Ahh, this brings back memories. Our MD2B
was configured identically. In the reverse gear/transmission there isn't much heat, our experience was that it was growth of marine
life rather than mineral scaling.
For mineral scaling you can use vinegar but it will take a long time (if you can keep it hot that helps) or muriatic acid (available at a hardware
or pool supply). You have to use more care with the acid, both for yourself and the equipment
. If you can figure out a way to circulate either that will help speed things along. Alternatively, a commercial
de-scalant (ALPACON DESCALANT, Alfa Laval
), while harder to find may do a better job.
growth we were able to rod and wire brush the water passage
in the gear. Get yourself a flexible wire brush about the diameter of the hose and push it through. Takes some work, but it can be done. The coolant
housing is a loop, so the curves make things difficult, but if you work from both the inlet and outlet side you can usually get it cleared out. Make sure you flush it well so that your loose crud doesn't end up in the engine passages when you re-connect things.
I would recommend also looking at the water passages in the exhaust elbow
at the same time. It's pretty easy to do, and even if you have good water flow now if the tranny is plugged then it is likely the elbow could use a little cleaning
Outwest (and anyone else confused),
On the Volvo
MD series with raw water cooling
the water comes from the seacock, passes through cooling
channels in the tranmission/gear, and then goes to the water pump inlet. As a result the tranny cooling is on the suction side of the pump, which means there isn't much head
available to overcome blockages (such as barnacles/muscles that decide to grow in there). The OP took the tranny out of the loop and everything was then OK with the engine (but the reverse gear is no longer cooled).