Volvo Kad 42 overheating

[Keithleask]

Have an overheating problem on the stbd engine, KAd 42 B. I bought the boat a couple of years ago and when on sea trial the stbd engine overheated. The owner had it lifted to check the intakes on the legs , all ok, changed the stats , x2 , still overheated , changed the impeller although he said it had been changed only several months previously, sea trial again and all was ok. The problem has returned again, it overheated when I was out on the Medaway only keeping to the speed limit so not a lot of load. Tricky getting it back thro the lock on an ebb tide with no power steering.When I took the impeller cover off the impeller had broken up. I took off the hoses and retrieved what looked like most of the bits. Fitted a new impeller run it up on the pontoon at around 1000/1500 rpm and it overheated again after about 30/45 minutes. The raw water pump casing was hot , I opened the filter top and sea water poured out , the header tank had a fair bit of pressure in it, so today I took off the heat exchanger to see how bad it was or if it was blocked with impeller debris, it looked remarkably clean really, though I cannot see if any of the tubes are blocked. One end plate is bolted , I am not sure if the other end actually comes off or not? I was going to soak it in vinegar and water but I need to know if the tube body actually comes out from the housing ?
The riser legs should be ok as I fitted new exhaust elbows last August and new bellows, the tops of the legs were a bit corroded but I fitted a volvo repair kit after cleaning up the tops of the risers so I am doubtful if the legs can be at fault? I am dreading the possibility that it might be a head gasket, is it worth using a flushing agent first to try and clear a lot of the crud out. The engines are circa 94 with 660 hours , very little use since August last, any help would be greatly appreciated, thanks

 

[aquatom]

I would take a look at the inside of the sea water pump. Check the inside of the cover plate and the back face of the pump body. If there is deep soaring the pump won't prime its self and will run dry causing the impeller to break up. It can happen if sand has been sucked up through the pump.

 

[andy59]

If the raw water pump is getting hot then I would say that its a raw water flow problem , blockage somewhere / collapsing water pipe on the suction side ? If you get a slight hiss of pressure from the header tank a day or so after the engine has run then that gives a reasonable indication of a watertight system . Have you checked water pump face plate / back plate for wear ? . Good luck with it . ( must type faster )

 

[Keithleask]

Hi thanks, I will check the pump tomorrow, I have a spare that I rebuilt after a seal went , that one looks good internally, I did notice the cover plate of the one in situ does have scoring marks / grooves in its face I didn't think it would make so much difference ?

 

[[2068]]

A badly scored cover plate can make a massive difference, because it can cause the pump not to self-prime.

 

[Keithleask]

Ok I see , thanks let's hope that's it then ?