1972 Mercruiser 165 overheating

[Ike-110722]

Took the boat out today for it's second launch. I have been rebuilding this boat for 8 months. I just replaced all the gaskets and bellows, had a new shift cable put in, so I wanted to check it out. Ran it around slow for about 15 minutes, everything seemed ok. I didn't take it over 1200 rpm, then the engine suddenly started over heating, put the needle on the temp gauge way into the red. Shut it down, took it home on the kicker.

Since I haven't replaced the impeller yet (the boat was sitting unused for years, maybe decades before I got it) I am assuming the impeller needs replacing, and while I'm at it I'll replace the water pump on the engine, and the thermostat.

Any advice would be appreciated.

This is the 6 cylinder in-line (GM 250). The sterndrive is a pre-alpha. Ser no 3216148,

Besides the impeller, what else may I need when I take off the lower unit? Gaskets, lubricants?

The thermostat should be what temp, 140? 160?

Any other suggestions (short of junk it) before I start taking things apart.

Thanks

 

[86 century]

Re: 1972 Mercruiser 165 overheating

The good news is you have in my worthless opinion the best engine ever converted for marine use.

The impeller has most likely torn into pieces so look everywhere you can for them.
Unless the circulation pump is leaking or making noise probably no need to change it just yet.

The thermostat I believe is a 160

Only one more thing you might think about is that eng has soft exhaust valve seats needs leaded fuel.

Good luck have fun with here

 

[86 century]

Re: 1972 Mercruiser 165 overheating

The good news is you have in my worthless opinion the best engine ever converted for marine use.

The impeller has most likely torn into pieces so look everywhere you can for them.
Unless the circulation pump is leaking or making noise probably no need to change it just yet.

The thermostat I believe is a 160

Only one more thing you might think about is that eng has soft exhaust valve seats needs leaded fuel.

Good luck have fun with here

 

[ziggy]

Re: 1972 Mercruiser 165 overheating

Since I haven't replaced the impeller yet (the boat was sitting unused for years, maybe decades before I got it) I am assuming the impeller needs replacing

that's a pretty good assumption. it's also the 1st place to look when ya get an overheat condition.

The thermostat should be what temp, 140? 160?

143*F t stat for a raw water cooled engine (no heat exchanger). 160*F t stat if ya got fresh water cooling system (has a heat exchanger).

Besides the impeller, what else may I need when I take off the lower unit? Gaskets, lubricants?

there's no gasket between the upper and lower. there is a quad ring that seals the gear lub passage between the upper and lower. all the parts ya need to do a water pump impeller replacement will come with the mercrusier replacement parts. part # 47-89984T 5.
you'll need 2 qts. of mercruiser gear lub. slightly more than one qt. will be used. maybe some spline grease for the lower drive shaft. ya might also want to replace the impeller housing so ya get a fresh set up for cooling. check for seal wear at the seal locations on the lower drive shaft. the bottom seal in the upper is easy to R&R and usually wears a grove in the drive shaft. ya can relocate to a slightly different location of the seal to gain a fresh surface if needed. when reassembling. ya can not bother with the o ring that sits at the top of the lower drive shaft. it can cause problems hanging up on the bottom seal in the upper during install. if you were to buy a replacement lower drive shaft. the o ring groove in the drive shaft would be missing, so you couldn't insall that o ring. it's not necessary.

might want to run a compression test to confirm all is well after the overheat.

flush water from the ft. to the back to remove as many impeller pieces as possible from the cooling system.

replacing the water pump on the ft. of the engine is most likely not necessary unless it's weeping out the weep hole.

take the exhaust bellow off between the riser and the exhaust pipe. there are flappers at the top of the exhaust pipe. these likely got melted when ya overheated. look for the remenents of the flapper in the exhaust system. they might be found in the bell housing stuck on the shift shaft in the bell housing.

sounds like ya jut had your drive off. assume ya did the pm while there. gimble brg. check, ujoint check, lug coupler splines, checked alignment, etc. if not. imho, it's much easier to remove the whole drive to do the impeller replacement. assembling the lower to the upper while on the boat is a drag. on the bench it's easy. like in a lot easier.

if ya need a book, use the oem service manual. here's a place ya can view it on line. ya need mercruiser service manual #1 for a '72.

 

[Ike-110722]

Re: 1972 Mercruiser 165 overheating

Thanks to both of you.

Ziggy, thanks for the details.

on the bench it's easy. like in a lot easier.

I wish I had a bench. I don't even have a covered place to work on it. Since I just had the gaskets and bellows replaced I guess I'll stick with just working on the lower unit. But thanks for the warning.

flush water from the ft. to the back to remove as many impeller pieces as possible from the cooling system.

How is this done? What do I need? Sorry to sound like a novice, but most of the work I have done on boats has been on sailboats and outboards. Never worked on an I/O before.

 

[ziggy]

Re: 1972 Mercruiser 165 overheating

I wish I had a bench. I don't even have a covered place to work on it. Since I just had the gaskets and bellows replaced I guess I'll stick with just working on the lower unit. But thanks for the warning.

i work on mine out in the open too. in a white rock driveway. sure is a bummer for sure. i do have a bench though... you could maybe cut open a box and lay the drive on the ground to work on it. just a thought.

How is this done?

i removed the lower only when i had an overheat. to flush i remvoed the tstat housing to get large chunks of impeller out of that area where the lodge in place. i also removed the hose that comes from the transom that feeds water to the engine. with the lower removed ya can flush from the ft of the engine where the hose ends, back and forth between there and the water pocket cover in the upper. be carefull with the water pocket cover in the upper. just looking at them 1/4-20 stailess steel bolts will break them off. let alone putting a wrench on them. don't disturb the water pocket cover if ya don't have to. but flush water fwd using the water pipe that goes between the water pump and the water pocket cover.
hope that makes sense.
i used a bucket to catch impeller pieces at both ends. you'd be surprised how many pieces can be flushed out.

used a garden hose to feed the water to flush out. from ft to back and back to ft.