OVERHEATING 302 Mercruiser....Please Help!

[conk3416]

I have a 1973 302 Mercruiser with a MERC 888 outdrive. The engine is in a 17' 1985 Sylvan open bow. When i am out on the lake the temp will slowly rise to 125-130 deg. Then it will quickly climb to 200-210 deg. I cant figure out what would cause this. I have changed the thermostat, put in a new impeller unit, then i took off the hoses that run to the thermostat and ran water toward the outdrive and everything seemed to flow good. Does anyone have an idea on what would be causing the overheating issue?

 

[beermunk]

Re: OVERHEATING 302 Mercruiser....Please Help!

Try running it at idle for 15 minutes in your driveway on muffs. If it still jumps in temp then you probably have clog in your system, bad thermostat or impeller.

If it stays at a reasonable temp then you know it's related to you running the engine at higher RPM's. Things to look for then would be clogs in your intake at the lower unit and your impeller.

I know you've changed out the thermostat and the impeller already. Most likely the thermostat isn't to blame as those are pretty solid. The impeller on the other hand, even if brand new, could be ruined by improper installation, wrong part or running it out of the water. If by chance the latter three situations apply to you, don't feel bad we've all been there before. Impellers are dirty little bastards.

 

[conk3416]

Re: OVERHEATING 302 Mercruiser....Please Help!

when i was cruising around lake at about 1200-1400 rpm's, that is when it started overheating. how do i check for a clog in the system? How do i know if the water pump is good?

 

[conk3416]

Re: OVERHEATING 302 Mercruiser....Please Help!

When i have it hooked up at the house when at idle it runs about 150 deg. Then i will hit rpm's up to about 1500-2000 and temp goes right down to 120 deg. ???? is it possible that i sucked a piece of plastic or something around the water intake on the outdrive and it covered the holes when i was on the lake. Everything seems to be fine now that im home?????? Thanks Andrew

 

[Bondo]

Re: OVERHEATING 302 Mercruiser....Please Help!

is it possible that i sucked a piece of plastic or something around the water intake on the outdrive and it covered the holes when i was on the lake.

Ayuh,.... Anything is Possible....
If that Is what happened,... It probably Smoked the Impeller from lack of Water,...

 

[Surfdancer]

Re: OVERHEATING 302 Mercruiser....Please Help!

If you believe you have good pickup pump water flow (test on the muffs and see how much water flows out of the side holes in the transom assembly), you may have crud build up in your exhaust elbows. That was the case with my 1976 model 888.

I unbolted the elbows, pulled the hose barbs and plugs, and gave them a good clean out with wire rods, pressure washing, etc. If they look like toast, replace them. They go before the manifolds, so bear that in mind when you examine them.

To do this, it takes some elbow grease (work, not a gasket dressing) and new elbow to manifold gaskets. Rather than try to save the stainless plates that are sandwiched between the gaskets, I replaced them too.

It takes some elbow grease to remove the stock gasket material, so be prepared to stuff the exhaust hole in each log manifold with some rags and scrape the gasket surfaces clean.

If there is a lot of rust packed into the water passages of the exhaust manifolds, pull them and clean / inspect them as well.

Overheating is caused by poor water circulation - usually traced to either a worn or damaged pump, or clogged exhaust passages.

 

[conk3416]

Re: OVERHEATING 302 Mercruiser....Please Help!

surfdancer, i have a 55 gal. drum at my house that i submerge the outdrive in. I have ran it 3 times since i was on the lake earlier and everytime it will heat up to 150 then go down to about 120. or if i rev it up to around 1500rpm it will run down to about 110 deg. Do you believe something is floating around in the exhaust elbows or Y-pipes and clogging it every now and then? I just find it strange that it seems to be running at proper temp now.

Thanks Andrew

 

[conk3416]

Re: OVERHEATING 302 Mercruiser....Please Help!

i will take the elbows off and check for any clogs. Then i am going to take it back to the water this weekend. I will let you know how everything goes on Sunday.

Thanks for your help, Andrew