1996 Merc 4 cyl 40 HP repeat overheat problem - Why?

[wharfcreek]

I have the subject engine on a small boat, and am now having what is I believe my 4th incident of engine overheat. What I find odd is that the engine will be fine and then just acts as if the water pump failed catastrophically....vs any kind of indication of a slow deterioration. I was out in my boat last Friday evening for perhaps 3 hours......trolling around in the Chesapeake near Baltimore, MD where I live. Then, went to go out Saturday and idles out about 1/4 mile from my house and noted that engine was sounding funny. When I turned to look at it I could see 'vapors'....so I shut it down and pulled the cover. It was smokin' hot!! This is the 3rd or 4th time this has happened like this......one day fine, the next day...nothing!! AND, last time I replaced the impeller, it STILL wasn't pumping water through. Then, just as if someone waved a magic wrench over it...... it started working.....and worked for nearly a year. Now...back to the same thing. I just removed the lower unit, and in reality the impeller looks pretty 'normal' to me. Clearly, I'll replace it again, but I don't see a problem with it, the housing, the lower plate or gaskets, the key was in place, the tube was fine, I don't see any blockage in the tube....and I'm told the engine doesn't have a thermostat.....so I'm thinking that can't be the problem either. Can I simply have a 'clogged block'? And if so...what does one do about that? How do you tell? Can you 'back flush' the block in some way? This is the C40ELPTO engine.....40 hp / 4 cyl, 1996....and aside from this, it's been a great engine. It did catch fire when the voltage regulator failed....but, hey, that's why the CG makes us keep fire extinguishers, right? Right!! Any help or suggestions here is really appreciated. Many thanks, Tom D.

 

[SoCalBoatGuy]

I'm no expert on all engines, but there could be some old chunks of impeller floating around inside the block that are intermittently clogging cooling passages. On my '87 50hp, I pulled the water jacket cover off of the back of the engine and there were bits of impeller wedged under the bottom cylinder. You can also pull off the exhaust baffle cover and access water passages in there.

Also, my engine gets pretty hot when it's idling. The water pump just doesn't send enough water through at low speeds. Does this always happen when you are trolling?

 

[gm280]

wharfcreek, do you know if your engine has any baffles in the water passages? I ask because "It did catch fire when the voltage regulator failed" stands out to me. It could have gotten so hot at that time that is melted a baffle in the water passages. When you replace the water impeller, try running water through the water tube installed to the power head of the engine first to see if it comes out with a nice flow. If not, you could have something blocking the water flow through the engine. I also would purchase the original factory engine specific shop manual and it will have ever thing you ever wanted to know about your engine. JMHO

 

[JDusza]

Hey.
Unfortunately, this is going to involve a teardown; Lower unit has to come off, and/or the power head needs to be opened.
Start at the pump with the pump test. I attach a drill to the drive shaft and look at flow coming out the water tube at the top. Couple things to check here. Pump function and condition of seals at the water pump and the top of the water tube entering the block, the sesls at each end of the water tube. These little black seals are notorious for folding and leaving floating pieces. From there, you'll have to get into the water passages inside the block. Try pushing compressed air through the block. Or try pushing a slug of water through.
Hope it helps.
J