thermostat question??????????

[ADulay]

Guys,

Well, there's been some interesting conversation with this thread on thermostats, but perhaps some of us may be thinking of the operation incorrectly.

If I may be so bold (being the new guy and all) to make a short and simple explanation of what you are seeing.

The thermostat is merely an "on/off switch" for coolant to move in a certain direction.

When the engine is cold, the coolant does NOT go through the radiator and therefore can warm up the engine as quickly as possible.

Once the coolant temperature reaches the design temperature of the thermostat, IT OPENS! Pretty simple concept, eh?

Once it opens, the coolant can then flow to the radiator and excess heat can be removed.

The key to remember here is that if it opens at 180 degrees, it will still be open at 200 degrees, 210 or whatever your cooling system can keep the coolant temperature stabilized at!

On my old 69 and 77 Corvettes, I did replace the original 195's with 180 stats. Yes, they did run cooler when running in normal conditions but when I pushed them hard (especially the 1969) the temps would run right back up to "slightly hotter than normal" no matter which thermostat was installed.

The bottom line? If your system is running at 200 degrees, the 165, 180 and 195 thermostats ARE OPEN. It's just that the smaller temp ones opened earlier.

If the theory of thermostats has changed since the late 70's then someone please correct me on this.

My current 2004 has the stock thermostat in it, which I will assume is a 195 and that's what it runs at for most of its running time.

Humbly submitted,

AD

[Rich Z]

Yes, that is my understanding of how a thermostat works as well. As an analogy, think about the thermostat on your air conditioning unit. When the temps in your house are cool, the AC is off and will remain off until the sun heats up your house to where the setting on the thermostat is reached. THEN once that point is reached, your AC comes on in order to cool down your house. When your house gets cooled from the AC, the thermostat detects that lower temperature and then shuts off the AC accordingly.

But there is a difference, of course, between this analogy and your car. The heat generated by a running engine will probably NEVER allow the coolant to drop down below the setting on the thermostat. Not unless you live within the arctic circle, I would guess. So for all intents and purposes, that thermostat stays COMPLETELY open from the time the temperature reaches it's set opening level, until such time that you turn off the engine, and the coolant then cools below that point to again close that thermostat.

In the analogy above with the home AC, if the heat (sunlight) being applied to your house were such that it was MUCH hotter outside than inside, your AC would likely run continually until daylight ended as the AC tried to bring the temp inside the house to BELOW the setting you specified on the thermostat. If the AC is defective (low coolant, for instance) it WILL run continually as it tries to meet that goal, but unable to succeed, as the AC is just not able to pump out enough cold air to cool down your house. Which is why when you get a HUGE electric bill some month, it is wise to have your AC checked out. It's probably running 12 hours or so a day trying to do it's job.

In both respects the THERMOSTAT in an engine and for a home AC unit will act in exactly the same manner.

[thebeepster]

Understand and agree w/ all the above...but once the engine is fully warmed up and operating (and thermostats are open), why do the cruising temps vary so much?

[davekp]

Originally Posted by Rich Z

Yes, that is my understanding of how a thermostat works as well. As an analogy, think about the thermostat on your air conditioning unit. When the temps in your house are cool, the AC is off and will remain off until the sun heats up your house to where the setting on the thermostat is reached. THEN once that point is reached, your AC comes on in order to cool down your house. When your house gets cooled from the AC, the thermostat detects that lower temperature and then shuts off the AC accordingly.

But there is a difference, of course, between this analogy and your car. The heat generated by a running engine will probably NEVER allow the coolant to drop down below the setting on the thermostat. Not unless you live within the arctic circle, I would guess. So for all intents and purposes, that thermostat stays COMPLETELY open from the time the temperature reaches it's set opening level, until such time that you turn off the engine, and the coolant then cools below that point to again close that thermostat.

In the analogy above with the home AC, if the heat (sunlight) being applied to your house were such that it was MUCH hotter outside than inside, your AC would likely run continually until daylight ended as the AC tried to bring the temp inside the house to BELOW the setting you specified on the thermostat. If the AC is defective (low coolant, for instance) it WILL run continually as it tries to meet that goal, but unable to succeed, as the AC is just not able to pump out enough cold air to cool down your house. Which is why when you get a HUGE electric bill some month, it is wise to have your AC checked out. It's probably running 12 hours or so a day trying to do it's job.

In both respects the THERMOSTAT in an engine and for a home AC unit will act in exactly the same manner.

Actually the cooling system can maintain a coolant temp lower than the stat opening temp IF there is sufficient airflow through the radiator and sufficient water flow through the engine. The problem arises when we sit in traffic and don't have any air moving through the radiator.
Also, AT CRUISE, the stat will be modulating the water flow, and in most cases will not be fully open all the time.

[Rich Z]

Originally Posted by davekp

Actually the cooling system can maintain a coolant temp lower than the stat opening temp IF there is sufficient airflow through the radiator and sufficient water flow through the engine. The problem arises when we sit in traffic and don't have any air moving through the radiator.
Also, AT CRUISE, the stat will be modulating the water flow, and in most cases will not be fully open all the time.

The term "modulating" here implies a non binary operation of the thermostat, which is what I am trying to determine. My understanding of how the thermostat works is that it is binary, i.e. OPEN or CLOSED. Not a variable flow device which fluctuates the amount of flow over a range from full closed to full open. And much like the settings on the radiator fans, the CLOSE temperature might actually be lower than the OPEN temperature, so when the engine would be cooling down, the thermostat wouldn't necessarily close at the same temperature that it opens up at.

But in any event, I seriously doubt that a 160 degree thermostat allows more coolant flow than a 180 degree thermostat when they are both fully opened.

[THOR]

Wow, This has been some interesting reading and I'm humbled by the knowledge base. So what does a neophyte like myself do? Do I keep the stock thermostat and not worry about the Florida climate or do I change to a lower thermostat? I went to Boca Raton and back from Bradenton last weekend and the coolant temp was bet 190-200. I have seen it up around 210-215 on other runs. Do I need to be concerned? thanks again, Mark

[ADulay]

No need to be concerned. It just depends on what type of driving you're doing. Easy city driving with stretches of medium speeds will most probably keep the temps spot on at 195-200. That's pretty standard for pretty much most automotive design these days.

Run it up to 110 mph for 20 minutes on the interstate and it most probably will increase a bit.

I'm sure that somewhere in the innards of General Motors are the heating/cooling calulations that say that at "X mph" the rate of cooling at X temp and X humidity results in X amount of BTU's being removed from the coolant.

Think about what the status of the thermostat is in a 180 degree rig.

At about 180 degrees, it opens up and allows the coolant flow into the radiator. So far so good. The radiator can disperse heat and return "cooler" liquid back to the system. If the cooling system has the capacity to actually bring the coolant temp back down below 180, the thermostat may well close again!! (But not for long)

Now, really start pushing the car and with the engine developing a major session of heat, the 180 thermostat is still open, the coolant is still flowing to the radiator and heat is being dissapated, but the rate of cooling is less than the amount of heat being generated so the temp starts to increase. The 180 (or 160, 195) thermostat is still open, it's just that the COOLING SYSTEM is now being tasked to remove more BTU's than it is capable of, which results in the coolant temp increasing.

The reason it doesn't keep increasing (hopefully) is because the entire system can handle something up to about 254 (234?) degrees before bad things can start to happen.

When I would run the 69 Corvette at 90 mph, the temp would routinely run up to something like 212-215 and sit there all day long. When I came back to 70 mph, the temps settled back down to "normal". This all with a 180 TS. If I had to come to a quick stop from 100 mph and sit and idle, the temps would just keep climbing. Why? Too much residual heat and not enough capacity while idling to remove that heat. My wife's Mercedes is notorious for THAT little trick. Hence the electric fans start coming on!! Good plan, eh?

The bottom line?

As long as the temps return to "normal" under "normal" driving conditions, it's working just fine.

You can get temps from 200-210 easily just sitting in traffic for 10 minutes down here in Florida. Start moving, cooling becomes more efficient, temps go back down. Standard stuff.

AD

[davekp]

Originally Posted by Rich Z

The term "modulating" here implies a non binary operation of the thermostat, which is what I am trying to determine. My understanding of how the thermostat works is that it is binary, i.e. OPEN or CLOSED. Not a variable flow device which fluctuates the amount of flow over a range from full closed to full open. And much like the settings on the radiator fans, the CLOSE temperature might actually be lower than the OPEN temperature, so when the engine would be cooling down, the thermostat wouldn't necessarily close at the same temperature that it opens up at.

But in any event, I seriously doubt that a 160 degree thermostat allows more coolant flow than a 180 degree thermostat when they are both fully opened.

The thermostat does in fact modulate, when the correct conditions exist ,i.e. adequate airflow through the radiator and adequate coolant flow through the engine. These conditions exist at normal cruise. This is why a 160 stat at cruise will maintain a lower coolant temp than a stock stat at cruise. This is the whole idea of a thermostat. It is not just to warm the engine up quickly, but to also control the operating temperature.
You are correct that all fully open stats flow the same when fully open.

[thebeepster]

OK Mark, there are your answers!! Actually, this was a great thread and very informative. I think what it boils down to is that many of us believe that we can out-engineer GM thru making mods to what is already an excellent performance machine. However, we should remain mindful that by definition we live in a SEVERE climate state. Those of us who run the 160 deg TS have little foundation for better performance, but simply are more comfortable shaving 18-20 degrees off the cruising temps on a hot day

[98 softtail]

Here is a thought, wouldn't it be more effective to be able to reprogram the fans to come on at lower engine speeds to help cool the radiator? I have lived here for over 30 years and we used to just remove the thermostat, but now it seems that if your engine doesn't reach operating temp the computer gets wacky.

Joe B.