Coolant Temperature Sensor. Help!!! Help!!!

I recently had an overheating problem with my 1991 XJ (4.0L, Auto Trans. w/ A/C). I just bought this car, drove it on a 150 mile trip (to pick up some free interior parts) and on the way back it overheated (260 deg. boiling over). I replaced the thermostat (on the road). Then noticed that the problem was that the engine seemed to have poor coolant circulation (i.e. the temp. gage would creep up on a long drive). I then replaced the water pump & radiator. This did not seem to make a big difference. Today I replaced the Coolant Temperature Sensor. I drove it for 5 minutes & my gage is reading about 250 deg.). I actually seemed to make matters worse. I have 3 questions. 1) what is the normal operating temp. 2) How did I screw up the C.T.S. installation (I did use teflon tape on the threads, although I don't see how that could have hurt). 3) If none of the things I have done to date fix the problem what would cause this poor cooling. Any help would be more than greatly appreciated (I am tired of waking my family up to drive me to work every morning). Thanks in advance.

I'd bet that your sensor is just fine. What I suspect is the problem is that you've still got an air bubble inside the head. [edit] I see you used Teflon tape on the threads. Not a good idea, the sensor grounds through the block. It's a brass NPT (tapered) thread so you shouldn't need any sealing material.

Temps in liquids won't go higher than the boiling point (note that this is dependent on pressure!) Temps in vapor phase will go MUCH higher. You see the spike in the gauge when the gauge sensor is in vapor phase, not liquid phase.

There's a few ways to deal with it. One is to raise the front or rear of the vehicle, and choose a bleed point to get as much air out as possible. If you lift the rear, the gauge sensor hole is a good bleed point. It's harder from the front because there's no good bleed point there. Even after you get as much out as possible, you'll still probably have a small bubble. The remaining bubble will eventually be driven out through the tstat through flow and heating/cooling cycles.

I had the same issue a few years ago, when I did a head gasket the night before I left for Moab. I watched the temp gauge bounce up and down for a couple of hundred miles until it settled somewhere in Iowa, and remained steady after that.

Thanks, I will remove the teflon tape. Is there any other way to remove the air (other than lifting the vehicle). This jeep didn't come with a jack (I should have took the one from the XJ I just sold).

Are you losing coolant? If there are no external coolant leaks but the coolant level is going down, you might have a blown head gasket or a cracked head/block. Check under the oil filler cap and see if there's any chocolate milkshake-like emulsion.

You wouldn't use a stock jack, you'll need to get the rears at least a foot in the air to have a shot at getting most of the bubble out. A couple of jack stands and a floor jack is the way to go.

The other alternative is just time. It will eventually blow through and be released through the pressure cap.

BE CAREFUL when you tighten down that sensor, it doesn't need much torque to seal. Brass is pretty soft and it will strip. Unfortunately, I can't find the spec in my 94 FSM, but it only has to hold 15-20 psi. When I tightened mine down, I used a 3/8" ratchet, and my hand over the ratchet itself (I've got a habit of over-torquing stuff.)

I took the sensor back off. There wasn't any teflon tape left on the threads. I tried to clean the sensor hole threads the best I could. I reinstalled the sensor & took a drive. I got about 3 miles & my gage read 255 degrees.

Beats me, then. Maybe I misunderstood your original post, I took it that the gauge went up and down. If the gauge just goes up, then you've either got a BIG bubble in the head, or you've got worse problems.

So after it overheated, and after you stopped and let it cool down again ... was the radiator FULL and some coolant on the overflow bottle, or was the bottle sucked dry and the radiator level below the filler neck?

Oh ... TIME OUT!

Your original post mentioned changing the water pump. Are you CERTAIN that you got the correct water pump? The old AMC 258 c.i.d./4.2L engine shares the same block, and the 4,2L water pump will bolt right onto a 4.0L. But the 4.2L doesn't use a serpentine belt, it uses conventional vee belts, so the direction of rotation is opposite. If you accidently got a 4.2L water pump, it isn't pumping anything, it's just sitting there cavitating.

While we're waiting on the water pump to declare itself - I've gone through the same problems, although overheating in 5 mins is much more severe than mine. I changed the coolant temp sensor, thought it repaired, changed the fan relay, and then just jumpered the relay plug - which worked. I note the FSM on the '90 shows with A/C there are diodes to control current flow from the CTS and A/C, so either can activate the fan, but not backfeed the other circuit.

Diodes can go bad - can losing current to the other circuit drop voltage enough to prevent the relay kicking in the fan? Most the time I just tap the relay and it kicks in just before I jumper the circuit.

Thanks for the replies.

I did get the correct water pump.
After I installed the water pump, Thermostat & radiator the car was working ok.
However, the temp. would creep from 210 deg. to 230 deg. and go back & forth between these ranges.
I was under the impression that 210 was the correct operating temp. so I thought a $10.00 sensor from NAPA would be an cheap & easy troubleshoot.

This is where I screwed up. Ever since I put the sensor on my gage will go up to 255deg.
It hasn't boiled over into the overflow bottle yet, but I am not sure if this is because I didn't drive it over a couple of minutes or because the sensor is giving me a false reading.

Also, to answer the previous reply; I do not have any water or gray emmulsion on the dipstick or under the oil cap, so I do not think I have a cracked head or head gasket.

If your engine really reached 255° F, it would give off a volcanic blast of heat when you opened the hood, and would be crackling like crazy while it cooled!

You probably do have an air bubble in the tstat housing, and repeated full warmups and cool-downs will eventually displace the air from the system. This is no consolation when your temperature guage is indicating you're about to meltdown!

Last I bought a temp sender it was more than $10. Are you sure you got the right temperature sender? There is one for the guage and one for the light.
be sure you got the one for the gauge.

I used the following technique to bleed the cooling system.

1. Be sure the coolant tank is almost full and let your XJ warm up until you get a reading of 210 degrees for a while.
2. turn off the engine and being "VERY CAREFUL", start opening the cap of the recovery tank to let all the air out. When you get the proper point, the air bubble will find it's way out, in the worst case after a few repetitions of this cycle.

This is for a closed system, an open system will probably react in a different way.

However, the gauge reaching 255 degres seems alot after only 3 miles, have you checked the coolant flow? Are you sure the thermostat is not getting stucked by something? Is the radiator getting hot?

schmiedel said:

I used the following technique to bleed the cooling system.

1. Be sure the coolant tank is almost full and let your XJ warm up until you get a reading of 210 degrees for a while.
2. turn off the engine and being "VERY CAREFUL", start opening the cap of the recovery tank to let all the air out. When you get the proper point, the air bubble will find it's way out, in the worst case after a few repetitions of this cycle.

This is for a closed system, an open system will probably react in a different way.

Click to expand...

The 91 is an open system, so this would not apply.

However, on your closed system, that bottle is NOT a "recovery tank," it's a pressure tank. And you should never, EVER open it when the system is hot and under pressure.

Vacalos -- I think you have air trapped in your cooling system from when you changed the temp sender. By the way -- which one did you change, the one in the back corner of the head or the one in the thermostat housing?