fuel filter + check for slime in it as you may have the diesel bug - do you add an algicide such as Fuelset or Soltron?
Also check the routing of the fuel lines - do any have a kink (flexible lines) or have they been flattened (copper lines) because of a heavy weight placed upon them?
 
Check all your fuel unions - you may have air getting in somewhere - I had a similar problem on my newly installed engine after about 50 hours - I decided it was fuel but it still happened after filter changes so then tightened all unions, cracked each injector to release any trapped air bubble, and it's run fine ever since.
 
Boat in Treguier with same problem recently. Local chantier mechanic called in... turned out to be the dreaded deisel bug. He had a full tank of fuel to get rid of. Dont know what happened next.
 
Fuel starvation: check 1. Fuel filters and lines - are you getting a good flow through at each point? Next if still in trouble, check all fuel joints. Wipe them dry, run up the engine for 10 minutes at at least half throttle. Then check each joint again with coloured tissue ; pink loo paper or coloured serviettes will do. If there is any trace of fuel the colour will darken, making leak spotting a lot easier! The slightest trace of fuel specially between the lift pump and injector pump can give the effect you describe. Is the tank breather blocked? When the engine stalls, take the filler cap off. There should be no vacuum.

Still in trouble? Check the fuel lift pump. The diaphragm may be leaking, or the valves faulty. Usually dirt cheap so replacement is safest option.

Stiull no go? Then the fuel line either to or from the tank is getting blocked - usually on the suction pipe from the tank - get the tank cleaned.....

If none of these work then you have a problem with the injector pump, which needs professional attention.
 
Know exactly how you feel! - happened to us on holiday earlier this month. Initially we bled the system and found air: unfortunately, could not see any trace or any reason for air to enter the lines.

For the next few days, the engine was only used at low rpm either to motorsail or to enter harbour. No problems.

HOWEVER, on our return leg I needed to make up some time to meet a tidal gate, so engaged diesel and opened up throttle to achieve fast cruise: then it started to die back again. This time I throttled back very smartly to a low tickover and the engine kept running.

When alongside a few hours later, a much more investigative search was carried out. By luck and the thoroughness of the engineer, air bubbles were spotted in the primary filter/water separator glass bowl. The whole filter was stripped down, a new filter bought, everything cleaned and the whole thing re-assembled with new seals/PTFE/etc on every junction. Result - no more air bubbles. We are now home, but very happy to have a sailing boat - if all else fails .....

Perhaps the seals had hardened over time. Who knows? But no water or dirt could be found on either occasion when the fuel system was checked.

Best of luck.
/forums/images/graemlins/smile.gif
Tam
 
If the engine simply fails to pull the anticipated revs I would start by checking the prop and hull are clean. If the revs fluctuate and then fall off I would go for diesel bugs [partially blocking the fuel pipe in the tank.
 
previously on here, some have commented on there being an unknown filter in their tank outlet which had clogged, particularly a Vetus tank I recall.

Could this be worth a look?
 
I had a similar problem with my engines. After lots of faffing about it turned out that at the bottom of the fuel pickup pipe in the tank little gauze filters were fitted which over time had picked up gundge and semi blocked.
 
You must log in or register to reply here.

Share this page

Top