120 Mercruiser cooling oddness

[MarkC]

Hello, I have an original equipment 1969 Mercruiser 120 (the 120/140 sterndrive, and chevy 4cyl engine) in my Glastron V164. The following happened a few days ago---about 10 running hours after a water pump total replacement (just for the **** of it, not to fix a problem).<br /><br />Until this happened (and even on the hose right before this trip), the engine used to hold a pretty steady temperature right in the middle of the green part of the gauge. Now, when I start it up, one of two things happens, both different from before: <br /><br />EITHER: it runs fine, but always higher than the middle of the green section and hotter (nearly red) when idling slowly. OR: it overheats at every RPM, and the rubber intake hose (between sterndrive and thermostat housing) gets hot (meaning, I assume, no water is coming in). When it overheats, if I let it cool down for 10 minutes and then start it up, it might well work fine (it seems random whether it'll happen or not). <br /><br />I pulled the thermostat and put it in a cup of boiling water, and it opened smoothly and quickly.<br /><br />I'm guessing that somehow the water pump in the lower unit is at fault, but I can't understand why sometimes it works and sometimes not, without ever going from working to not-working or vice-versa without restarting the engine...<br /><br />Any advice would be greatly appreciated!<br /><br />Thanks,<br /><br />MarkC

 

[MarkC]

Re: 120 Mercruiser cooling oddness

still looking for ideas---i guess i'll pull the lower end off and check out the impeller.

 

[Walter]

Re: 120 Mercruiser cooling oddness

Did you replace the water pump on the engine or in the outdrive? As I'm sure you know, the hose leading from the stern drive to the engine water pump is the sea water supply. I don't understand how that hose could get hot??<br /><br />If these problems occurred after you replaced the engine water pump...be sure it's the correct one for your engine and plumbed right. I still can't see how hot water could get into the supply hose unless something was hoaky with the engine water pump...<br /><br />If you replaced the outdrive water pump...then I suggest you pull the gear case off and double check everything.<br /><br />You're topic description is correct...this is odd.<br /><br />Walt

 

[Trent]

Re: 120 Mercruiser cooling oddness

Your going to have to drop the lower again. Did you replace the housing and wear plates? If not you need to. Check to make sure you got a good connection on the water tube. Also check the "heater hose" that runs from the drive to the transom plate (outside the boat) Look for a collapsed hose or split in it.

 

[MarkC]

Re: 120 Mercruiser cooling oddness

Walter, I replaced the water pump in the sterndrive. I'm guessing the supply hose gets hot just by hot water migrating out of the hot engine, but i hadn't considered the engine water pump... <br /><br />Trent, yes I had replaced the entire water pump, including the base, seals, plates, impeller, and housing.<br /><br />Thank you both very much for your advice. I'll look at the hoses and pull the lower end tomorrow, and I'll report back!

 

[Capt Ken]

Re: 120 Mercruiser cooling oddness

Mark, when you drop the lower unit, do this. Check the copper tube that goes from the pump to the upper gear housing. Sometimes the tube gets pushed too far into the rubber grommet on the topside and it doesn't fit tight enough into the grommet on the pump housing when you put the two together. I always like to pull the tube out and replace the rubber grommet on the top just in case. This also gives you a chance to check the tube for holes which is possible in a unit this old. Kick the outdrive to the right and check the 5/8 heater hose going from the outdrive to the gimble housing. If its never been replaced, I bet its pretty dry rotted by this time.

 

[MarkC]

Re: 120 Mercruiser cooling oddness

Thanks, Ken! That hose is wrapped in some kind of black tape that doesn't look so old---maybe it's been replaced (I've had the boat for 2 years)?<br /><br />I've pulled the lower unit and here's what I've found so far. <br /><br />-- The plastic guide tube had melted to look like a candlestick holder! Caused by the hot exhaust without water, I imagine.<br /><br />-- The metal lining of the upper housing of the water pump (a can-like thing) was apparently drilled wrong, and came in contact with the drive shaft. This melted some of the metal lining and some of the housing plastic, and the lining partially warped, squeezing the impeller from above, which made the impeller a bit burned on top and bottom, and melted the base a little where it hits the impeller. Also, the melted metal of the lining WELDED ITSELF onto the drive shaft, so that I couldn't even get the impeller off without a great deal of work to remove the extra metal. Sounds pretty dramatic, but nothing was damaged all THAT much, in terms of how much material was removed or how far it was twisted.<br /><br />Anyway, I'm hoping that the situation is, the damage allowed air into the pump so that cavitation could happen, and that's the whole story. I've ordered a new waterpump, base and all, and I hope that my supplier (The Shad Connection) will send it free in the circumstances. Anyway, I'll keep you posted!