I have a little MD5A 7.5hp (late 70's) in my 26 foot keeler, which has recently started overheating
after owning for 6+ years (incl. partial rebuild).
Very quick recent history
The boat had been out of the water
for a year, during which time I ran the engine
3-4 times to keep everything coated in oil
etc (Cooled with water
hose). Still starts perfectly, on one or two turns (clearly very good compression).
After relaunch, but before going further than the mooring
, I replaced the thermostat housing, cooled exhaust elbow
, and oil
(heavier grade - 20W50 + Moreys). The thermostat housing and exhaust elbow
both are custom, but I paid close attention to not changing or restricting flow. I also pulled off and refitted the head
, as I suspected a fuel
leak from the injector nozzle sleeve.
Under idle, the engine
sits at a healthy (for raw cooled!) 60-65 degrees, if left alone. Under moderate - high load, it climbs as high as 95degrees celcius (bad), or 80-85 degrees at lowish load. Once it's gone high, it doesn't really want to come below 80 again, even if left at fast idle in neutral for 5-10min (don't want to do that too much anyway).
I have checked the water inlet and poked through it - no obstruction. Replaced the impellor (old one was fine). Replaced the thermostat (old and new tested fine in hot water - to specs). checked the head
for obstruction - clean as a whistle. Taken out the block drain-cock (lowest point of cooling
circuit), a little bit of black crud came out with poking, but it was not blocked at all.
Run through a descaling liquid in a closed loop for about 20-30 min or so. checked block for hot spots with thermo laser - nothing.
The whole time the exhaust seems to still be spitting out plenty of water.
Engine has minimal smoke, little bit of greyish smoke when cold, virtually gone when warm - not worse than previously to my memory.
What else can I check?
Is there any scenario where the head gasket
could be at fault? eg water in to cylinder on downstroke, or gas in to the water channels? Doubtfull as it starts so very easily.