Mercruiser Model 470 Overheating

[jaleistra]

I have a 1983 Model 470 (4 inch heat exchanger) with no cooling issues up until this year. I think the temp used to max out around 150 to 160, but temp is now maxing out at 190 while running at 3000 to 4000 RPM pulling skiers. Idling engine at 2000 to 3000 RPM will reduce the temp down to 160 oC in 1 to 2 minutes. Anybody know the temp setting of this thermostat? Lower end unit water pump impellor replaced last fall when bellows was replaced. No overheating issue prior to water pump impellor replacement. Could mechanic have installed incorrectly? I have never removed a lower end unit and I am concerned about getting the shift linkage correct on reinstallation.
Fixes tried to date as follows:
1) Replaced thermostat
2) Replaced temp sender
3) Backflushed voltage regulator on freshwater side
4) Backflushed freshwater side of heat exchanger
5) Confirmed freshwater flow out of heat exchanger exit tube. Not sure how to measure if sufficient flow.
6) Flushed glycol side of heat exchanger
7) Removed and flushed all glycol hoses, but did not remove inlet fitting to glycol pump as I did not have a glycol pump inlet gasket available.
Any recommendations on next steps? Could I have a restriction in glycol passages of exhaust manifold? The only inspections I have not done on the glycol side are removing glycol inlet cover to water pump, removing glycol reservoir & exhaust manifold.

I would appreciate any insights. Thx.

 

[Greytfun]

Re: Mercruiser Model 470 Overheating

Same engine here, on a Four Winns 190 (1987). I would recheck the install of the lower unit impeller first. If the plates are not in correct order and the impeller is jamming, there's your problem. Also, I never use an auto thermostat. I've had too many issues and now I only use the correct Mercruiser part. Check to make sure your thermostat isn't stuck. I also removed the heat exchanger tube and checked each small tube for blockage (no blocks). I replaced the end cap seals while it was disassembled.

Questions for the coolant side:
1) Are you losing coolant?
2) Is the coolant reservoir full or empty?

My engine was doing the same as yours and the answer for me was doing both the impeller and correct thermostat. I think (trying to recall) the therm. should open at approx 165 deg. My gauge usually reads around 180 on a real hot day at WOT for a while.

 

[scv76]

Re: Mercruiser Model 470 Overheating

A lot of people mention "flappers" blocking the exhaust, which I guess creates back pressure and or can block the flow of water out the engine?
How are you measuring the water temp?
Maybe your gauge is indicating incorrectly because it is expecting a different resistance than your replaced temp sender is putting out?
If you have a laser thermal reader you can verify the temp of the coolant...or a thermo-couple and a reader.
I think the OEM thermostat is a 160 degree one. What temp thermostat did you install?

 

[NHGuy]

Re: Mercruiser Model 470 Overheating

Has the problem been there the entire summer? If so recheck the impeller service. Also check that the cool water hose from the inside of the bellhousing to the transom is free of kinks. It is usually removed &/or replaced when a bellows is done. It's easy to get a kink in it, which will restrict water flow.