I recently bought a boat with a BD722 engine fitted with a TMC40 gearbox. The Installation dates from 1999 but the engine hours are less than 500. There is also a Halyard Marine Aquadrive unit fitted to the propshaft.
The engine starts and runs very well but after about 45 minutes normal usage starts to labour, and unless put into neutral , will cut out. After 10 minutes or so the engine will run normally in gear for a while before the same thing happens.
The engine temperature does not increase, there is no weed, rope or other obstruction to the prop. It is possible to turn the propshaft easily by hand when in neutral and the stern tube bearing does not get hot. There is no undue vibration when the engine is running in gear.
I feel the previous owner has possibly damaged the gear box in some way but before I get involved in fitting a new gear box I would welcome any advice regarding other possible causes/solutions.
This may not be a transmission issue. The transmission will run hot, probably hot enough that they seem to be overheating. If this is your problem, then I am sure you would notice a discoloration in the paint on the transmission.
Another possibility is a fuel feed problem as the problem you describe could also be a fuel starvation problem resulting in a reduction in power, ultimately stalling the engine.
1 - Check and make sure all you fuel line connections are snugged up. Although they may not show signs of leaking, they can draw in air and cause a similar problem.
2 - If you have rubber fuel hose between the fuel tank and engine lift pump, consider replacing it as it is possible to have the inner layer of the hose collapse and restrict fuel flow.
3 - If you have a 'Racor' style primary filter, make sure there is no water showing clear sight glass and that the filter is not clogged.
4 - Make sure the fuel tank breather/vent is clear and not restricted. If blocked it does allows a vaccuum to build up in the fuel tank and fuel pump cannot overcome this vacuum to pump fuel. You can judge this being a problem if the problem occurs quicker when you have a full tank than a partially full one.
5 - Make sure you tank is free of debris which can be sucked up by the fuel pickup pipe in the tank as a larger piece of debris may be picked up but not be drawn into the pickup causing a blockage. This may be more intermittant and variable in it's nature, particularly in rougher seas.
I will certainly try renewing the primary filter - the fuel in the sight glass is as clear as a bell but changing the filter will eliminate that possibility. I will also check the tank vent is operating OK.
The fuel lines to the engine are in copper up to the primary filter and the tank was drained and flushed at the start of the season so those two items should be OK. Both fuel filters were also changed at the start of the season.
The stern tube bearing is of white metal with a pumped grease feed - this works well with no play in the bearing and the propshaft does not get hot near the bearing or at any other point.
However, there is no discolouration of the paintwork on the gearbox so it could well be that my problems lie elsewhere. I will check all possible causes before resorting to a new gear box.
Sorry I've been slow in gettting back to you but the problem was only resolved last weekend.
All the suggested items were very pertinent and helpful and I'm in the process of going through the list. The first Item I checked was the fuel tank vent but found that it operates OK.
The cause of the problem was the air supply (suggested by John). There was a slight leak between the exhaust elbow and the rubber hose (wet exhaust). The standard position for the air filter housing inlet is just below this joint and, of course, moisture and exhaust fumes were being drawn into the air inlet. It was a very small leak but enough to cause some rusting in the air filter housing and contaminate the air supply to the engine.
I cleaned up the filter housing, fitted a new filter, directed the air inlet away from the leaking joint and the problem seems to be solved. Last Friday I ran the engine for four hours under load without any trouble. The boat is due to go into the yard next week for the winter lay up so the leaking exhaust, filter changes and fuel line checks will be done over the next couple of months.
The advice received through this Forum has been invaluable and enabled me to solve what I thought was a serious transmission fault with very little effort.
Many thanks to yourself, Stanley and John for the help.
