1990 5.7. 260 Overcooling at idle.

[jimmbo]

Get a Temp reading gun and aim it at the Water passages on the Intake Manifold and see what the Temps are

 

[Mkos1989]

Temps are right on par with temp gun I borrowed from work. On the rare occasion itll bounce up to 160* and back to 100 within 5 seconds and do that for a few minutes then lay back on 100. Like I said its been doing this for 10+ years but its finally bugging me to the point that it shouldnt be doing this LOL. All new hoses since last post, thermostat, Alpha water pump and actually a new sender unit I had laying around from a car project Ive been working on.

 

[jimmbo]

I will bet the Highest Temps alternate between the Port and Starboard sides of the intake. Engines often do that instead of Being uniform and even temps.

 

[Mkos1989]

I will bet the Highest Temps alternate between the Port and Starboard sides of the intake. Engines often do that instead of Being uniform and even temps.

Starboard is always hotter in my case by a good 20*

 

[kenny nunez]

I see 2 things that could be the cause. #1 your intake manifold has no heat passage and stays cool.
#2 just for kicks move the temperature sender to the port side of the manifold away from the in coming water supply.
Where the water exits the thermostat housing does it have the 2 plastic balls connected with a long bolt through the housing and has springs that hold them against the outlets?
As far as the flame arrestor it is the correct one.

 

[Mkos1989]

It does not. Per the parts guide and post #11 there are no balls and rod on the Type III cooling version. The balls and rod is on the type II thermostat housing.

 

[Mkos1989]

Should I move the temp sensor to the intake manifold or leave it in the thermostat housing?

 

[alldodge]

Unless the thermostat was made incorrectly from the factory the senders should be separated from direct in coming cold water. Should be on the motor side

Being in Erie just put a 160* in it, won't hurt even if you were in warmer waters

 

[Mkos1989]

Unless the thermostat was made incorrectly from the factory the senders should be separated from direct in coming cold water. Should be on the motor side

Being in Erie just put a 160* in it, won't hurt even if you were in warmer waters

I did replace it with the newer 160. So I should plug the T stat housing hole and put it in the intake?

 

[alldodge]

You need the small hole in the Tstat so the air will bleed out. The one small hole doesn't have much effect on cooling

Since it does have a 160 then I have no clue

 

[jimmbo]

Unless the thermostat was made incorrectly from the factory the senders should be separated from direct in coming cold water. Should be on the motor side

Being in Erie just put a 160* in it, won't hurt even if you were in warmer waters

A 160 is too hot for a Raw Water Cooling System. Usable in a Closed System, but a 180 would be used there

 

[alldodge]

Jim he's in fresh water so 160 shouldn't be an issue

 

[jimmbo]

While there isn't as much salt in fresh water, there is still plenty of it and other minerals to come out of Suspension at that temp.

 

[alldodge]

Ok but Merc has been using 160 Stats in open cooled motors for years
Here is about a 2010 MPI open cooled
https://www.mercruiserparts.com/bam/subassembly/31751/12840/190

 

[jimmbo]

Perhaps Mercury is putting a Coating on the Water Jackets and Passages, they certainly do in the Intake Manifolds.

 

[alldodge]

My understanding is it was all 140 because of the main issue was salt water. When EFI and MPI came about they needed high temps so 160 came in. Merc recommended salt folks get close cooling.

The service techs here can clarify like @achris @Fun Times @muc

 

[jimmbo]

Wise of Merc to push Closed Cooling on the Engines used in Salt Water, but a lot of Fresh Water is still loaded with Salts(not just NaCl) that will come out of Suspension, Brackish takes it to another Level. Unfortunately a lot of Builders will not choose the Closed Cooling System because of the Extra Cost. And a lot of Boaters are ignorant of what happens in the Water passages at higher temps.

 

[alldodge]

Mkos, I'm as puzzled as you are. The block gets cool water from the circulating pump when the TS opens. It opens which allows water to be pumped in and warmer water comes out around the TS.

Only way for cool water to go into the block at out the TS housing is:
The housing was not made correctly during manufacture
The housing as a crack so its not sealing off
The TS is stuck open slightly
The TS parts were not installed correctly in the housing

Other then the above I have no idea why it would do what its doing

 

[Mkos1989]

Im wondering if I should pull the risers and see if I got all 4 slots open like the merc bullitin. If I do that Im half temped to go back to the restricted.. Keep it at 900-1000 rpms, zero issues

 

[alldodge]

Im wondering if I should pull the risers and see if I got all 4 slots open like the merc bullitin. If I do that Im half temped to go back to the restricted.. Keep it at 900-1000 rpms, zero issues

Could try that but I don't see how that will help. As before the thermostat housing should keep the water closed inside the block until it opens.