I have volvo penta md6b inboard diesel. When I would start it up it
would run right up to 2200 rpm's when in neutral. But when put in gear
it would only go up to 500 rpm's and take about 15-20 minutes before it
would go up to 1500 rpm's.Max rpm's shoould be about 2400. Then one
day I accidently left I forgot to open the raw water inlet value and
overheated the engine. I had a so-called mechanic come out and told him
the situation. He found water in the oil but said it was not a head
gasket problem but was due to water back siphoning through the exhaust.
He changed the oil several times to get the water out and said it
should be fine. When I went out again I still had the rpm problem so he
never even addresed that correctly. I decided to dive the prop and
removed and incredible amount of barnacles. However the rpm problem
persisted and I checked the oil again and found WATER in the oil AGAIN!
The engine does not overheat but am I right in thinking it is the head
gasket? If so are there any manuals that explain how to replace the
head gasket. I'm tired of paying overpaid mehanics who don't know what
they are doing and don't show up half the time.

thanks for any help