Engine overheat above 2,200rpm
Q
I have a Birchwood 29 Sedan fitted with a Mercedes OM636 engine. It has a Borg Warner Velvet Drive gearbox, ratio 2.10.1, driving a 15in pitch R10 propeller. My problem is that when I go over 2,200rpm the engine overheats. So far I have fitted a new impeller, cleaned out the gearbox oil cooler and all the water hoses, and also cleaned out the heat exchanger. Thereafter, I removed the cylinder head and had it pressure-tested, but the overheating still persists. I was wondering if fitting a smaller pulley to the intake water pump would solve my problem? PAT MANLEY REPLIES:
The water-pump pulley, unless it has been changed at some time in the past, will be correct and designed to run the pump at the correct speed. There is no need to change it. An engine of around 40 years old may well have its internal waterways corroded, but this is probably not too much of a problem as long as regular changes of antifreeze have been observed. There are a number of potential reasons for the overheating: presumably you just have an overheat warning light and no gauge?
■ The overheat switch may be malfunctioning: it will be difficult to check directly unless you buy a new one. You could however try feeling the thermostat housing as the engine nears the overheat rpm, and again when the warning activates. Once the engine reaches normal running temperature the thermostat housing should maintain a constant temperature. Ensure that you do not get any clothing or body parts entangled in the engine! An infrared remote operating temperature gauge is very useful for this type of job.
■ The thermostat may be malfunctioning. Remove the thermostat, place in a saucepan of water with a suitable thermometer and bring the water to the boil, observing the operation of the thermostat and the temperature at which it opens. The correct opening will be printed on the thermostat.
■ The engine raw-water intake may be partially blocked. Check the intake and water strainer for fouling.
■ The water-pump front and/or back plates may be worn so that the impeller is not gripped properly, reducing the pump output. Also, the pump cam that compresses the blades may be worn, likewise reducing pump output. A new or reconditioned pump may be the answer to this, although spares may be available for a DIY solution.
■ The exhaust injection bend may be partially blocked, especially round the water injection point, which can reduce the water flow. The bend can be removed and cleaned or replaced as necessary. ■ The exhaust pipe/silencer may be partially blocked. This is more likely to produce too much back pressure rather than reduce water flow.
■ The prop may be too big. This could cause engine overloading – your prop is slightly undersized, so this is not a problem unless the prop is fouled.