3.7 Mercruiser Running Hot

[Sarah75555]

We’ve got a ‘86 Thundercraft with a 3.7 Mercruiser 170hp that keeps running hot. Last summer we had a cracked head and replaced it, new impeller & water pump seals. Have ran it a couple times and it’ll get up to 190 and we quit. Cleaned the heat exchanger and both sides are patent through it. Husband says fresh water is flowing good but doesn’t think coolant is flowing through the motor. Any suggestions on how to test patency through motor and make sure pump is working?

 

[Eric 3.7LX Alpha 1]

Welcome. Last summer you had a cracked head, or head gasket? The gasket is common to fail on these if they overheat, the head cracking isn't very common to happen.

I don't believe the engine circulation pump is often the cause of an overheat on these either. Bad thermostat, impeller in the drive, timing off.. there are two pages of possible causes of an overheat in the Mercruiser service manual. When does it run hot? at idle, mid RPM? while on the trailer and running on muffs? How old is the impeller in the drive?

 

[kenny nunez]

When the impeller was replaced did you inspect the plastic pocket cover in the upper housing? If it is melted and distorted you will lose water pressure and have a overheating problem. Also a partially blocked exhaust riser. Connect a 3/4”garden hose from the heat exchanger outlet hose to the inlet of the riser and there should be no restriction and free flowing.

 

[Scott Danforth]

when was the impeller last replaced? all the missing blades found?

check the cover as Kenny mentioned

its not the circulating pump.

no way to see water circulating within the motor, however it is.

 

[Sarah75555]

Welcome. Last summer you had a cracked head, or head gasket? The gasket is common to fail on these if they overheat, the head cracking isn't very common to happen.

I don't believe the engine circulation pump is often the cause of an overheat on these either. Bad thermostat, impeller in the drive, timing off.. there are two pages of possible causes of an overheat in the Mercruiser service manual. When does it run hot? at idle, mid RPM? while on the trailer and running on muffs? How old is the impeller in the drive?

Yes, it was a cracked head and we put a new one on. The thermostat is working, we’ve taken it out and checked on the stove. The impeller was replaced last year and we’ve only ran it a couple times since and always runs hot at high RPM, but even at lower RPM it’s running hotter than it should, about 190. On muffs it never gets hot.

 

[Sarah75555]

when was the impeller last replaced? all the missing blades found?

check the cover as Kenny mentioned

its not the circulating pump.

no way to see water circulating within the motor, however it is.

Impeller replaced at end of last season and old one was intact. Husband said cover looked good.

 

[Sarah75555]

When the impeller was replaced did you inspect the plastic pocket cover in the upper housing? If it is melted and distorted you will lose water pressure and have a overheating problem. Also a partially blocked exhaust riser. Connect a 3/4”garden hose from the heat exchanger outlet hose to the inlet of the riser and there should be no restriction and free flowing.

Yes he said cover looked good and he said he’s done that and water flows with no resistance. He’s pretty good at working on stuff but this has him stumped. Said he feels like he’s replaced or tested almost every part of the cooling system that he can.

 

[achris]

Yes he said cover looked good and he said he’s done that and water flows with no resistance. He’s pretty good at working on stuff but this has him stumped. Said he feels like he’s replaced or tested almost every part of the cooling system that he can.

It's not flow that you're checking for, it's leaking.

Water pocket cover - the how, the why and the what.

WATER POCKET COVER. What is it, where is it and why is it important. The water pocket cover is located in the bottom of the drive shaft housing, at the top of the exhaust cavity, of all MC-1, R, MR and Alpha One drives. It’s a plastic cover that has the copper tube from the water pump going...

forums.iboats.com

The other item to check is the exhaust elbow. They clog with rust and restrict flow through the heat exchanger, causing a high speed overheat...

Chris......

 

[kenny nunez]

With the boat in the water remove the drive pump water supply hose on the heat exchanger, start the engine and try to stop the water with you hand at idle speed. You should not be able to hold it back. If you can then there is a pressure leak between the drive pump and the heat exchanger. Either the pocket cover of possibly the hose from the bell housing to the gimbal housing.