Chrysler 318 cooling problem Sherwood D60

"My Sherwood D60 (double i

"My Sherwood D60 (double impeller pump) is pumping only on the rear impeller. Pump is rebuilt w/ new seals and impellers, engine is RWC, boat is on trailer.

Forward pump has good suction when I disconnect the inlet hose. There are no blockages in the intake - when I pressurize the intake with a garden hose, water flows freely through both hoses into the pump. No impeller pieces, or any other crud in the system.

I think I have narrowed the problem down to the cast inlet "manifold" below the pump which distributes raw water to the two impellers, and also receives water from the thermostat housing.

raw water intake manifold"">

The lower left hose is raw water in, the next hose from the left feeds the aft section of the pump, the next feeds the forward pump, and the hose with elbow on the right is the return from the thermostat housing.

Raw water enters the manifold, but only wants to go the rear pump route (when I route that hose to the forward pump - then the forward pump makes water).

I am suspicious of the thermostat return hose - farthest to the right in the picture - that enters the manifold right below the outlet that leads to the forward pump. When I disconnect it from the thermostat housing, the forward pump pulls hard against my hand, but still will not pull water.

I'm out of ideas. What have I missed? Thanks/john"

"I have had the same problem,

"I have had the same problem, and actually overheated my engine slightly, as the forward pump would not pick up a prime although the exhaust manifolds were cool,and water was observed exiting the exhaust. This happened after winter storage, and I suspect that perhaps an air lock had occured. I "solved" the problem by pouring water into the thermostat return hose and with the supply line to the engine removed, noticed that the forward pump immediately pumped. I reinstalled the hoses, tapped the thermostat housing with a hammer, and didn't have any more problems throughout the season.

Perhaps the problem is related to the 'stat.

Anyone else?"

"OK - that helped. I removed

"OK - that helped. I removed the hose between the two molded elbows in the thermostat return run, and inserted plugs in each elbow. This ensures no air (or water) can enter the pump from this source. Now the pump moves water through both sides.

plugged"">

In the process of burning as much cash as possible to solve this problem, I replaced the thermostat - not easy to find, got a used one that looked pretty good. It seems that whether the thermostat works or not, though, it would not by itself introduce air into the system. So either there is enough air in the engine to cause the pump to lose prime, or the pump is drawing air through the hoses that are meant to discharge from the thermo housing to the exhaust elbows.

thermo housing"">

There is a check-valve between the thermo housing halves that looks like it is meant to prevent that. It seems to be functional, but maybe that is my next spending opportunity?

Any additional ideas greatly appreciated."

"I have the same set up on min

"I have the same set up on mine, every year at first start up I have to prime the pumps by filling that bypass hose, with the sea cocks closed, then reattach the hose , open the cocks & start the engines. I also put some dish soap in with the impellers, helps with a dry start, plus the bubbles usually show up at the transom before any water does"