99 XJ, 4.0
Recently I've started to have this issue with the XJ where even when I've been driving enough that the engine should definitely be up to full operating temperature, the gauge is reading almost 2 full clicks below the 210 mark.
I replaced the water pump, thermostat/water housing. Don't recall if I replaced the sensor at that time as this was about 6 years ago. The Jeep gets only about 3-4k a year clicked off on the odometer, so pretty low usage.
My concern is if the thermostat is not opening correctly. Not sure what is causing it. Fortunately the parts are dirt cheap to replace.

I don't want to load the parts cannon... what do y'all think?

 

If you suspect the thermostat, remove it and place in a pot of water on the stove, with a thermometer, and heat it up and see at what temp it fully opens

if good, reinstall, it bad get a new one

if still getting too hot, well assure your gage is correct, get a secondary reading on the coolant temp.

if really hot, then consider doing a back flush of the system, maybe it is dirty and clogged.

assure fan / pump belt aint slipping, check belt tension

assure the crank pulley is not slipping on the harmonic balance, check by making a mark on it across the hub to pulley with a sharpie then run it and then check to see if the mark slipped

Make sure the fan clutch is engaging.

of course check you aint having a dragging brake, or bad fuel mixture or tranny slip, that there can cause a hot engine.

 

that is all over it like a duck chasing a June bug. Great advice!

 

Thanks for the insight on this with things to check.
Will do so soon as I can.

 

That is great advice for running hot....original post is running 50 degrees too cold. That can only be a stuck open thermostat, if actually that cold. Verify with a temp gun . Also if you have a code reader...when at ambient for several hours or in the morning...verify that coolant temp and IAT temps are within a few degrees. But since happens when warm, temp gun is the best.

 

How about real world opinions from experience on what temp thermostat is best to use in our old XJ's with the 4.0.... I know stock is a 195 F I believe. But is that really best? Or should it be allowed to flow at say 180?
I ask because I know that on our old 4.0s the cooling system only works best when watching it closely and maintaining it properly.

 

In Ny where its winter...i run the Robert Shaw 195 balanced thermostat from flowkooler....it runs a steady 192-196 at highway speeds and goes up to around 205 in traffic, not really moving...thats at temps 40 and below..In summer I run their 180 stat. Keeps me 180-190 at highway speeds but depends on ambient temps and traffic..upper 90's and i can top above 210 with only mech fan in stop and go but the aux fan bypass brings it down below 200 in those conditions. The 180 did not give enough heat in winter for Vermont ski trips and stuff. DISCLAIMER....I do run a stroked motor with 20% larger injectors than stock...so i burn about 20%more fuel and run hotter. I have owned 2 jeeps since 1995 and have run them both to over 325K each...built several motors and strokers. I have a 5 gallon bucket filled with brass from the hundreds of thermostats i have run over the years. Most of them sucked. I do not have ac installed...siezed the pulley one year and since in NY summer heat and traffic I could NEVER keep it from overheating no matter what thermostat I ran if I was in traffic for an hour..normal for new york. ..both trucks 4 motors...numerous radiators...hence the bucket of BRASS. I never reinstalled.

 

Here in the deep South... I'm an hour drive away from the Gulf of Mexico. Summers here get pretty hot.. a lot of days over 100 F. And it's a huge city so lots of stop and go traffic to deal with most of the time.

 

It's not out of the realm of possibility that the T-stat failed with that few miles on it (I personally replaced the water pump, t-stat and housing when I bought it) I've worked remotely for 5 out of the 7 years the XJ has been under my care. And in that time I've averaged a whopping 3k/year on her. So those parts have very little wear.
It only has about 120K on the clock. So she's still got a lot of life left in her.

If the t-stat has indeed failed, at least it failed like it was supposed to.... but...... please correct me if I'm wrong... but even if the t-stat failed in the open position... once I've been driving long enough for the engine to be at full operating temp., it would stand to reason that the temp gauge would read where it normally has on mine with the 195 F t-stat. just a hair below the 210 mark on the gauge. But it is staying a full tick mark lower on the gauge.
so what gives? Maybe the temperature sensor at the t-stat housing is taking a poo? To my knowledge I believe it to be original to the Jeep.

I do have a desert class radiator installed. But I don't believe that to be in play here as it's been in since I replaced the above cooling system parts.

 

Thats the purpose of verifying the actual temp with the temp gun. You can remove the sensor and drop in a pot of boiling water, while reading the sensor resistance...should be 650 to 750 ballpark I don't remember the exact numbers. If the sensor is accurate, then the cool temps can ONLY be a stuck open thermostat. You need to hook up both terminals to your meter and read while its in the boiling water, if you take it out to read it cools off very quickly and will give a false reading. The higher the resistance, the lower the temp reading

 

I'll have to see if I can borrow one... I have a lot of tools and decent amount of specialised ones for the XJ and my other vehicles. but a temp gun isn't one of them. A timing light/gun yes.. but not temp.

My question for anyone that may know is still the following - If the T-stat is failed open, then once the engine is up to full operating temp, shouldn't it read on the gauge correctly? So that's what is making me think the sensor is the culprit here.
Thoughts?

 

I think that if you looked at it with no thermostat, then the engine would run at a temperature that it can maintain with in the parameters and the abilities of the existing cooling system. The manufacturer assumed that the Jeep under most driving conditions can maintain a engine temp under 195 degrees. The open/closed loop system of engine management I understand is triggered by the 195 degree temp reading to the ECU. So to maintain the 195 a thermostat is used for getting the engine to that temp and maintaining it even though the engine could run cooler that 195. I haven't run my Jeep without a thermostat, but under normal driving it stays around the 195 mark, on hot summer days (central Florida) it will climb to the 200-205 mark in stopped/slow traffic. If you drive your Jeep without a thermostat and it is still running 210...you may need to to some work to your cooling system. You can get a cheap IR temp gun and measure at the thermostat or even invest in a cheap aftermarket gauge to test with...Also if you have a App like Torque, it will tell you what the PCM is receiving as a temp...

 

With a 96 late and up...any jeep with only the sensor in the thermostat housing.....the temp displayed is what that sender puts out regardless of what the actual temp is. Its one of the failings of single source instrumentation. The purpose of the thermostat is to slow coolant flow into the radiator that the radiator can only cools a small amount of coolant and the temp goes up. When the temp hits the preset temp the stat opens up and allows cooling again and the temp's stabilize. This assumes there is enough airflow across the radiator to cool the flow rate. If your gauge is incorrect, then code reader / scanner will show what the sender is putting out. If your sender is "off" you will never know without independent verification. If you block off airflow into the radiator and you still show cold...problem is indication....BIGGER problem with that is you might overheat and cause damage before you can correct it. Either test the t-stat temp by some alternate means....test sender in a pot of boiling water...test the tstat in hot water, might be stuck open when you remove it...but you have to do something if you want to fix this. Other question...what is a desert rated radiator...I need one

 

Not sure if this is what he means, but Jeep did have a heavy-duty cooling option on the XJs that got exported to the Middle East and perhaps elsewhere. The radiator core is about 1/4" thicker than the regular one.

 

To answer your question.... I replaced the OEM radiator a couple of years ago with upgraded/aftermarket one I have now. It was designed for hot desert climates. It works better than the ageing OEM one I had.