More Diesel drama

[chas]

About this time last year, I had some very good advice when my Thorneycroft T90 decided to increase revs without warning and eventually became almost uncontrollable. As a result, I had the Fuel Distribution Pump and injectors reconditioned by Lucas CAV. I have had, until last week, a troublefree season with an engine that has behaved perfectly. The problem has now come back. The symptoms are a sudden increase in revs - usually returning to normal within about 10 - 15 seconds. A couple of times the revs have not reduced, leading to a tickover of 1200 rpm plus rather tahn the normal 500 - 600 rpm. I topped up the tank with 25 galls of fuel before this happened. Could this have had any efffect? Can it be fuel related rather than something to do with the governor or FDP? Any help or advice would be very welcome!

 

[Guest]

Post deleted by danfoley

 

[stevebirch2002]

Had a similar problem on a Thorneycroft 345. Traced the fault to very dirty fuel, tank and filters. Try feeding the engine from a can of diesel with new filters. This should prove if it is the primary fuel system at fault.

Regards

Albin Vega "Southern Comfort" V1703

 

[tr7v8]

Overspeeding is too much fuel not too little. Aside from governor and pump the other possibility would be the fuel return, if this gets blocked then you'll over fuel an that'll cause a runaway. Have you topped up with oil recently? cos diesels run just as well on lubricating oil.

Jim
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[jleaworthy]

Often an increase in engine revs means that air has got into the governor of the fuel pump. Try bleeding your fuel system again and if that works try finding out where the air is entering. It will be somewhere on the suction side, probably at a joint in the fuel line or a lift pump diaphram. Best of luck.

 

[boatone]

Interesting. I also have this problem once in a while with my Volvo AQD32's. Usually running at around 1500rpm (river speed0 engine will suddenly rise to 1800-1900 for a few seconds and then return too normal. Engineer aquaintance suggested dirty injectors etc and suggested I dose the tank with a diesel cleaner additive and its now far less frequent infact quite rare.

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[Castletine]

Don\'t forget the Water Trap

I too have had over-revving and it turned out to be air entering the fuel system via the water trap.

I checked all filters, but it turned out to be a seal - could be worth a try.

Jonathan

 

[30boat]

Re: Don\'t forget the Water Trap

by water trap do you pre-filter?

 

[Castletine]

Re: Don\'t forget the Water Trap

Yes,

The set up is as follows;
from the tank to filtre 1, then to filtre 2, then to the water trap, then to pump and injectors.

This set up has been working faultlessly for 10 years (save for the over revving) and has taken the boat down to the Med and back twice.

 

[PeteMcK]

Almost certainly correct

My T 90 was prone to doing this when rolling with less than about 5 gallons in the 45 gallon tank. The T 90 has a hydraulic speed governor in the CAV DPA pump (some DPAs have mechanical governors, e.g., in Landrovers and tractors). With air in there, it thinks it's running too slow and opens up the injection delivery, over-revving for a few moments. I replaced my lift pump recently, just as a matter of course (at 15 quid, why bother cleaning the old one!) and the over-revving has gone - the engine simply falters when it draws air. (My next move is to replace the tank with three small ones!)

Check for air leaks on the suction side of the lift pump, or more realistically, remake all the joints. Backflush the pipe from the tank - it might have a blockage. Strip and clean the water separator. And I'd get a new lift pump from AMC/Thornycroft, for all they cost. Change the filter anyway, even though it's on the pressure side. Now!

It COULD be fuel related too, as grotty fuel could be blocking your filters and strainers, and any air in the DPA will not be getting flushed out so readily. The air has to get in somehow though.

 

[Guest]

Re: Almost certainly correct

Good advice and probably hits the nail on the head. But check your engine oil also because if the fuel lift pump diaphragm has failed you could be both sucking air and also losing diesel into the sump. A faulty shaft seal on the DPA pump could also cause similar problems.

 

[Guest]

Re: Almost certainly correct

Good advice and probably hits the nail on the head. But check your engine oil also because if the fuel lift pump diaphragm has failed you could be both sucking air and also losing diesel into the sump. A faulty shaft seal on the DPA pump could also cause similar problems.

 

[chas]

Re: Thanks for some excellent advice

Thank you all very much indeed. I do not really fancy taking 45 galls of diesel out of the tank (I had just filled it for the winter). I will start with a new lift pump - mine is original - and a filter change and then work along the fuel system. If I suceed, I will let you know!
Thanks again

 

[Kurrawong_Kid]

Re: Thanks for some excellent advice

I had this sort of roblem on a Golf Diesel car. The engine almost revved itself to destruction! A friend of mines with a similar car did just that on the motorway-very dangerous! The cause was found to be a blocked outlet filter in the fuel tank. Apparently the pump calls for more and more fuel and then the suction produces such a rush of fuel that the engine overrevs--I suggest you look for fuel starvation problems first before spending lots of money.

 

[Guest]

Post deleted by danfoley

 

[richardandtracy]

Oil as Fuel

I once read that David Sinnet-Jones connected his diesel engine's crank case breather vent to the air intake. There was enough oil in the crank case air on a couple of (lumpy) occasions to cause a run-away. This may be worth checking.

Regards

Richard.

 

[chas]

Re: Cleaning dirty fuel/tank

Thanks, Brian - I will investigate.

 

[oldharry]

Re: Oil as Fuel

Diesels run quite happily on their own engine oil if it gets past the pistons, or via the crankcase breather if it is connected to the air intake as it is on the Perkins Prima for example. Normally oil should not get up the breather tube, but an engine that has worn rings/bores will build enough pressure in the crankcase to force the engine oil up the breather pipe, in to the air intake - and hey presto! unstoppable runaway engine. Had it on an old Perkins engined van once.

This form of runaway is characterised by emitting huge clouds of dirty blue smoke. Not nice in the middle lane of the M4....

 

[chas]

The Solution!

I hope...... Thanks again for the advice. I was hoping it would be a job for the winter but went out yesterday for a last sail before laying up. Engine started racing and eventualoply went to full revs with no control. Then it stopped (in the middle of the fairway). managed to start it. Went to full revs and then stopped again. Kind fisherman gave a tow back to the mooring.
Replaced fuel filter element and fuel pump. Discovered blockage in fuel return line to tank so blew it out. Engine now running perfectly (touch wood).
The fuel filter was totally blocked. Suspect the 25 gallons of fuel that I picked up 10 days ago was to blame (absolutely no problems up to then).

 

[DepSol]

Re: The Solution!

Just another lesson learnt that the fuel you get is not always tip top as the manufacturers suggest. Maybe the use of an additive might prevent all this hassle in the future....after all prevention is better than a cure and wasting your time.


Dom

I am boating again ;-)