1984 Merc 5.7L operating temp?

[rdm190]

So now that I have got most of the kinks worked out of my new toy I have another question.

My 260HP Chevy based Merc was running ~100-110F while trolling and only reaching ~120F while crusing at 3200rpm. I know this is waaaaay below the nominal operating temp for a chevy 350 of this vintage. I pulled the thermostat and it was stamped at 190F, and very old (obviously not working). Anyway the information I was fed told me I needed a 140F stat. I replaced the broken one with the new 140F. I started it up on the muffs and it got up to 170F (then it appears the stat opened up) and it settled right down to 140. I didn't want to run it too long and cook the drive so I don't have a good idea of where it was running now.

Any input on what the best operating temp for this motor is would be great! Thanks guys,

-Ryan

 

[Don S]

Re: 1984 Merc 5.7L operating temp?

Just idleing on muffs, 140 will be about all you get. Once under load you will see between 165 and 175.
If you were on muffs when you did the test, that may have been part of your problem. When was the last time the impeller was changed and the pump inspected?

 

[rdm190]

Re: 1984 Merc 5.7L operating temp?

Just idleing on muffs, 140 will be about all you get. Once under load you will see between 165 and 175.
If you were on muffs when you did the test, that may have been part of your problem. When was the last time the impeller was changed and the pump inspected?

The low numbers ie. the 100 and the 120 reading were both done, under load, in the water. This is what caused me to inspect the t-stat. I just replaced it yesterday and got the 140-170F results on the muff's (have yet to get it out on the water with the new t-stat, I am going out tomorrow).

I changed the impeller myself last week, the engine water pump appears to be in good shape at a glance.

What should I expect to see out on the water tomorrow? And secondly, was the 140F t-stat the correct choice?

Thanks as always for your quick responses Don.
-Ryan

 

[Don S]

Re: 1984 Merc 5.7L operating temp?

Under load means in the water and running, which should be between 165 and 175 when using a 140? thermostat. If your engine is raw water cooled, a 140 stat is what you want. If it has a closed cooling system (with heat exchanger and antifreeze in the system) then the 160? stat would be better.

 

[rdm190]

Re: 1984 Merc 5.7L operating temp?

Under load means in the water and running, which should be between 165 and 175 when using a 140? thermostat. If your engine is raw water cooled, a 140 stat is what you want. If it has a closed cooling system (with heat exchanger and antifreeze in the system) then the 160? stat would be better.

It is raw cooled. So the 140 was the right choice. Thanks Don.

-Ryan