Engine overcooling issue

Hello all. I have recently bought a 1999 s10 pickup 2.2L rwd
And I’m having a problem with the engine overcooling. No matter how long I drive the temp Nevers gets much hotter than 160°f.
I was told the thermostat was stuck open so I replaced it twice with no difference.
Then I was told the temp sensor could be bad so I replaced it and also didn’t fix it.
I also check the Coolant Temperature In the radiator after it has been running for a while with a thermometer and it was also only reading about 160°f
My heat seems to work fine , it gets hot. And I don’t see any Coolant leaks and my Coolant level stays the same.
I took it to a friend and he wanted to take the radiator cap.off and start the truck when it was nice and cold to see if there was circulating Coolant ( I guess he said this was a way to test for a open thermostat and he said there was not any ciculation so be said my thermostats are not sticking open.
My thermostat is a 195 so I should be expecting the temp to get to atleast 195 I would think…
Also my check engine light is on coming back with the Insufficient Coolant Temperature code.
I have no idea what could be wrong? I have also replaced the fan clutch with no luck.
And by the way I wanted to add the guy I bought the truck from said he had a new head gasket instslled.
But from all the info I’ve been hearing and reading is saying if there was a bad head gasket or a air pocket or a Coolant leak that the engine would be overheating.
Thats why I’m so confused by this?? Any help is appreciated. Thanks

Is the coolant level correct?

If the coolant temp sensor is not immersed in coolant, you could certainly get that fault code you mentioned

if the pressure cap is on the radiator . . . not sure with this engine . . . remove it with the engine COLD and make sure the radiator actually is full

For what it’s worth, I have had some bad luck with aftermarket thermostats. Meaning some of them flat out don’t work correctly

A T-stat can easily be tested with a hotplate, a pyrex vessel, a cooking or lab thermometer, and a piece of wire. Hang the T-stat into water in the Pyrex vessel, put your temp measuring device in there too, and heat it up on a hotplate. Then let it cool. You should be able to see the T-stat open and close at the appropriate temperatures.

I strongly recommend safety glasses and oven gloves. Pyrex should be absolutely safe at these temperatures, as should a cooking or lab thermometer, but you should always ensure safety when working with heat. God forbid you should get the one-in-ten-million vessel that’s defective and get glass in your eye. Or accidentally burn yourself while working with this.

A T-stat can also be easily tested by removing it from the engine, rinsing it off, place in a pot of water on the stove with a thermometer, and heating the water up to see at what temperature the T-stat opens.


I just said that except I used a hotplate and Pyrex.

Go buy yourself a simple non contact infra red thermometer… see what the temp REALLY IS…

Then move forward with parts… I think your temp gauge is wonky… If you dont specifically ask for a 160 degree T stat… I believe yours would normally be like a 195…or gasp a 210 T Stat…

A cool or cold engine is inefficient…160 is too cool.


I agree that those gauges in the cluster aren’t terribly accurate

My coolant level is normal full.
The thermostat is a 195/degree I bought 2 brand new higher end brand thermostats that are both 195 temps and still having the overcooling issue.
I have replaced the sensor with a new one and still didn’t fix the issue.
Then I took out the thermostat and the sensor from my 98/s10 that I know are working fine and installed into my 99 s10 and it is still overcooling. I used a friends inferead thermometer and tested the actual temp and its reading 158°f tops. So I’m completely lost???

Is the condensor fan always running?

This isnt very common… LOL… Is the truck acting up in any other manner? Driveability issue?

What is making you even worry about this? Perhaps it always was this way?


I believe this truck may have only fan, mounted on a fan clutch . . . ?

In other words, I don’t think it even has a dedicated condenser fan

“What is making you even worry about this? Perhaps it always was this way?”

Hmm . . .

P0128 from day one . . . doubtful, in my opinion

The check engine light is/was almost certainly on, so there might be a problem getting the car smogged/inspected

And you know the truck isn’t getting up to proper operating temperature in a timely manner . . . perhaps not at all. So fuel is being wasted

The truck seems to run fine other than it uses alot of gas and it seems to lack power.
Everyone I have talked to said that with the engine running cold all the time will cause greater gas useage and could cause a drop in horse power. That is really the only reason why I am worried about the issue.
I know its better than running too hot lol.
The truck could have always been like that . its very possible, I’ve only owned it for about a month .
Its just super puzzling and driving me crazy .

And yes it only has one fan mounted to the fan clutch. I bought a brand new fan clutch and had it replaced also.

How is the coolant getting to the radiator if the thermostat is closed? A 195F thermostat should be closed at 160F. Is there noticeable circulation when the engine is cold? The thermostat should not allow coolant to go to the radiator until operating temperature.

I’m not really sure? A good friend of mine took the radiator cap off when the truck has been sitting overnight so it wasn’t ran for a long period of time and everything was cold. And then had me start the truck and he was looking for circulating Coolant and he said there was no Coolant circulating from a cold start . supposedly that told him the thermostat was not stuck open. I’m not to car savy so I apologize if I’m.not making to much sense on explaing myself lol… But anyways that’s why I ruled out the thermostat because of the test he did he said the thermostat was not the issue .

Some wild thoughts . . .

This truck doesn’t by chance have an incorrect radiator . . . too large . . . installed?

This is the standard fan clutch for ac


This is the standard fan clutch if you DO NOT have ac


Which one do you have?

The fan clutch I bought was WITH A/c my truck DOES have air conditioning so that’s the one I got.
As for the incorrect radiator I never thought about that … Is there a way to tell if it’s the wrong radiator just by looking at it?

Possible you have a defective temp gauge. Go buy a thermometer, start the engine and let it warm up. U can leave the cap off, it won’t hurt it. If the thermometer reads above 160, U know its the gauge. Simple as that. Coolant doesn’t circulate if the thermostat is closed. That’s said and done.

Which temp sensor was replaced; the dash gauge sending unit or the engine management ECT sensor?

Any chance there could be 2 issues here; a flaky gauge along with a flaky ECT sensor or circuit?