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Sorry for the redundancy, I posted this over on Boat Diesel.com and I'm trying to use the boat today and need help.


1993 250hp 6BTA, cpl 1322.

I lost an impeller the other day on my port engine. I Shut the engine down as soon as the alarm went off. Prior to this the engine was running great. Cruise would see about 190 degrees normally.

I replaced both impellers and went through the hoses and transmission cooler up to the heat exchanger, (my 250hp is fresh water cooled at the after cooler) thinking no way debris was going to get through the trans cooler. I found some chunks in the trans cooler and flushed all the hoses and cooler, including the hose up to the heat exchanger.

I checked the hull strainer and the seacock strainer which was all good.

Yesterday I went out and that engine started over heating at cruise. I would throttle back and the temperature would drop back 190.

I thought okay, there must be something in the heat exchanger blocking enough flow to overheat at higher RPM. This morning I took the exchanger apart expecting to find some blockage of some sort. Nope, it was clean. I inspected the tubes with a flashlight and they were all clear. I ran water from a hose through and the flow was good. So, I went back and pulled the hoses from the trans cooler again thinking I missed something but they were all clear.

I went out again this morning and the problem persisted. The funny thing is that the port engine was slower (not by much) than the starboard engine getting up to 190, and the EGT´s were lower. Got up to 200 degrees and throttled back. Got quite a bit of white smoke when at 200. It was smoke, not steam.

Engine coolant good, not using any.

What am I missing? Seems like anything outside those issues like a thermostat would be a highly coincidental thing.

Might be too late, but try to see if you found all the impeller veins. If you missing one you know you still have to find it.

If you broke the connections from pump to raw water intake…make sure itstigh5and not sucking air. Also the housing…gasket or coring in good shape…housing bolts tight? Sucking air I a problem


Last thought as I just got back from my boat after changing the thermostat. New thermostat that I changed last year and made a world of difference. This year motor was running warm again. Not overheating,but no the normal 190 either, more like 197. I did changed my coolant and flushed.
…I heard that can cause issue with thermostat so changed tow 160°…now motor is running too cool. Apparently it was the thermostat. I’ll buy correct and change

I would post this problem on sbmar website. That is SeaBoard Marine website/forum They are Cummins experts and you will NOT get better advice anywhere.

That "smoke" is prob steam.

Did you check your seacock? Or thruhull valve??? I dont know what you have and if they are very old there could be an obstruction.

Last year I had a similar problem and it turned out to be the old "gate" valve that I had for my raw water intakes. I replaced both of them with new seacocks

Like you, when I came up to cruise she would start to overheat and steam. Once I figured out that the raw water was obstructed from entering the engine the problem was solved. I needed to upgrade anyway, so I did it.

Thanks for the replies!

I was thinking it had to be a restriction in the raw water side somewhere, and as mentioned was probably a piece of impeller.

On the morning of the 4th, I put a mask on and checked the hull strainer again and it looked good, next I checked the seacock and strainer (again) inside and they too looked good. Then I took a garden hose and shoved it in the strainer and back flowed water through it. It seemed to flow good with no water backing up. Then I checked the elbows going into the muffler which were clear.

Later I had a few people on board and found out I didn't put the cap back on the strainer basket. With the people on board it lowered my water line and flowed water through the cap and into the bilge for a few minutes before I found it and corrected it.

Well, I am not sure what the trick was but no more overheat! It ran perfect (about 190) at cruise all day. I'm guessing it was some kind of buildup in the through hull?

Sometimes it pays to be lucky!