Volvo Penta engine revs not responding to morse control when engine hot.

[simont140v]

Single Volvo Penta TMD31A (100HP)
Norfolk Broads
The problem occurs once the engine is hot. The engine revs are brought back to below 1100 for manoevouring, but will refuse to increase above this if more revs are needed. If the engine morse lever is pushed further, then the engine might respond with a sudden surge or it may not. Quickly bringing the revs back to above 1100 should it respond, will allow control again, but go below and you are back with an unresponsive engine above 1100. Bringing the morse lever back and then forward quickly might cause the engine to become responsive again. The boat has dual control positions and it happens in both. If the gear box is disengaged and the revs brought up, there is no problem. Running the boat with “say” 1400 revs and bringing the revs back to below 1100 and then increasing immediately back to 1400 is fine, but let the boat speed drop below 3mph and then try and increase revs above 1100 and there is no response. It might appear to be that the force of water over the propeller, keeping it turning as the boat moves through the water at higher speeds, assists the engine in maintaining revs. With the engine stopped and still hot, the shaft can easily be turned by hand and there is no undue heat from the stern gland.

So far, the following remedies have been tried:- Fuel tank cleaned to remove any sludge or debris entering the fuel probe. New engine fuel cut off fitted. All fuel filters have been replaced. Lift pump removed and internal filter replaced with unit found serviceable. All fuel pipe washers replaced to ensure good seals. The air filter is new and there is no black smoke from the exhaust. The morse cables are old, but work well with no visible signs of snagging. The Volvo dual station unit moves freely. Gear box lubricant has been renewed.

Please, can anyone point me to a solution.

 

[Momac]

So does it go perfectly well when the engine is fairy cold?

 

[simont140v]

Yes, there is no problem when the engine is cold or as it comes up to temperature.

 

[kashurst]

Are the fuel supply pipes rubber or metal? And are there any relatively tight bends?

 

[simont140v]

The fuel line from the tank, through the oil/water filter, then a debug filter (which has been dismantled and checked) to the lift pump are rubber. The rest are metal as fitted to the original engine. There are no tight bends in the rubber hose.

 

[kashurst]

The fuel line from the tank, through the oil/water filter, then a debug filter (which has been dismantled and checked) to the lift pump are rubber. The rest are metal as fitted to the original engine. There are no tight bends in the rubber hose.

Rubber fuel hose is cheap. I would change it and see if it helps. You might have a section that has gone soggy. Its oknwhen cold but it under negative pressure when hot, it is collapsing in on itself. Easy to change.

After that given everthibg else I would be looking at the fuel injection pump but they are generally really reliable unless contaminated.

 

[simont140v]

That's interesting. Some of the pipe work is relatively new, but other sections are old, well before I bought the boat. I'll measure up and renew and include the return line. Is there anything else I could look at, apart from the fuel injection pump?

 

[harvey38]

Might be a dumb observation but is the actuator at the end of the throttle linkage moving correctly, ie it's definitely an engine issue and not a problem with the control system?

 

[simont140v]

I'm preparing to clean the linkages, the dual station unit and the Vetus remote control units. Once the linkages are clean I will be able to see more clearly if there is any wear and they are moving correctly. The morse cables may well be replaced. Thanks for the suggestion.

 

[Momac]

Just a thought
Could the fuel tank breather be partially clogged and could this cause the symptoms described ?

 

[simont140v]

Thank you for the thought. This was looked into as a possible cause, even to removing the fuel filler cap to see if there was any change.

 

[simont140v]

Thank you all for your replies and knowledge. I’ve been away on the boat and have been through all your recommendations and ideas. Re-ran and renewed the fuel lines, re-checked the breather etc. However, on returning to base, the engine refused to increase revs above 1100/1200. (Out of drive it was fine). Even stalled at one point when changing from lower to upper steering position.


When I last renewed the fuel filter last year, it was not brimming with fuel, it was half empty. On eventually returning to base, I bled the system and found that I needed to pump the lift pump considerably before fuel started to appear from the bleed screw. I think that the lift pump may not be working properly and not able to keep up with demand, so am going to replace it.

 

[volvopaul]

Thank you all for your replies and knowledge. I’ve been away on the boat and have been through all your recommendations and ideas. Re-ran and renewed the fuel lines, re-checked the breather etc. However, on returning to base, the engine refused to increase revs above 1100/1200. (Out of drive it was fine). Even stalled at one point when changing from lower to upper steering position.


When I last renewed the fuel filter last year, it was not brimming with fuel, it was half empty. On eventually returning to base, I bled the system and found that I needed to pump the lift pump considerably before fuel started to appear from the bleed screw. I think that the lift pump may not be working properly and not able to keep up with demand, so am going to replace it.

If it revs out of gear no load but wont Rev under load there could be a fuel supply problem as the engine requires more fuel under load .
One question, have you had the sea water pump off for rebuild ?

 

[volvopaul]

Check the banjo fitting , I read you have had fuel lines changed , the banjo fitting on the outlet pipe can be changed over with the inlet , if it’s wrong way fuel flow is restricted. The outlet fitting will say OUT on the hex head .

 

[simont140v]

I follow your idea of the fuel supply problem in that the engine requires more fuel under load. If the fuel filter is only partly full then there could be a problem with the supply.

The raw (sea) water pump seal to the engine was replaced 14 years ago and has never been a problem since. The impeller is removed every winter to avoid distortion, and replaced with new every two years.

The lift pump was carefully removed and all parts re-assembled in order. The filter inside the lift pump was replaced with new, as well as the gasket and new washers used on the cover securing screws. The copper washers on the banjos were replaced with new on assembly. I will check the hex heads, but the problem started before the lift pump was serviced. Thank you for your feed back and when I replace the lift pump I will ensure the inlet and outlet fittings are correct.

 

[volvopaul]

I follow your idea of the fuel supply problem in that the engine requires more fuel under load. If the fuel filter is only partly full then there could be a problem with the supply.

The raw (sea) water pump seal to the engine was replaced 14 years ago and has never been a problem since. The impeller is removed every winter to avoid distortion, and replaced with new every two years.

The lift pump was carefully removed and all parts re-assembled in order. The filter inside the lift pump was replaced with new, as well as the gasket and new washers used on the cover securing screws. The copper washers on the banjos were replaced with new on assembly. I will check the hex heads, but the problem started before the lift pump was serviced. Thank you for your feed back and when I replace the lift pump I will ensure the inlet and outlet fittings are correct.

The hex banjo with OUT on fits on the injection pump not the lift pump.

 

[simont140v]

Thanks

 

[James jameson]

If it revs out of gear no load but wont Rev under load there could be a fuel supply problem as the engine requires more fuel under load .
One question, have you had the sea water pump off for rebuild ?

Quick question, Paul why would having the raw water pump off cause a problem ?

 

[volvopaul]

Quick question, Paul why would having the raw water pump off cause a problem ?

If the shaft protrudes the incorrect depth it pushes against the injection pump timing gear causing low fuel injection pressure.