Can someone please explain to me exactly what a thermostat does. I always assumed I knew but I suspect I'm wrong, which is common with assumptions.

 

A thermostat is a spring loaded valve that opens up at a certain temp, and helps control flow.

Mine is a 195° so it opens at that temp only. Thermostats only control minimum coolant temp.

There are two kinds of fail modes for a thermo, most fail in the open position.

Did that answer the question?

 

Yes, thank you.

So if the engine becomes too cool while driving, the thermostat will close, but is otherwise open after the engine reaches normal operating temperature, correct?

Also, does the fan receive it's signal to engage from the coolant temperature sensor, or elsewhere?

 

hope that helps

 

It does, thank you Jakbob and rompnXJ. I now have a better understanding of my cooling system.

 

If I may, I'll just add one little tidbit regarding the operation of the t-stat. The t-stat will open slightly and slowly when coolant 1st reaches the t-stat's designed open temp. If coolant temp cools, the t-stat will again close. If coolant temp continues to rise, the t-sat will continue to slowly/gradually open more. At a coolant temp that is +/-15 degrees above the t-stat's designed open temp, the t-stat will be fully open. When the t-stat is wide open, it essentially is doing nothing. In that +/-15 degree window above the t-stat's designed open temp, the t-stat "hovers" in an infinite number of positions between closed and open.

As stated above, the t-stat does one thing only.....control minimum coolant temp. All of the other cooling system components and their condition along with ambient temp and driving conditions determine max coolant temp.

 

I'm trying to figure out how to fix my over-heating problem. She always ran at a few ticks below the 210 mark but the water pump went out last month so I replaced it, along with most of the hoses, the thermostat (195º from NAPA) and the thermostat housing. I flushed the system and refilled. Now she runs a few ticks above 210 and, if going up a hill or driven in warm weather, the needle constantly hovers near the red mark. I assume the radiator is okay, it doesn't leak and it was working fine before I replaced the water pump (NAPA) and all the other stuff. I've tried burping the system by letting her run with the heater on and reservoir bottle cap off but it made no difference in operating temp.

Despite running hot, there are no signs of leaking coolant. I'm at a loss. She'll idle all day and as soon as it starts to get warm, the fan comes on and the coolant temp immediately drops but if you're driving and the engine is under load, it gets hotter than it should. I was driving through the desert last week (probably 110º ambient), going 65-70 mph on flat highway and watched the needle slowly creep up to the danger zone... the e-fan was running but it made no difference. I had to pull over and wait for her to cool down before continuing. I just had my windows down, I wasn't using AC.

 

Just read through some of CherokeeOutlaw's post about basically the same thing...

https://www.cherokeeforum.com/f2/ove...angles-141309/

I'll read through the rest of that for ideas instead of wasting valuable forum space with my identical problem.

 

Ugh... what an unnecessary mess that thread became. Still, I take from it that I need a new coolant pressure bottle. I thought the bubbles and hissing around the cap were normal. Closer inspection reveals hairline cracks all over in the top of the bottle which is, I'm sure, allowing the system to suck air...

Cheaper than a new radiator!! WooT!

 

On a Renix closed system like yours, that bottle holds radiator pressure.

Radiator pressure raises the boiling point of the coolant.

 

That's what I figured. When I replaced all that stuff I washed out the bottle and noticed some deterioration in the plastic (tiny cracks/weathering) but paid it little attention since it had been working fine. I probably washed off all the gunk that was sealing the cracks in the bottle, LOL.

So, basically, even small, pinhole leaks lower the boiling point of the coolant and cause overheating. I'll order up a new bottle this afternoon. I assume that altitude doesn't help, these problems became more pronounced at 10,000+ feet... which also significantly lowers the boiling point.

 

Nearest I can tell, the boiling point of water at 10,000 feet is ~194ºf, or about 1º below my thermostat rating, LOL.

 

Also, from my experience, look under the water inlet (thermo) housing. I had a little leak and it was causing symptoms you have. Also, try removing the coolant temp sender on the back of the head to get some of that air out. Another few things i would look at;

Fan clutch. And i didnt see it covered, but renix has two temp senders and one sensor.

One sender is mounted on the driver side of the radiator, the other on is on the back of the head.

The sensor (what the ecu reads) is on the block, drivers side, under the exhaust manifold.

 

Correct, but the purpose of ethylene glycol (antifreeze) is to lower the freezing point and a side effect of that is a higher boiling point. Couple that with the pressure within the cooling system itself, you have more than enough.

Now that i think of it, sounds like you just have air in there. Pull the temp sender on the head, open the reservoir tank, and make sure that a steady stream of coolant comes out. Not bubbly.

Make sure you do this on a cold system. And when i say cold i mean cold.

 

Don't you mean lower boiling point?