Not reaching 195-210F - Not thermostat...

Hi everyone,

I've been browsing a lot regarding my "problem" on a 93 xj 4.0 HO. It doesn't reach normal operating temp (195 to 210F). It stays right above or below the 160F mark on the dash (70 deg C). When the engine reach this temperature, the upper radiator hose start to have some hot coolant in it.

What I've done so far
- I've changed twice the thermostat (195F rated and not fail safe junk ...) I've tested both of the new ones in a pot of water and they do open at the right time. I've controlled the temperature on both ECU sensor and dash sender and they correspond to the displayed temp (the coolant is actually at 160 degrees fahrenheit)

- I've tried to put more sealant to avoid leakage around the thermostat inside the housing, no change.

- I've also burped / bled the system in many ways to ensure no trapped air and replaced radiator cap that was leaking with a 16lbs oem one.

The engine only reach operating temp when doing some offroad with very hilly mountain and when putting lot of load. Then it stays in the neighborhood of the 210f mark on the dash.

What I'm thinking :
I could try using a flow restrictor to prevent from loosing pressure to fast in the block.
Both of the 195F stats opens earlier when put in the engine?



I don't know where I should dig from now :/ What could it be ? if anyone has anything I could try to make it run the right temp that'd be awesome!

Love the jeep apart from that

Are you sure this is a problem?

160f is right about where mine runs most of the time. Like yours I only reach 210 when I am pushing the engine. I don't see it as a problem. It is nice to be able to go wheeling with the AC on. It is not as if the engine doesn't get hot enough to run the heater when needed, so I am not worried about.

However, I will suggest if low temps bother you then do not ever get a 12 valve Cummins. On that one, my normal temperature situation is a constantly swinging gauge, going back and forth between 140 and 190f.

Thanks for your reply Anak but I'm not a diesel kind of guy, so cummins won't happen

The engine is designed to run 195F, 160f is really low and decrease engine's efficiency (which is already pretty low and rudimentary...).
What are your mpg figures with these temps?

15.26 mpg.

That is an average taken from 10424.4 miles driven, 682.8 gallons of fuel.

FWIW I have a roof top cargo box which lives up there full time. 3.07 gears and 235/75r15 tires.

I agree that it seems impossible. If the thermostat doesn't open, the coolant is routed to the heater core. You're definitely not cooling the engine with the heater core.

I've found that when running dash command on my '96, the dash gauge is easily 10 degrees cooler than what's reported to the computer at the other end on the thermostat housing. I'd find it hard to believe that you've verified that those two temperatures match.

Have you tried changing the coolant sensor anyway? Have you verified the temperature of the coolant with an IR gun on the thermostat housing? IR guns are cheap and might help you determine if what you're seeing on the dash is actual temperature.

Lastly, you could block off part of the radiator to see if you can get the temperature up. But it certainly seems like you've got a thermostat problem. Thermostats are designed to keep the coolant inside the engine until it heats to operating temperature.

160 is not normal. That is what it would run if its constantly open. 200-210 is what you should see it run at when hot on warm days. 190 on cold days. Anak - your going to lose some 2-3 MPG if you are running it at 160 on a 75F day. It should sit closer to 195-200 and creep to 217 if your sitting in traffic.

It sounds like a stuck t-stat. If it was me, I would change it again despite that you have already changed it. There are not many reasons that its going to run cold unless its being cooled, thus t-stat.

What brand thermostat did you get? Some thermostats open wide and stay that way allowing the engine to run too cool.

Is there a hole (drilled in the thermostat)? If the hole is too large it have been known to cause over cooling.

Are the fans running constantly while driving? Check if the heater controls are set on defrost or if the e-fan is cycling/running.

Personally, I find the OEM thermostat to work best in my XJ. Others have had good results with other (specific) brands. Not all thermostats are created equal.

Lastly, try blocking part of the radiator and see what if it changes. I am sure it will.

The only reason the engine can run cold is the thermostat. You have corrected that, so...

Suspect the gauge and/or temp sender.

Your 93 has a separate gauge sender at the back of the head, and it frequently misreports. Especially if you replace with autozone crap part.

You really need to verify the temperature. Easy way is get an IR temperature gun and point it at the thermostat housing, then compare it to the gauge reading. Another thing to do is find a shop with a good quality OBDI scanner with a Chrysler connector and read the engine temp reported by the ECU (that is the sensor in the thermostat housing).

Hey guys, thanks for the kind replies. I havn't take the jeep that much lately.
I'm going to give a try to a third new thermostat, that one OEM from the dealer, since I can't find an oem one elsewhere.

I don't know the brand for these thermostat, one might be a crown one, the other one, I have no idea.
Regarding the gauge and temp sender they seemed fine with the checking I've made.
What I've done. Measure the ECU sender unit resistance while cold (overnight seating) and compare it to actual air temperature and required ohms for this temperatures in FSM, all good. Then compared the ECU sender unit ohms, related temperature in FSM and displayed temperature on dash, a bit off but no more than 5 to 10% off. So it is good to me.

I've done it this way since I don't have a laser thermometer, don't have access to one and don't want to buy cheap stuff...

Also, Yes there is a hole in one of the two thermostat, the one with hole display slightly lower temperature, but both are still far from operating temp


Hopefully the dealer's 40euros thermostat will fix that, otherwise I'll have 60euros of working thermostat sitting around XD

I'll keep you in touch when I receive it (It is mercedes that sells MOPAR over here and they don't have stocks for Jeep parts...)