Exhaust Pressure in the cooling system

[narnone]

I have a problem with a 1990 OMC 351 open cooling system with the impeller in the outdrive. I am getting high pressure exhaust into the cooling system. This happens at RPM?s greater then 2500. It is blowing back the water though the water pump in the drive and destroying the impeller.
I am not getting water into the oil and the engine runs great. There is no white smoke out of the exhaust. I believe that rules out a head gasket. There is no water getting into the cylinders. The engine start right up. I believe that eliminates a crack in the exhaust manifolds. If there was water would enter the exhaust section of the manifolds and end up in the cylinders. Usually locking the engine. The bellows are getting burnt. The Exhaust is getting to the ?Y? pipe, but the water is not at high RPM?s.
The engine has gone through 3 sets of impellers and exhaust bellows.
I placed a pressure gauge on the hose doing the dump into the manifolds. AT higher RPM?s I am getting 10 lbs of pressure at RPM's greater then 2500. And there is no water. I believe this to be exhaust pressure. The water hoses going to the exhaust gets very hot when the engine starts to over heat. I do know that the impeller is just about done. We burnt another one while testing.
I believe my problem is the flappers in the exhaust have become dislodged and are placing a big restriction on the exhaust. This causes the exhaust to push the water out of the system and blow exhaust out the water pump.

Anyone have any other ideas?

 

[bruceb58]

Re: Exhaust Pressure in the cooling system

Put some clear hose in the hoses that go to the manifolds and see if you see one side backing up more than the other. Might also put one in the feed to the thermostat from the outdrive.

 

[45Auto]

Re: Exhaust Pressure in the cooling system

I'm not real familiar with the OMC set up, but I bet it's got some kind of rubber radiator hose going from the outdrive pump to the engine. I would expect the hose to burn up at the same time as the impellor.

If the intake hose isn't burning up, I very seriously doubt if you've got exhaust gases going back through the water pump. My Mercruiser runs at over 20 PSI on the water pressure gauge at mid-range engine RPMs. A couple of pounds of back pressure is enough to choke an engine (old banana in the exhaust trick - 20 PSI in a 3" exhaust pipe would be over 550 pounds on the banana - the engine isn't capable of putting 20 PSI into the exhaust or it would blow the banana out).

I would look for restricted water flow burning up the impellor, then immediatly overheating the exhaust. Doesn't take but a couple of seconds under load without water to burn up all the rubber in the exhaust system.