Foamy oil is a worry in 1GM10 motor
Qi have a 30 year old Mg spring 25 with a Yanmar 1gM10 engine and shaft drive. i believe it to be the original engine fitted to the boat but the head was replaced by the previous owner about 10 years ago.
The engine has been totally reliable until very recently when it started to cut out if the revs were dropped too quickly. This could be overcome by dropping the revs gently.
The following weekend the same problem occurred a couple of times but later, once it had been under load for a short while, the power suddenly dropped and white smoke was seen coming from the exhaust.
We were in no position to stop the engine straight away as it was too dangerous to, so we limped out into the river and got sailing as soon as we could.
When i checked the oil i found it had turned into a grey, foamy mess.
Talking with club members and a marine engineer the consensus of opinion was the head gasket had gone, so my mechanic son replaced it. We checked the block and head but saw no signs of any cracks and the valves appeared to be seated correctly. Also the combustion chamber was dry and clean.
We got it started again but it ran for only about 30-35 seconds, hunting up and down the revs until conking out. We saw some smoke coming out of the air filter intake, and removal of the intake showed the same grey foamy oil coming out of the intake port.
Obviously the issue is far more serious than the head gasket. Any ideas? Adam Gray, by email Stuart DavieS replieS The exhaust elbow is a known weak spot. They can block with a salts and carbon mix. This allows seawater to back up into the exhaust port and corrode it. They also corrode and crack on the inside tube of the exhaust elbow and again seawater can get in to the exhaust port.
In extreme cases it corrodes through the wall to the pushrod tunnels and then seawater gets in to the sump – which gives the symptoms you describe.
There are also reports of cracked exhaust valve seats which causes lost compression and raw water leakage in to the internals. So remove the head again, check the exhaust port and valve seat very carefully and also check the exhaust elbow for blockage, internal cracks or corrosion.
This is likely to be an expensive episode: a new head may be needed. There was a four-page feature on a rebuild in PBO January 2018. When looking for parts shop around – some parts could be cheaper sourced from the USA than from the UK.