Thermostat Talk

Well, some people say that a 160 or other low temp thermo is the way to go.

While some of you say that a factory 195* thermo is the way to go....

I'd seriously like everyone's take on this..

The reason is, Ive got a 93 XJ, 6 month old GDI 3 core (before the company switch) , new water pump, new idler pulley, and new clutch fan, yet my frickin XJ STILL overheats.... but!! i have a 160* thermo in my xj...

Could the thermo be my WHOLE problem? It runs fine at highway speeds, but gets hot on the trail and at idle...

Well before the watchful eyes of the Eagle gets this. I run a 180 t-stat in mine. It works great on the trail/traffic/highway. I don't like it to get up to 210. But that's me. This past weekend I was out at Paragon, climbing up a long stretch of trail at a slow speed. I did get up to 200/210 with the weather about 40degrees. Once I got to the top and could cool it down. Right back to 180. So just my take. Juice

I'm not enough of a "problem solver" to help you out too much on your
overheating problem... but I *will* take a moment to explain to you WHY
I think the thermostat (and it's temp) have NOTHING to do with it!!

The Thermostat's job, in a nutshell, is to bypass the radiator during engine
warmup. This helps the engine to warm up much faster. Once up to full temp, the thermostat should be open, and all the coolant should be flowing thru the
radiator.

So, your lower-temp thermostat is simply letting coolant run thru the radiator at a lower engine temperature... which will slow down the warmup process. (This is why many folks recommend that you stay with the factory 195).

A thermostat CAN cause overheating, if it fails to open. This is quite common.
Even when installing a brand-new thermostat, folks usually make sure it
operates properly before driving away. This kind of overheat usually
happens within 5 minutes of startup (just as the engine gets to full operating
temp). Sometimes it'll just go slightly overtemp (you'll see the gauge go
into the red, or smell coolant) before the thermostat snaps open at some
"higher-than-designed" temp. (I think mine is doing that right now... the
gauge is going to 210+, and I smell a whiff of coolant, before it settles
down.) But since you have 35 degrees of "fudge factor", that's probably
not your situation.

A thermostat can also cause overheating if it's not opening fully, so that
the coolant is never getting a full flow to the radiator.

Did you have the same sorts of problems BEFORE the new 'stat was installed?
If not, I'd point the finger at the thermostat. A replacement is cheap.

Otherwise... there's another problem somewhere. It could be a water pump,
a blocked (or collapsing) hose, wrong radiator cap (if the system is not
pressurized enough, the XJ's normal tendency to run hot (220+) will
cause it to boil over.).

Cooling problems can be a major pain. But they're almost always logical
once you find the problem! You've already replaced most of the expensive
parts... it should all be downhill from here!! (Unless it's the water pump).

Den

No problems before really, but the thermo was stock, 10 yrs old, EEK!! But we replaced it when we did my radiator just b/c it seemed right to do.

I find one flaw in your theory, but I , grant it, COULD be wrong..

If the thermo opens up at too low a temp, it pulls the water/coolant thru the rad. too fast, causing it not to cool as well, how true is this?

That is the case if you use no thermostat...some road race cars from the 70s (still today? I don't know) used no 'stat, but they did use a restrictor plate in its place for that very reason

I too run a 180. I am uncomfortable with it because I strongly believe that a 190 or 195 should work fine. It did for many years, even with towing (I have owned mine since new).

I have done nearly all the fixes found on this forum with the execption of going to an open system. There may be one more issue, Air/Fuel ratio. I have been running with high NOx for a number of years. I don't know why. I believe adding a header, back cat and K&N has improved my volumetric efficeincy. The MAP should have accounted for it, but I am not sure. last week, I installed the MAP adjuster for a number of reasons, one of which was to pass emissions test. Since I have fixed the NOx problem, I am running much cooler.

If you run too cool a thermostat, you may fool the computer, via the water temp sensor, that you are in start up mode with a "cold" engine. Your MPG will suffer from too much fuel.

I am believe I am close to solving the cooling problem. When I do, I will swap back to the 190.

180 t-stat here too after doing the open system conversion on my '89. I run around 180 to a max of 195 in traffic on the freeways now. Much happier with these temps and so is the engine.

I switched over to the 180 as well. Makes a huge difference in keeping the motor cool.

Another problem I had was that I kept getting an air lock in the system, which would usually find its way to the heater core, then I would have to "burp" the system. Try filling the block and radiator separately, then remove the heater hoses and fill one while the air bleeds out the other. Fill completely and quickly put the heater hoses back on the thermo. housing. Its messy. but worth a try.

Just a thought. I had a long time Jeep mech. tell me this after trying many other things to remedy my overheating troubles. However, the 180 thermo made the biggest difference.

If it cools fine on the road, but overheats on the trail and in traffic your not getting enough air flow through the rad. to keep it cool. Check your fans. JIM.

160 is too cold. 160 is below the temp where the ECU switches from open loop ("warmup mode") to closed loop ("run mode," with the mix controlled by the ECU and O2 sensor). You'll be running constantly rich, wasting gas, and burning up the catalytic converter.

180 is marginal but should be warm enough to get you into closed loop operation. However, remember that the function of the thermostat is to keep the temperature UP, not down. The engine was designed to operate with a 195 degree thermostat. Physically, thermostats are the same except for the temp rating of the spring. Once they're open, they flow the same (disregarding specialty "high flow" models). If you cooling system isn't capable of keeping the temperature down to 195 with the thermostat fully open, how the hell do you think it's going to keep it down to 180?

There is zero reason not to run a 195 thermostat. That's what the engine was intended to run.

I'm no expert, but there's a lot of :bs: here.

Using a 180* thermostat does NOT mean your engine will run at that temp. That means your thermostat will OPEN at 180*. It will not help your engine run any cooler.

The 4.0 is designed to run at 210*. It may sound high as compared to other engines, but it is the norm for the 4.0.

Flame on...I know someone will prove me wrong.

-edit- Sorry Eagle, you beat me by a couple of minutes. And you explain it much more eloquently.

I ran into the same problems as you with a new modine 3-row. My '96 4.0L, AW4 still overheated after total swap out of cooling system and 180* t-stat, usally under loaded conditions (towing or lots of people and gear) and high outside temps (90+*). I concluded two things (with the help of much input):

1. When the tranny is being strained, the hotter than usual tranny fluid going through the internal radiator tranny cooler is actually preventing the rad fluid from cooling to it's max potential, and

2. I have in front of my radiator, an AC condenser and an OEM tranny cooler. The air is probably hot by the time it gets to the 2nd and 3rd rows of radiator, which has 5/8" tubes (very narrow). I replaced the 3-row with a 1-row aluminum rad w/much larger diameter tubes and overheating is a thing of the past. I found the aluminum rad at http://www.alumrad.com for $140.

Point #1 is hard to fix unless you own a tranny shop and don't mind risking frying it by bypassing the radiator internal tranny cooler. You'll notice that almost no one with a manual tranny has overheating problems.