I too have Volvo
MD7A with salt water cooling
and a heating
problem. The boat is quite far, far away from both me and parts
sources so I need to figure out what I have to buy to bring with me to fix her.
runs perfectly with one exception, it occasionally overheats and I can't figure out what the heck is going on. I bought the boat used and the previous owner had fitter her out himself. Every time she overheats, I find something to "fix" after which she is fine for a spell then she will overheat again. Clearly I'm not getting to the root cause. Generally she will seem to be running along just fine and all of a sudden the pressure gauge just goes up, up, up. It seems to be a bi-state kind of thing, it either works perfectly well or not at all. Its almost as if there is a ball valve or something somewhere that is sticking. It's obviously not a bad gage.
goes like this:
Tap to the transmission/engine
Tap to secondary pick up from bilge
Tap to sinks
Here is some of the history
and what I had done, in each case I have also checked/changed the impeller. They usually look just perfect.
1: Recently fitted salt water
strainer seemed to be blocked with debris from its construction. It is home built out of some interlocking pieces of PVC pipe, the inner piece has a zillion small holes drilled in it. I dumped this stuff out and she was fine.
2: Found a small leak in the copper pipe from the transmission
to the impeller. I thought that maybe it was sucking in air and the impeller was getting air bound. Repaired the leak.
3: Moved and lowered the salt water
strainer. This was mounted pretty high on a bulkhead with real long hoses. I moved it to the engine
compartment and lowered it. I thought that maybe there was just too much suction head
to get the water going.
4: Threw thermostat overboard