First hand observation on thermostats. Cooling.

When I did my engine rebuild I bought the OEM 195F stat from the dealer. After that the dash gauge always went to just above 210 while the mechanical gauge that is in the top hose would go to 205, both would go up from there when idleing for long periods or at highway speeds above 75MPH. This week I swapped a 180F stat back in and the temps now are just below the 210 mark in the dash and 190 on the mech gauge. This is what worked for me, YMMV, but it is a simple enough of a swap that it's worth a try for those who are running a little warm. TC

The problem here is that you believe 210 is "a little warm," when that's actually the temperature the engine was designed to run at.

I still don't understand why people can't understand that the factory used a 195 degree thermostat because that's the temperature they want the engine to operate at. The gauge runs at 210 because it's at the back of the head, which happens to be a localized hot spot. This is demonstrated by the fact that you got a bigger drop with your mech gauge than you did with the dash gauge.

You can get away with running a 180 but it's not helping you any. Those who run a 165 are just plain screwing themselves.

I still have the closed system.....it is maintained properly, 50/50 mix, flush it every so often, stock radiator, fix things when they break...with stock parts. I run at 210 all day long........no problems. I agree with Eagle....it is designed to run there, let it. If it boils.....something is wrong, just fix it.

Rev

I have an 89 on which I installed a 91 thermostat cover to obtain a port large enough to insert a mechanical sender. My temp runs at 190 at idle and 180 breezing down the road. It rises to 200 stationary with the AC on 90 degree days and heat soaks to 210 when you cut it off. The problem is, this is at the inlet of the engine rather than end of the circuit through the motor to the water pump. I'm prob. running 205 or 210 at the stock sender location(but its an idiot light). Everything in my system is new but the thermostat and radiator and I've added an aux. trans cooler. I gather that I must have a lower temp. thermostat than OEM but if its hot enough-it does not matter- the capacity of my system can only remove so much heat and I'm headed for 190 to 200. Even if you took out the stat you are still headed for 200 only you will mess up perform. and mpg during the warmup phase. That's why you should look elsewhere than an aftermarket cool stat if your truck runs hot.

Don't overlook the clutch mechanism at the base of the belt-driven fan. These wear out. Sometimes it's hard to tell that the fan isn't fully engaged when things get hot. I replaced mine and it made a real difference. I happend to note that the docs with the new clutch said 'replace every 5 years'

Eagle said:

The problem here is that you believe 210 is "a little warm," when that's actually the temperature the engine was designed to run at.

I still don't understand why people can't understand that the factory used a 195 degree thermostat because that's the temperature they want the engine to operate at. The gauge runs at 210 because it's at the back of the head, which happens to be a localized hot spot. This is demonstrated by the fact that you got a bigger drop with your mech gauge than you did with the dash gauge.

You can get away with running a 180 but it's not helping you any. Those who run a 165 are just plain screwing themselves.

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Couldn't agree more!!!

I ran the 180 for a couple of years before the rebuild, when I went back to the 195 so I've got experience with both. One thing remains constant on mine no matter which stat I run; the temp will rise in the range of 10 to 30 degrees hotter when stressed. 10 over on hot days here and there but dropping back down quickly on deceleration, 30 over on very hot days climbing a long hill with a load for example. When the temp rises a lot I start the counteracting routine, you know AC off-heater on-backing off the speed etc.
30 over 190 (220F) isn't bad, 30 over 210 (240F) get's my attention. The motor run's fine with either temp stat, I even think a little better with the 180 but I don't like to make claims I can't back up, but with the lower one I'm not constantly watching the gauge wondering when it's going to shoot up. One thing for sure about a motor that starts to get hot, the hotter it gets the faster it climbs and the harder it is to get it back under control. Keeping it from getting to the hot end of the scale in the first place is worthwile IMO. I for one do understand that 210 is normal and that's what the factory engineered them to run at stock. The relative term here is STOCK, when we add weight, height,bigger tires,accessorys, age and so on, the limited cooling capacity becomes overworked pretty quickly. This I believe is how the 180 helps, at least in my experience. TC

I understand how the thermostat can control the coolest temperature that the engine can run at. I don't understant the thermostat's role when the engine is at its highest operating temperature. Wouldn't both a 180 or 195 degree thermostat be fully open above 210? If so then the actual thermostat wouldn't be important...?

I would think that the highest operating temperature would come from a balance between the radiator/fan's ability to move heat to the surrounding air (which increases as the radiator temperature rises above the ambient air temp) and the motor's production of heat.

mhead said:

Wouldn't both a 180 or 195 degree thermostat be fully open above 210? If so then the actual thermostat wouldn't be important...?

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Absolutely correct. Once the thermostat has opened, it doesn't make any difference what temperature is stamped on the flange. If the cooling system is inadequate, the vehicle's going to overheat.

In other words 'The thermostat does not open any wider as it gets hotter' once it opens it's open all the way, putting in a cooler thermostat only causes it open sooner and in the winter time will reduce the heat output of your heater, in the summer it will just keep it in cold loop longer.
My 87 Trooper had a piece of ty-wrap stuck in it keeping it open. I got NO heat at all and the engine never warmed up at all. What a PIA swapping out a thermostat at 8am in a parts store parking lot at -10F, the indian head gasket cement was freezing before I could get it on the gasket

Also..running a thermostat below 190 degrees makes the fuel mixture richer.You don't want that...