IF YOUR FUEL COOLER LEAKS DO NOT TRY TO REBUILD IT, JUST REPLACE IT!
In viewing these posts I can't believe I have been screwing with my fuel coolers since 2005. What a waste of time and money.
The cooler on an 8-71 and a 6-92 are the fat round jobbies surrounding the raw water line as the last thing on the engine before this raw water is injected into the exhaust line at the showerhead. The 6V92TA cooler cools two things, first the returned fuel (about 2/3 of the fuel sent from the fuel pump to the injectors is used to cool the injector rather than being burned, then this hot fuel is cooled and returned to the tank.) Behind the fuel cooler portion of this jobbie is the transmission cooler which takes the high pressure oil from the transmission and does the same thing. Both coolers are a distinct series of horizontal small pipes looking like a honeycomb through which all the hot raw water of your engine passes on its way out of the boat. My fuel pressure is about 60 psi going to the injectors, probably a lot less returning to the tank. My transmission pressure is zero in neutral, but 250 psi in gear. I didn't think water could get into these fluids against these pressures, but I was wrong. When my "new" engines were sea trialed in 1993 the raw water pressure varied from 10.5 psi at 2300 rpm down to 5 psi at 1200. No measurement at idle, but even at low rpm the raw water pump seems to be pushing a lot of water. There is bound to be some pressure there.
So way back when I had my stbd. fuel cooler rebuilt. Fixed that problem. Last year, in the Exumas, I was doing my daily pre-start engine fluid check and find nothing showing on the stbd. transmission oil dipstick. Added a gallon or two of engine oil, started engine, no apparent leaks, nothing in the bilge. Went through 10 gallons of oil getting back to the US. Sent cooler out again for a rebuild. First shop said it was fine, but I learned they only could test it to 60 psi. Second shop found it leaking at 250 psi, allegedly rebuilt the cooler. With new oil boat went about two miles each way to get bottom painted and new zincs. Two days later I arrived to go to the Ocean Reef Vintage Weekend. Checking the stbd. transmission I found it was white, with the emulsified consistency of toothpaste. My first thought was #@%&*##@, followed by, "Well, there goes Ocean Reef." By replacing the 23 year old cooler (air freight) and flushing much lube oil through the transmission until it came out clear we got to the show only three days late.
My conclusion is that water can pass from the raw water line into the cooler, small amounts into the fuel, probably larger into the transmission because this can have zero pressure in neutral. Hot sea water erodes the cooler's little tubes and once this starts happening the best thing to do is replace it. The cooler does not look like it comes apart. The paint looked intact so I don't know how it was rebuilt. I am hoping it did not ruin my transmission. Twin Disc recommends removal and complete rebuilding. Since the water was only in a few days I will see what happens this spring. One marine engineer recommends replacing all the clutch plates in the forward and reverse linkages as he says the material will begin delaminating by next year.
I have had better days.
