Engine won't stay at hi revs in fwd gear but revs up by itself

jellyellie

jellyellie

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So, boat with an Albin AD2 two cylinder 20hp diesel engine. It has a problem with the revs in fwd gear.

Even with the throttle set to maximum, it only reaches revs of about 1,500rpm. When motoring with the throttle set in this maximum position, occasionally (once every 10 mins or so? it's not regular, but frequent) it suddenly revs right up to about 2,000rpm but only for maybe 10 seconds, then drops back down. When it drops back down there is a noise almost as if something is slipping.

In reverse gear it happily revs up to around 2,000rpm but never tested this long enough to see if it revs up by itself too.

Thought it might be fuel-related, we've changed filters, had the injectors serviced (new nozzles), but neither have made the difference we were hoping for.

Any ideas? What can we try next?
 
I do not know the engine, so just general thoughts.

While it could be a governor fault, that's unlikely unless its recently been touched, its probably not enough fuel/air in fuel/water in fuel.

You've new filters, did it happen before or after they were first touched?
While clean filters, could the line from the tank be restricted?
BTW, does it rev fully in neutral?
If not sorted try
Inserting a transparent section of fuel pipe as near the HP pump as possible in the LP feed to view any air passing.
Inserting a vac gauge between LP pump and filter to detect a restriction, continuous or intermittent (ie gunge in the tank wafting past the pick up)

Good luck and let us know-

Nick
 
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This isn't a fouled prop is it? Fouling preventing the engine from reaching peak revs but then occasionally aerating/cavitating to allow full revs? Especially if it will rev fully in reverse.
 
jellyellie said:
So, boat with an Albin AD2 two cylinder 20hp diesel engine. It has a problem with the revs in fwd gear.

Even with the throttle set to maximum, it only reaches revs of about 1,500rpm. When motoring with the throttle set in this maximum position, occasionally (once every 10 mins or so? it's not regular, but frequent) it suddenly revs right up to about 2,000rpm but only for maybe 10 seconds, then drops back down. When it drops back down there is a noise almost as if something is slipping.

In reverse gear it happily revs up to around 2,000rpm but never tested this long enough to see if it revs up by itself too.

Thought it might be fuel-related, we've changed filters, had the injectors serviced (new nozzles), but neither have made the difference we were hoping for.

Any ideas? What can we try next?
Click to expand...

Others will comment soon, I'm sure, but if nothing else this response will bump your thread!

This sounds very much like air in the fuel supply or fuel starvation, particularly as you are occasionally getting surges to 2000rpm (is this your expected max rpm?).

I think the noise you are hearing is probably a red herring.... (they swarm round the prop and slow it down.. ha ha).

I had exactly the same trouble recently - first the filters became blocked which reduced the fuel available to the engine. At low revs it was fine, the filters allowed enough fuel through even though they were constricting the flow. But when higher revs were demanded the engine tried to suck even more fuel through the filters and gradually a fuel vacuum built up after the filters - the engine slowed, the demand reduced, the vacuum relaxed and more fuel became available - which the engine happily began sucking through again till the same sequence started all over. I thought that changing the filters would sort it. It did, but I had failed to bleed the fuel lines right up to the injectors and air in the fuel lines caused similar probs - unable to rev to max, wavering speed. I bled the system thoroughly and all returned to normal. It took a while but it worked - I hope it works for you too. Don't be hasty to blame the engine before you're sure it is getting all it needs. Have you checked the air filter? Could it be clogged? Run the engine with it out for a short while, or preferably replace it if you haven't done so for a while. Diesels needs are simple and few - they need absolutely clean fuel (no dirt or air) and clean air.

Edit: what Nick said in essence, I was typing at the time!
 
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Your astern could be lower ratio than ahead, so whatever the cause it would account for differing max revs. Have you got an occasional maladjusted/slipping clutch in ahead? Fuel starvation could do the same, mind. When it revs up is there a noticeable increase in propwash and speed? If not, slippy clutch.
 
Er.... If the clutch was slipping the engine would over rev, not slowdown surely? I think you are probably right about the load in reverse though. It may or may not be a different ratio but as props are designed for max efficiency in forward gear the load in reverse will be less anyway. This is almost certainly fuel restriction or air in the fuel as others have suggested.
 
Cool, thanks for the responses everyone.

She has a lot of underwater growth at the moment (but is used regularly) and will be hauled out in the next week anyway, but I don't think that is the cause; she has the problem even when tied up alongside so can't see how the prop could be cavitating to reach full revs?

Expected max rpm is stated as 2,200rpm in the manual, but it's an old engine, so 2,000rpm would be about reasonable?

Definitely still think it's a fuel problem from what you've all said. Will try changing fuel lines.
 
This could be an expensive governor problem however it is far more likely to be a simple fuel issue. The reving up and down is known as hunting. It could very well be air in the system but I would also check to see that the tank suction is not fouled with debris or dirt or that your fuel lines have not been squished somewhere. What state were your filters in when you changed them? Were they black? I ask as the symptoms are also associated with MBC aka Diesel Bug blocking the filters.
 
funny that I get the same problem with my prop shaft my misses says why do you keep speeding up and slowing down !

mechanical noise you say oh er could be governor, air in fuel system would make starting the engine difficult
cheers
mick
 
Again no Knowledge of your engine but you could make sure that the vent into the fuel tank itself is not being crushed periodically and starving the engine of fuel until the vacuum is released. Even when the fuel system is bled it could still give such a symptom (and would be a cheap afloat check!)

ianat182
 
The main point is that it runs at 1500 fairly happily, and every ten minutes revs up for ten seconds. If it were a fuel starvation/air in fuel problem I would expect 2000 revs with occasional slowing down until it purged the air, or slowing down and dying unless:
(a) the throttle is reduced to stop the intake of air through a leak, or to match the throttle and revs to the available fuel, or
(b) the air is purged via a bleeder.
It is less likely that it has too little fuel which occasionally becomes enough.
 
Could be a bent regulator rod, but doubtful if you have not had the injection pump apart. If you are heavily fouled, I would personally go for fouled prop. To get rid of any doubt about fuel, fit an outboard tank with a 12v pump, to take the lift pump out of the circuit.
 
Hi,

I'm sure the answers given so far are in the right direction, but one thing that can give slightly similar symptoms

would be a partially blocked exhaust elbow right after the manifold, which gives the wrong back pressure and especially on an older engine can effect the incoming fuel mix due to only partially exhausted fuel from the cylinder.

It's fairly quick and easy to undo and check too.

S
 
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