Car coolant running too cool. It MUST be the thermostat, right?
 

Water cooled car, obviously.

It generally runs at 1/4 on the temp gauge, which I measured to be 160 degrees.

Half on the gauge is 180. Where it should run.

If I unplug the electric cooling fan (there is no engine driven fan) and let the car idle, the temp will continue to rise. When I plug the fan back in, or if I drive the car at speed, the temp will go back to 160.

Gauge is confirmed to be working accurately (by separately measuring coolant temp).

It really has to be the thermostat, right? Either the wrong one (unlikely, as I don't think there is only 1, a 180 degree, available), or it's stuck open.

I normally would just swap a new one in, but on this particular car it's a relatively expensive part.

My opinion is that you are correct it being the thermostat. Most are designed to fail in the open position.

Sounds like classic stuck thermostat to me.

At what temp is the cooling fan supposed to turn on? I would guess it's 200+ deg.F.

This is a long shot as I don't know which part of the US you are in. Perhaps too much antifreeze? I did that once. Added too much antifreeze and there was no heat...
Just a thought. Likely what the above says a dud thermostat.

Temperature sensor.

Just tell us , what kind of car is it ? . Sure sounds like a classic t stat stuck open to me also

Pull the t-stat. If it's open when you pull it (cold engine) then it is obviously bad.

If it is closed, toss it in a pot of water and bring to a boil. It should begin to open when the water approaches the temperature of the t-stat (e.g. 185 or 195, etc). When the water is boiling the t-stat should be wide open. As the water cools, the t-stat should close on its own.

This is assuming that you have actually measured the car's coolant temperature (not relied on the guage) at 160. If it really is 160, that is way too cool and you indeed have a problem to chase.

angela

And you wife will LOVE the fact you are boiling a antifreeze covered thermostat in one of the good kitchen pans. Wifes think that is just hysterical. Wait, they tend to get hysterical with the wonderful aroma of antifreeze in the kitchen.

They are CHEAP. If you are going to the trouble of pulling the old one, just replace it with new. And get a new gasket.

DO like Glen said. Also the cooling fan should not come on until the system reaches around 210F on the gauge and off again around 190F. Make sure the defrost nor AC are on as those cause the fans to run as well.

For heavens sake man, RINSE the car part off before you bring it in the house! No smells and no damage to the pans! By the way, the rinse before you bring it inside is an especially important point if you are going to put her dishwasher to parts cleaning duty! ;)

angela

When the car is warming up at idle, there should be a big temp difference between the two hoses.
(I think there is a small bypass 'always open' to relieve some pressure built up from the water pump)

When the T-stat opens fully, the hose temps will be the same, and then fan kicks on.

Might want to check the fan switch continuity if it runs all the time.

Update: Replaced the thermostat, solved the problem. It now runs straight up on the gauge, around 180 degrees F.

For my car, they aren't particularly cheap (around $125).

I took the old one, and boiled it in a pan on the stove (after wife had gone to sleep), while monitoring the temp.

The thermostat wasn't stuck open, it did open and close as it heated and cooled, but it seemed to be "off" a bit. Opened just a bit too soon, which let the car run too cool.

Wow, 125 bucks!

I was curious so I checked. I can get a OEM AC Delco thermostat for my car for $7.23.

https://www.amazon.com/ACDelco-12TP1E-Professional-Coolant-Thermostat/dp/B000IYROXE/ref=sr_1_6?ie=UTF8&qid=1476818412&sr=8-6&keywords=El+Camino+thermostat

Ya gotta love cheap small block parts. :D

OK, to be fair I have to buy a new gasket and that is about 5 bucks for the silly gasket. Just highway robbery! ;) It would take almost 10 minutes to do it.

As they say, the ///M stands for Money.

I put the old and new in the pan for testing. It's nice to know/confirm that the new t-stat opens & closes as expected. (once the new one did not)

Incidentally, "Money" was the first song that brought 1960s Motown into fruition.

https://www.theguardian.com/theguardian/2014/may/01/motown-detroit-soul-berry-gordy
Barret Strong has had but one minor hit in his career: in the first week of July, 1960, his "Money" made No. 23 in the Billboard charts. The year before, an obscure record producer named Berry Gordy had founded a record company in Detroit with $800 borrowed from his family. A black man, grown up in the ghetto, Gordy had nowhere to go but up. "Money" was the first hot action he owned;