ONAN MDJE coolant leak

[Genevatexan]

I have some antifreeze coolant collecting in my engine pan below the ONAN MDJE 7.5. She will run for 4 or 5 hours and then shut down for overheat. We add about 3 cups of antifreeze to the tank, and away she goes. I have checked all the hoses on both sides of the water system and I cannot find any leaks. I did notice however, that when I shut her down last week that the overflow tube at teh radiator cap spit a small amount of antifreeze into the engine pan. Questions:
1) Should the radiator cap not allow that, and if so, can I assume the cap is bad? 2) If it is supposed to release some due to pressure should I add a recapture bottle? 3) Where else could I lose coolant into the engine pan if none of the hoses are leaking?
If I am also losing into the heat exchanger and sending it out with sea water, can the core of the heat exchanger be disassembled and repaired on this generator? there does not seem to be an easy place to access the guts...
Thanks,
Genevatexan

[dottieshusband]

My survey says....fresh water circulating pump.. Check the bleed hole at the pulley shaft..

[bigbill]

it should not spit out at the cap if everything is o k.. you may have a bad cap, or the unit is getting hot and building up too much pressure and bleeding off the excess pressure through the cap. check the belt tension on the fresh water pump.

[doc g]

if the coolant is not visible anywhere , as in under the genset, it is leaking from a perforated heat exchanger or a bad head gasket . These are the only two places I know of that you could lose coolant and not inside the boat. If you leave the cap loose and therefore the system unpressurized do you still lose coolant?.........................Pat

[doc g]

there is also a way to test if salt water is in the coolant side but it escapes me right now.......Pat

[dottieshusband]

Clean up the coolant in the pan. Refil the reservoir. Leave the front cover off the sound shield. Line the pan with something light in color, and absorbent... Start the Genny, and put a bright light on it.. It's dripping into the pan from somewhere. I had the same issue on my Onan, and like I surmise yours is, it was leaking from the weep hole for the freshwater pump. But where ever it it is leaking from, the drip path will point you the right way. It's hard to see the weep hole behind the belt pulley... But, that's where my money is.

[bobk]

The weep hole on the pump is a good possibility. I like to tie a wad of paper towel over it and check to see if it gets wet. If so, that is one source. Others may still exist. Does the HE have a zinc? If not or if it has not had regular changing, you might just have a perforated HE. It should be easy to remove and have tested.

Bobk

[Bob Bradley]

Auto parts stores sell coolant additives that are picked up by IR light. I used that approach to track down an antifreeze leak on my mains a couple years ago. It's a very small amount (1/2 oz or so) and several years later, it is still very present in my engines.

If you decide you need a new heat exchanger, I have two from my old MDJE available.

[Genevatexan]

Good feedback. Will get out there Saturday and check weep hole and other risks. Parts diagram shows zinc on left side, will track down and check. I like the ideas of getting a light on it and material under to highlight leak and bit of paper under weep hole.
Btw, raw water pump turning well, impeller good, water volume out the hull exhaust good. Since coolant collecting under engine, had assumed not compromised HE, because I expected in that case the water to go into the exhaust and out the hull, not under jenny.
If it is HE, though, I may could use one of those extras....
Cheers,
GT

[Finalee]

Another common thing is impeller disintegration. The pieces will plug the HE and restrict flow even after a new impeller is installed. That leads to overheating which causes pressure build up that comes out through the cap.