The Stovebolt Site Forums The Shop Area Engine Shop Motor started running cooler🤓

The motor used to run 180 F. Now suddenly it’s showing about 160F.
I recon I need to replace the thermostat in the head?
IL 6 Chevy.

Use an infrared thermometer to verify the actual temperature of your engine rather than relying on your dash gauge.

If you really want to know, install a temporary gauge under the hood at the sending unit port or petcock port.

Normal engines will run about the same temp with a stuck open or removed thermostat as they do with a good working one, they just take longer to heat up since the coolant is free flowing through the radiator when the engine is cold. Thermostat only control how hot the engine gets before the radiator can start working, once open, the engine will keep heating up if the radiator isn't working properly.

Check your gauge or fluid level.

If your engine requires a thermostat to run 180, consider yourself lucky everything is working so good. The engine is running cool, radiator is working good, and the thermostat is regulating the flow just right to maintain 180 degrees. Very few engines will run this efficient in the summer time.

Personally, I would change the thermostat. A 180° thermostat as Joe pointed out, should be closed to heat the engine up then be able to open or close to maintain that temperature. Engines are more efficient at 180°. If it’s dropping to 160°, it’s not working properly.

My thermostat was working perfectly then decided to not open for ten to fifteen minutes. Once it opened it did maintain 180°. I was concerned that if it took so long to open, it might get stuck in that position. I changed it out and now the gauge starts to move in about five minutes.

I replaced the thermostat over a year ago because it did not open soon enough. It has been working great since. I have an electric cooling fan over the radiator that works manually. I only use it when at idle for several minutes. Temperature gauge stayed at 180 until last week its reading around 160.
So I’m thinking the motor is hot as usual which leaves the gauge on the dash is malfunctioning or it’s the probe in the head?
Thanks for your input!

A couple of things, Glenn:
1. You know the thermostat rated temperature doesn't mean that's the max temp your engine will run, yes? It just means it won't run any cooler than that. I'm certain you already know this, but sometimes it is overlooked.

2. Please avoid using a thermostat sold under the name Fail-Safe. It is sold by a company named Moto-Rad and the problem is there is a little ratchet device built into it so that if your motor overheats it will set up a Rube Goldberg reaction and will lock in the open position...and the thing will stay open forever if you don't remove it and fix it. I've seen them stick open at a mere 10 degrees past the rated temp value. What you have to do is take it out and use a pair of needle nosed pliers to get in there and carefully disable the ratchet part you paid extra for. I've had two of them and they both stuck wide open and the same thing happened to a friend a couple of years ago.

I don’t know which name it has in it. I’m thinking to test I could let the motor idle a while. It normally will run hot after several minutes at idle without the manual cooling fan on. Even if it’s stuck open it should more than likely run hot?
I can use the infrared temp to measure too.
Also I’ll post these 2 pictures of two different temperature probe areas. One on the thermostat housing (the one piece I’m using now)
Then there is one in the head that I don’t use & don’t have one to use on it. ??

Glenn - Your thermostat housing temp sensor may or may not accurately register because the sensor feed wire going to the gauge is so close to the exhaust manifold. If there is enough flexibility to your gauge feed, I'd suggest you relocate it to run along side the valve cover, as far away from the exhaust manifold as possible. I see your original electric temp sensor is still present in the side of the head, just back of the forward leg of your intake manifold.

If you want to double check your temp sensing, you could get an electric temp gauge and hook it up to the sensor in the head and see what it records in your test runs.

Checking temps today using my using my handheld infrared. Everything reads right on even with the cooling fan off for 10 minute. 178 is as high as I saw.

Temp probe at the thermostat housing will give a more accurate reading of the coolant, down on the head will be hotter due to how close it is to the cylinders. Both will work, but out at the thermostat it will be a truer reading. My 250 engine shows close to 15 degrees different.