Even though this was not an issue on Detroits but 1400hp Cat 3412Es, it could happen on any diesel so I figured I d post here too

For the past few months, I ve noticed the stbd tranny running a little warmer than the port at low speed only but 15 59 20 deg. On plane, temp would come down and both would be close

I checked the obvious ( strainer, impeller, debris in the cooler inlets...) and couldn't find anything. Engine coolant temp was fine. It was one those things that wasn't broken enough to start doing expensive things like pulling coolers etc

Then one day after a start i realized the raw water pump cover was heating up and not priming. That was quickly addressed by re priming thru a plug just down stream but clearly there was a problem

Next day i did some testing and with nothing else running in the ER i heard a gurgling sound when shutting down the engine, coming from the raw water pump area.

Took me a couple of hour to find the cause... Turned out to be a very small hole in the fuel cooler which on these engines is on the intake side of the raw water pump. It was almost impossible to see being in a hard to inspect part of the flange and at first it just looked like a paint chip.

Being on the intake / suction side, it wasn't leaking but would cause water to drain back out the thruhull. It wasn't big enough to cause overheating, just a slightly warmer tranny at low speed only.

A bit of 4200 on the hole temporarily fixed till a new cooler could be ordered. Sure enough, both trannies are now running at the same temp.

So when all else fails, listen carefully and turn other things off if you have too. Air con compressors or a generator would have covered that sucking sound...

And if you have start/off switches in the ER, it s a good idea to use them, at least for the first start and the last shut down of the day . Starting from the ER also recently alerted me to a starter that was about to go instead of being caught later with a dead engine. I ve also developed a habit of touching the raw water pump cover after the first start of the day