5.7 merc wants to run hot

[glmtrophy2352]

I'm starting to run out of ideas here. I have 1992 trophy 2352 with 5.7 mercruiser I/O alpa, engine rebuilt 1 month ago, bored .030, heads redone and all new pistons and rings bearings ang gaskets, new manifolds with new risers were installed new waterpump in outdrive,new thermostate and hoses. We have checked all hoses and have good water coming into waterpump on engine. running 143 degree thermostate and it is opening. But boat wants to heat up to about 175-190 degrees after ebout 7-10 minutes of running on the garden hose. Any help would be appreciated. I 'm good with auto engines but this is wanting me to pull my hair out!!!

 

[Randy169]

Re: 5.7 merc wants to run hot

Since you know the engine is getting water (you said you have water at the engine water pump)

This is what I would do:

Swap out the thermostat anyway. Cheap and easy to test.
Check the temp sending unit, gauge, and wires running to these.

I don't what else it could be once these are checked.

Let me know.

 

[bruceb58]

Re: 5.7 merc wants to run hot

Make sure your garden hose is giving enough water. You can turn it up pretty high becuase excess water will leak out the muffs.

Get an IR temp guage and measure the temps of the block near the temp sender and the manifolds. If the manifolds and the engine are too hot, it is a water supply problem. If it's only the engine, could be a thermostat.

Was the engine ever run without water since you put in the new impeller?

You should also do a serch on this topic. There is a recent thread that all of this has been discussed within the last week.

 

[QC]

Re: 5.7 merc wants to run hot

I 2nd bruce's thoughts on the IR Gun as I would want to confirm the gauge before I did anything else. Can you touch the risers? Also, turn the water on full flow no matter what. What RPM?

 

[glmtrophy2352]

Re: 5.7 merc wants to run hot

I'll be replacing sending unit first of the week, gauge seems to be fine , wires also. I'll let you know after I replce the sending unit. Thanks

 

[glmtrophy2352]

Re: 5.7 merc wants to run hot

Thanks for the input, I'll try and get a IR temp gauge and check it first of the week,it wasn't run without water,it seems to equally heat engine and manifolds. at 2000 -2500 RPMs there is no water escaping at the muffs, risers are hot to touch but not hot enough to burn you, but you wouldn't want to leave you hand on them longer than 2-3 seconds. thanks

 

[glmtrophy2352]

Re: 5.7 merc wants to run hot

Risers are hot to touch, not enough to burn you but you wouldn't want to leave your hand there long, 2-3 seconds and you move it.I'll try IR gauge first of the week, water was on fully, RPM 2000 -2500, no water escaping around the muffs after 1800to2000 RPM. replacing sending unit first of week also, gauge seems to fine but will verify and repost results after first of the week.Thanks

 

[bruceb58]

Re: 5.7 merc wants to run hot

If there is no water escaping at the muffs, you probably don't have enough water supply, especially at 2500 RPM.

I wouldn't change the sender yet. Check with the IR gun first. You can get them at Harbor Freight.

 

[JustJason]

Re: 5.7 merc wants to run hot

it is raw water cooled or do you have closed cooling?

 

[QC]

Re: 5.7 merc wants to run hot

Too fast and you're probably cavitating the pump. Try at 1000 RPM only with good solid flow. Also, changing a sending unit will not confirm that the gauge is good or visa versa for that matter . . . Only valid test is in the water as there is no load in neutral either.

Edit: It's a trimulcast . . . Exactly like a simulcast only different . . .

 

[gd8boltman]

Re: 5.7 merc wants to run hot

Did you replace/check the engine water pump?
I think 2500 rpms on muffs is a little too high of RPM's, does the temp. only get that high at that many RPM's? It would be a good idea to verify temp. guage, sending unit accuracy, you could also go to the parts store and buy a cheap mechanical temp. guage, screw it into the cyl. head and see what info. that provides, good luck to you.