Diesel running slowly

T

TimfromMersea

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14 Apr 2005
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Boat at West Mersea, Essex. Live in Wivenhoe, Esse
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Strange problem last weekend. Engine (Volvo 2020) connected to Brunton's Autoprop via standard shaft, i.e. not a sail drive. Running perfectly to leave moorings at West Mersea, then when approaching Woolverstone, could only get 1900 rpm under load. It span to 3750 rpm in neutral. Running OK though, no vibration etc.

Tried ahead and astern several times, still the same, could only get 1900 rpm irrespective of throttle opening.

Arrived Woolverstone, everything appeared normal.

The next day, topped up with diesel. Then leave early AM with no wind, and everything is fine - and remained so.

Any ideas? tank breather seems fine so unlikely to be a vacuum in the tank.

Baffled me and so any advice/thoughts welcome!
 
maybe somthing in the tank blocking the pick up tube and it either moved or was sucked up into the filters,leaving it to settle/taking the suction of may have disloged it,had a similar problem once on a lorry,turned ouy to be a bit of plastic,someone had put a tube in it to fill with fuel and when they pulled the pipe out the tank had shaved a sliver of plastic of which every now and then sat over the end of the pipe,took a while to sort that one out as all seemed clear when the engine was stopped,and it only happened occasinally. if you have a tank access hole it might be worth checking the next time your level is low,maybe if it happens again it might be worth stopping the engine then starting again to see if it clears it.
i assume filters etc are all clean as a blocked filter could do the same/similar
 
I had similar symptoms a couple of months ago on my Volvo 2020 with saildrive and Volvo three-bladed folder. After happening intermittently it has now become a pertmanent problem. The fuel looks clean, but I changed both filters just in case. I also got a diver friend to polish up the prop - this gave me an improvement from 1900 rpm to 2100 max in gear.
I am now about to start paying for someone else to continue the diagnosis.
 
Could it be prop fouling? Or the Autoprop going too coarse? A few years back on previous boat with a 2 blade folder we lost 500 rpm that way; maybe the run up cleared something? Were you getting black overfuelling gunge out the exhaust?
 
It's either the fuel supply or the fuel supply.

Change filter(s) check for leaks or faults in the lift pump.

Assumed you've checked the basics like prop damage and travel in the injecion linkage.
 
When my filters start to blind a drop off in revs is noted. When alongside the egine runs ok on no load but will start to die on load as the demand for fuel increases. If it is a load problem youd be getting loads of black smoke etc.. as the engine stalled.

I'd try changing the filter regardless, a cheaper alternative to more expensive investigations that may well cure the prob!
 
Agrre - its a fuel fault. Not enough fuel is getting through to the injection pump. The obvious likely cause is the filters. I have had a large piece of muck block the fuel pipe in the tank causing the engine to slow. Once suction was relieved and the tank stirred up a bit it freed up, and did it again a couple of days later. Tank out for cleaning was the only permanent solution!

Could also be a very slight leak somehwhere in the fuel line allowing tiny amounts of air in - not enough to stop, but enough to slow down the engine.

All these faults usually allow the engine to run at full revs off load because the engine needs much less fuel running light.

As others have said, if the problem was excessive loading at the prop, the engine would give off black smoke as it labours.
 
I agree maybe air and as the problem went away after topping up I wonder if an O ring is leaking very slightly on the fuel filter..I had this problem.. Fuel never leaked out as the tank was never full but after losing ower a couple of times realised air was leaking in.

This can happen if using the wrong filter in a Volvo promary filter.. The Cav etc are a trifle shorter and so the centre bolt can become thread bound and appear tight before compressing all the seal rings. Either fit expensive volvo element or add washers under bolt head with after market filters:)
 
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