High temperature thermostats versus low temperature thermostats?

KIWASABI1

MEMBER
2001 MITSUBISHI ECLIPSE
2.4L
4 CYL
2WD
AUTOMATIC
233,000 MILES

Hello 2CarPros,

I have a question about thermostats. I had ordered a Motorad 422-190 thermostat a while back and just put it into my car along with a new water pump. My mechanic warned me that these types of thermostats allow the car to get a little too warm, but then the temperature eventually goes down. Well, he was right, that's exactly what's happening. My temperature gauge gets about 2/3 of the way up and holds there for a while until it finally decides to go down. From talking to him and doing some research online, it looks like there are 170-, 180- and 190-degree thermostats. Evidently this particular one is a 192-degree thermostat. Should I switch to a low temperature / 170-degree thermostat so that it opens sooner and doesn't allow my engine to get so warm? I do understand that there's a risk of an engine running too cold all the time causing extra wear. But in this case, I would say it's definitely getting too warm before the thermostat opens up.

Thanks,

Adam

5 Replies

STEVE W.

MECHANIC
13,475 POSTS

That is how a thermostat is supposed to work, the 190 is the correct temperature part for that vehicle. The lower temperature thermostats will only make it take longer to warm up and during that time it can cause higher fuel use and cylinder washing from the extra fuel. Also, the thermostat doesn't set the temperature the engine operates at, it simply sets the point at which it opens. The radiator and cooling system capacity set the operating temperature.

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Thursday, September 22nd, 2022 AT 9:40 PM

KIWASABI1

MEMBER
198 POSTS

Hi Steve,

Thanks for the helpful response. So, given what you said and the fact that I've replaced the water pump, drained and replaced the coolant, and pressure tested the whole system, what could the problem be? The radiator itself? There are no leaks that I'm aware of. It was the water pump, which was leaking, hence why I replaced it. The radiator is 3 years old at this point. That said I think I've replaced the radiator basically every 3 years the last 3 times I've done it. 2012, 2016, and 2019.

Thanks,

Adam

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Thursday, September 22nd, 2022 AT 9:50 PM

STEVE W.

MECHANIC
13,475 POSTS

That system should operate around 212-215 degrees by design, so it sounds like it's operating properly. If it actually overheats and dumps coolant is when you need to worry.

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Thursday, September 22nd, 2022 AT 10:21 PM

KIWASABI1

MEMBER
198 POSTS

Thanks Steve. Now that I think about it, this happened when I first got the car used back in 2005. So, it seems to be more of a design flaw in the temperature range of the gauge, rather than a cooling system problem. This has only just now dawned on me after 17 years with this car. It appears it's 2/3 of the way towards overheating, but it's actually operating at an ideal temperature. That's a relief.

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Thursday, September 22nd, 2022 AT 10:35 PM

STEVE W.

MECHANIC
13,475 POSTS

To see the real temperature, you could use an OBD II reader to look at what the sensor is showing the temperature as. Most of the dash gauges are optimistic guesses unless they are numbered, even then you will find them sort of close but everyone is a bit different. It's one of the reasons why race cars will have gauges all set at different angles. They will take the car out, get it fully warmed up and make sure all the systems are correct, then they will rotate the gauge in the dash so that all of the pointer hands are the same position, like straight up or straight across. The gauge numbers sort of become meaningless you just look for a straight line or whichever pattern you choose. At least on your vehicle it has an actual working gauge. There are many out there (Fords especially) that don't use a true gauge, instead they have a simple switch that activates the gauge to a fixed position. That was done because the service departments were getting large numbers of complaints about oil pressures that would vary depending on engine rpm or coolant temperature changes. IE as the engine RPM goes up and down the oil pressure will follow that, but many thought that meant there was a problem. The answer was to convert the real pressure gauge into a fancy light. They added a resistor to the gauge and a switch instead of a real sender. As soon as the oil pressure activated the switch (usually 10-15 psi or so) the gauge will pop up to some point in the "Normal" range and stay there. No more complaints.