ENGINE WILL NOT REV WHEN WARM

[dsw]

Cummins 6bta 5.9 td (Bosch pump) with prm 750d box.

after half an hour or so when my engine is running at normal temperature and the gear box is only warm to touch and I then slow down but when I go to speed up again the engine will not rev up, but if I knock it out of gear the engine revs fine but put back in gear then it will not rev any more than just over tick over, I have checked the oil level in the gear box and I can not see any leaks within the diesel system, and I don’t want to start doing things to the engine if the box is at fault and vice versa so any ideas would be much appreciated

 

[nimbusgb]

What type of prop? Could it possibly be feathering and getting jambed in the trail or fully coarse position?

 

[dsw]

the propeller is fine along with shaft ect and it only happends when she warms up .

 

[Twister_Ken]

Guess.

When stuff expands with the heat of the engine it opens up a tiny leak in the fuel system which allows air to enter, thus starving the engine of red???

 

[wotayottie]

sounds like fuel starvation but why it should only occurr when warm, I do not know. On the theory of starting with the simple possibilities, I would change all fuel filters and check the flow from the lift pump. Also check that the throttle is opening fully and staying open, not slipping.

 

[1955patric]

Could also be a problem with the linkage. Have you tried moving the throttle lever on the engine? If it revs Ok when doing this it's definitely a linkage problem - either cable or lever. If not other things to check are:

1) fuel sytem - leaks or blocked filters, lift pump
2) Air filter
3) Compression. When engine warms up piston, rings and cylinder expand and Lub Oil is thinner. Any leakage aroundthe piston rings will be greater when warm. However this usually causes problems when starting.

 

[dukes4monny]

Another guess..........is the fuel tank vent blocked? When the fault occurs again, try opening the fuel tank filler, if it's difficult to open, or there is a sudden inrush of air, then you need to check the breather.

 

[dsw]

I have changed the filters air and fuel and linkage is a ok but after checking the lift pump there dose not seem to be much pressure when I pump it, just a little bit when the leaver is almost at the bottom, and as I said when out of gear it revs up fine and when its cold every thing works fine but when I go to stop in reverse after its warmed up it just blurrrs under load so it’s a good job that I have a swing mooring or it would not be much fun trying to moor up in a marina with 12 ton of steel that would not stop.

 

[tinkicker0]

A long time since I worked on a B series and then not a marinised one, so I may be leading you up the garden path but.
Some truck applications had a boost pressure hose going from the turbo to a sensor diaphragm in the pump.
This was meant to stop overfuelling and black smoke whilst building up road speed.
When the hose got blocked / kinked / damaged, this would cause symptoms similar to yours.

likewise incorrect valve clearances could also cause the same problems.

A sticky governer in the pump - same again.

Bad lift pump, inj pump transfer vanes, regulator, leakoff valve and or worn pump ditto.

Impossible to diagnose on a PC screen though. You need a techie.

 

[jimbaerselman]

Your symptoms say 'too little fuel when hot', rather than an overload problem.

Gear box load, or variable prop misbehaviour will give you black smoke and a lot of noise when you demand revs and don't get them. Whereas a limited fuel supply will give you revs when there's no load, then no revs under load.

So, what's restricting the fuel supply? Filters, pipes, pumps, control levers. Air leaks usually give surges and gaps in power delivery. Mechanical faults usually have other symptoms as well (smoke, temperature, rattling cans . . .)

 

[dsw]

you was right it was a fuel problem i.e. pipe from filter to lift pump gone very soft and when it was hot not letting enough diesel through when sucking but enough to let it rev with no load and i thank you all whom gave their thoughts.

 

[Twister_Ken]

Blimey - I wasn't that far out. Maybe the diesel maintenance course was worth it after all!

 

[wotayottie]

Was that the RYA diesel maintenance course or the one we get every day on here. To say that we mostly are yotties, we do discuss an awful lot of engine probs.