Engine losing revs - why?

[MissFitz]

Our engine (a Sea Panther Watermota) has developed a strange problem - when we put it up to full revs, it will go for a minute or so & then suddenly the revs will drop off. If I throttle back & then put revs back on again it works fine - but then does it again a couple of minutes later. If I don't touch the throttle, the revs also come back.

It's not a blocked fuel breather (I've checked) & it's not something round the prop (ditto). It looks like we have a very small diesel leak from the banjo valve at the top of our secondary filter - could it be very small air bubbles that are causing the loss of revs, or would any air stop it stone dead?

Or, er, any other ideas?

All suggestions very gratefully received, thanks.

 

[sailinglegend420]

Saw this on a Jeanneau 43DS and it was the pick up tube in the diesel tank partly blocked with bits of plastic offcuts!

 

[nimbusgb]

Partial blockage of the water seperator filter element did it for me a couple of weeks ago.

 

[Leonidas]

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All suggestions very gratefully received, thanks.[/QUOTE]

Sounds like a clogged filter or fuel line. I suggest you check that the fuel shut off cock at tank side is fully opened and the fine filter before the injection pump is not clogged with debris, slime etc.

 

[boatmike]

Our engine (a Sea Panther Watermota) has developed a strange problem - when we put it up to full revs, it will go for a minute or so & then suddenly the revs will drop off. If I throttle back & then put revs back on again it works fine - but then does it again a couple of minutes later. If I don't touch the throttle, the revs also come back.

It's not a blocked fuel breather (I've checked) & it's not something round the prop (ditto). It looks like we have a very small diesel leak from the banjo valve at the top of our secondary filter - could it be very small air bubbles that are causing the loss of revs, or would any air stop it stone dead?

Or, er, any other ideas?

All suggestions very gratefully received, thanks.

Could be any number of things but what you describe is certainly due to fuel starvation. Causes
1. Blocked filter
2. Blockage in tank or line.
3. Blocked tank breather.
4. Contamination (fuel bug)
5. Contamination (water)
6. Air leak
Only you can tell by eliminating each cause in turn. One thing is clear though. It ain't getting a sufficient flow of clean fuel!

 

[PeterGibbs]

Diesel engines will tick over with faulty fuel supplies and appear well-running, but the real operating test is when they come under load, and fuel is a-surging! Load is defined as prop engaged! What you describe is typical of starved fuel input, through blockage or air intake.

Check the filters and discard any that look even slightly smutty. Diconnect the fuel hoze from the tank at the first filter and "blow" down it - to dislodge any blockage in the takeup pipe in the tank. Use a piece of garden hoze to connect to the pipe - you don't want even a drop of diesel on your face!

Then bleed the system thoroughly.

If the problem recurs with crumbed up filters you might have to drain the tank and purge - you could have contaminated fuel and there is no short cut to fixing that one!

PWG

 

[2nd Wind]

It was a faulty lift pump that did it for me 2 weeks ago. It was 26 years old though!!

 

[oldharry]

It looks like we have a very small diesel leak from the banjo valve at the top of our secondary filter - could it be very small air bubbles that are causing the loss of revs, or would any air stop it stone dead?

Yes, and No.

Yes, the lift pump suction is drawing a tiny amount of air in through the leaky banjo and causing the engine to slow under load, and No, microbubbles in the fuel from this leak will slow the engine, but not kill it stone dead - until the leak gets a bit worse and the bubbles get a bit bigger!

Fix that first, and you will be pretty unlikely to have to do all the other nasty jobs suggested!