50 GL Overheating

"My 2002 5.0 GL has developed

"My 2002 5.0 GL has developed an overheating problem. Two months ago I had the raw water pump impeller replaced as normal maintenance. Boat ran fine before impeller work with no signs of overheating. I did not get to run the boat after the impeller replacement and had to leave the country for two months. Now first trip out and it overheated a few minutes after starting. I have (carefully) loosened the raw water discharge hose with the engine idling and a good stream of water comes out so it seems the raw water pump is working OK. The boat is on an over-water hoist so I have checked the water discharge from the stern drive exhaust ports while the water intake is submerged. There is no water discharge while the engine is running, but there is a normal discharge from the exhaust ports if I run water from a garden hose to the engine flushing connection. The first flush water that comes out does have what appears to be fine black solids in it.

In reading other posts I gather the exhaust manifold water passages could be clogged. The boat is run in salt water (Galveston Bay) and the manifolds have not been replaced or boiled out. However, the discharge of flush water and the fact that both sides have suddenly stopped gives me hope that perhaps the problem may just be a stuck thermostat. Would a stuck thermostat give this kind of symptoms? I have not pulled the thermostat housing yet so I am not sure what the water flow arrangement is.

Have out of town guests coming in for fishing over the Memorial Day weekend, so I would appreciate troubleshooting assistance and hopefully quick fix ideas.

Thanks for the help."

"Tom, even if the thermostat w

"Tom, even if the thermostat was stuck, there is a bypass hole in the thermostat housing that would allow water to reach the exhaust manifolds and the sterndrive exhaust. If you are sure that the raw water pump impeller is OK and was installed correctly, your best bet would be dismantling the thermostat housing and inspecting it internally, as well as the thermostat."

"Not sure where the water was

"Not sure where the water was coming from when I first tested the raw water discharge, but more testing by removing the raw water discharge hose from the thermostat housing shows that the raw water pump is not delivering any water.

Is it necessary to prime the raw water pump somehow? I have read that the impeller can be damaged by even very short periods of running without water. I have tried to prime and check the raw water suction line using a garden hose and flushing "ears" on the sterndrive water inlet, but the ears do not seal well enough to push water up to the pump inlet.

Looks like my next move will be to remove and inspect the raw water pump impeller.

Thanks for the quick response El Pescador."

"Tom, years ago I learned that

"Tom, years ago I learned that it takes 7 seconds to ruin an impeller if the pump is ran without water. There is no need to prime the pump, as it is of a positive displacement type pump; but one thing that VP recommends is to coat the pump casing with non-petroleum based lubricant (such as glycerine) when installing the new impeller, so that the pump can safely run a few seconds without water. One thing you may want to do is, instead of using the "ears", install a large pail under the drive and fill it with water from the garden hose until the cavitation plate level, then keep filling it while you start and run the engine."

"Removed raw water pump impell

"Removed raw water pump impeller and it did not look bad. Very minor wear but nothing that looked like it would cause the pump to not function. Replaced the impeller anyway and used plenty of glycerine when putting it in. Also put electrical tape over the joint between the plastic hose connections and bronze pump housing in case there was a leak at the joint. Even pre-primed the pump before starting the engine using the flush connection, but the raw water pump still does not produce any flow when the engine is running and the stern drive water intakes submerged normally.

Have tried to check for air leaks on the suction side of the pump. Using ears pressed against the stern drive I can get water out of the pump suction and discharge with no signs of leakage from unusual places. Have also tried taping over the sterndrive inlet holes and sucking on the pump suction hoses. Section from pump inlet to oil cooler seems to hold suction pretty well while the section from oil cooler to stern drive inlet does not seem to hold as well, although it is hard to tell if it is a leak in the line or just a poor seal where I am trying to suck on the end of the hose.

Looking at sterndrive diagrams there are a lot of connections in the water inlet path between the inlet ports and the transom. It also looks like you have to disassemble the sterndrive to get to most or all of them. Am still hoping for ideas on how to positively identify and fix the problem, preferably without major expense and downtime.

Thanks again for your assistance and ideas El Pescador."

"Tom, the only major component

"Tom, the only major component I see between the transom shield and the pump is the oil cooler plus the hose fittings. Have you checked if something is blocking the oil cooler, or if some clamps are loose, or if a section of suction hose is somehow disconnected from its fitting?"

Oil cooler is clear as I can e

Oil cooler is clear as I can easily blow through the raw water pump suction line from the pump inlet to the inlet ports on the lower unit. All hoses are connected and clamps are tight. When flushing through the garden hose flush connection water comes out what appears to be an exhaust port at the bottom of the outer transom plate and from the water inlet ports in the lower gear housing. There is no sign of leakage from hoses or connections when flushing. Flush water coming out the water intake ports does seem to indicate that water is able to flow fairly freely backward through the raw water pump when it is not running. That surprises me some as I thought the pump would seal better in its housing.

I continue to be baffled as to what the problem is. Thanks for continued help.