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.
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.