I often give advice on troubleshooting engines via email, phone and through our forum. Overheating is a common subject that I am asked about. In the case of an engine that seems to run hot or overheat I often ask if the engine has recently had impeller problems. If water flow is cut off, an impeller can burn up or disintegrate. When this happens pieces of the rubber impeller can take up residence in the cooling system and cause overheating issues.

Recently I received a report from a customer on troubleshooting his engine’s overheating. Stay with it as there’s a twist about how impellers can be the source of overheating towards the end.

A while ago, I wrote to you about an overheating problem I had on my Universal M3-20 diesel. I feel compelled to tell you what happened with the overheating.

After tearing every part of the seawater cooling system off, replacing the hoses, replacing (and upgrading the size of) the heat exchanger, replacing the impeller a couple times and going through the fresh water side with a fine toothed comb, I came to the conclusion that there had to be a restriction on the inlet of the sea water pump.

Since I had taken the system apart piece by piece several times, I was certain that it had to be the thru-hull was sized too small for this engine in the temperature water we sail. In a last ditch effort to prove or disprove this, I decided to run a test.

A large tank (read: Garbage can) would be placed in the cabin, a submersible pump would feed that via a garden hose and a volume of water would be maintained in the tank to preclude any restriction in flow. A hose would be taken directly from the sea water pump on the engine to the tank. Since I needed a long hose, I would complete the operation by again replacing all the hoses on the engine.

Since this hose is far from cheap, I decided to start by removing a small section of the hose to take with me to make sure I got the right size and type. The shortest section was the one between the thru-hull and the strainer, so this is the one I removed.

On a whim, after the hose was out, I placed my hand over the top of the thru-hull valve and opened it. There was sufficient water flow to push my hand up! I looked at the volume of water coming in and realized that there was a helluva lot more water coming in than had ever come out the back end of our boat!. I shut the valve and decided to do some tracing. I put the hose section back in and removed the one from the outlet of the strainer. Opening the valve, I was again surprised to see a torrent of water! I pulled the hose off of the inlet to the seawater pump and repeated the process to again be surprised by the volume of water pouring out. Realizing that with the impeller blades blocking the way, I would not see a repeat of this were I to remove the hose on the outlet of the pump, I decided to once again investigate the inside of the pump.

The pump in question is an Oberdorfer with a 90 degree pipe elbow on the outlet and a 30 degree pipe elbow on the inlet. The inside of the pump was as clean as a whistle, and I again expected to see another flood of water when I opened the thru-hull, but instead, there was only a dribble! I had cleaned the outlet elbow on several occasions, but this was a new one on me.

My wife, looking over my shoulder with camera in hand, asked, “what is that in the inlet?” “Oh, it can’t be anything in there. It would have to force stuff BACKWARDS against the flow if there was anything stuck in there.” Just to prove her wrong about it possibly being fouled, I got out an Allen wrench and poked it into the 30 degree bend. Out came a palm full of old impeller parts! After digging them all out, I replaced the hose and opened the valve again. This time I had an awful lot of water. It didn’t take me long to reassemble the engine cooling system and fire it up. For the first time since I have owned this boat, a bolt of water shot out the back of the boat about 6 feet long. We took the boat out on the bay and slowly throttled up. Prior to this, 4 1/2 knots, even in the winter, was about all we could get to. In short order, we were doing just under hull speed with the temp gauge acting like the needle had been nailed in the 160 degree position. Since then, we have upgraded to a 3 blade fixed prop and now reach hull speed, and the temp still doesn’t go above 160.

About now you are probably thinking, “Well, this doofus fried an impeller and just didn’t get it all out!” That would be a pretty fair assumption except that the engine was 10 years young when I bought this boat 2 years ago. At that time, the previous owner told me that he had always had temperature problems with this boat and that I would always be restricted to below 5 knots! I am hoping that between the removal of the “lifetime restriction” and the larger heat exchanger, I will not be temperature limited in this boat, no matter what temperature the seawater in the gulf gets to! The idea of impeller parts getting jammed into the INLET has left several respected mechanics scratching their heads at the end of this story. That, at least, allows me some degree of ego retention.

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