Onan Cooling

On my Onan 7.5 MDJE, the fresh water coolant isn't flowing.* I checked the stat, it has already been remove by orig. owner.* Then, I remover/diss.*the water pump; everything looked O.K..* I blew thru the hoses to check for blockage; everything seems to be open.* What am I over looking.

How do you know your fresh water coolant isn't flowing?* When you run the generator with no load and no thermostat it should be around 155-160 degrees.* With 60-80% load it should be around 185 or so.* If your unloaded temps are running high and the previous owner has taken out the thermostat,* there is a circulation issue.* Good luck.*

I changed the coolant when I bought the boat a few months ago. The gen has shutdown a couple times since; high temp. I check the coolant an each time the tank is half empty. When I refilled it this time I left the cap off an watched for flow in the tank; none. I'm just guessing there should be some flow like in an auto radiator. Oh, my gen has only an amp and oil pressure guage; no temp. guage. My Onan man. doesn't show where I can install one.

The Onan cooling system apparently had several configurations through the years it was manufactured. There is typically an engine-driven pump at the bottom right corner of the unit (when looking at the injector side of the engine) and a belt driven pump on a sort of platform higher up on the engine. On our MDJE the engine-driven pump-- a Jabsco flexible impeller pump--- is the raw water pump and the belt driven pump-- a metal impeller pump--- up above is the coolant pump. But I have heard of similar setups that were reversed: coolant pump down below and the belt driven pump the raw water pump. What is the arrangement on your MDJE?

Similar symptom with an MDKD was caused by a stopped up heat exchanger.

Mine has a Jabsco flex impeller on the bottom for raw water heat exchanger and a belt driven/metal implr pump for the eng. coolant. I rodded out the heat exch. best I could; blew through the intake pipe an had a fair amt. of air flowing. I have been unable to find any restrictions anywhere on the eng.; so far.

Sorry to keep asking questions in answer to your question, but I'm assuming the raw water side of the cooling system--- the salt water intake, pump, heat exchanger, muffler (if there is one) and exhaust is working as advertised? You have good raw water flow out the exhaust outlet in the side or transom of the boat?

dhiggins wrote:
Mine has a Jabsco flex impeller on the bottom for raw water heat exchanger and a belt driven/metal implr pump for the eng. coolant. I rodded out the heat exch. best I could; blew through the intake pipe an had a fair amt. of air flowing. I have been unable to find any restrictions anywhere on the eng.; so far.

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On an MDJE 7.5 kw your raw water pump should be a gear driven Onan #131B152 pump, rubber impeller. *The coolant circulation pump is an belt driven Oberdorfer P50 or P51. *The Oberdorfer is a slinger style pump and as long as the bearings are tight it should work fine. *I have the details for both if you need them. *I had a similar problem on mine and the heat exchanger, tube stack was gone. *The coolant is probably going out through the exhaust. *

Take the exchanger to a radiator shop and have it pressure tested. *The heat exchanger replacement is available through Seacamp (AKA. The Manifold Wearhouse) on line. *

When you get tired of rebuilding that POS Onan gear driven pump, which is getting difficult to find parts for and expensive when you can. *Throw it away and put a cover over the PTO. *Buy a nice Jabsco brass 3/8" inch belt driven pedestal pump and mount it on the opposite side of the generator from the Oberdorfer pump on the same platform, get a larger drive belt and you are good for years to come.

Larry--- Good suggestion about the replacement for the raw water pump when it dies. I'm going to make note of your idea in the front of our MDJE manual. Thanks.