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Thread: Is my 4.0L really running hot or is gauge sender in hot spot?

  1. #1

    Question Is my 4.0L really running hot or is gauge sender in hot spot?

    1989 Comanche 4.0, Pioneer pkg. Running hot according to temp gauge but not in red. Without AC on, temp climbs slower but eventually goes past 210 mark on gauge. Had also bypassed the thermal switch in radiator with a jumper so that auxiliary fan runs all the time.
    Thoroughly cleaned cooling system and replaced thermostat, auxiliary fan switch in radiator, sender for temp gauge and a couple of hoses. Looking into head through thermostat opening, everything is clean and I have reason to believe radiator is too. No head gasket problems. No leaks. No restrictions of air flow through radiator and condenser. Engine temp slowly increases past the 210 mark on temp gauge in stop and go city driving and on highway. Hasn't gone into red zone on gauge yet but if accurate, must be about 225 degrees. Not comfortable with it running that hot.
    Have driven AMC's with inline 6's for 40 years and never had an overheat problem with everything clean and in good shape. A slight increase in engine temp is expected when air temp climbs into the 90s but not this much.
    I noticed that the senders for the temp gauges on older engines have been located at the front of the head below and to the right of the thermostat housing. On my Jeep, the sender is directly above the #6 exhaust port on top of the head. Not much room between the sender bulb and the water jacket wall above the exhaust port. Temp readings taken with infrared pistol grip type thermometer seem to indicate normal operating temps at front of engine, on hoses and radiator. However back at the sender, the temp reading is much higher. Almost 30 to 40 degrees. Radiator shows only a 10 degree drop from one end to the other with the AC on. Lately we have had air temps near 100.
    Questions: It has been recommended that I install a new radiator mostly due to age but before I do that, has anyone else with this engine experienced this much extra heating in the summer months and cured it with a new radiator? Could I be getting a false reading on the dash temp gauge due to the location of the sender? Is my engine really running that hot? Thanks.

  2. #2
    Registered Don Stephens's Avatar
    Join Date
    Aug 2009
    Yorba Linda,Ca.


    Welcome to the forum Ruckle, first off I would suggest moving to the Cherokee/Commanche section, for a faster and more accurate response. My experience with the 4.0 is that it is a hotter running engine and I would check on how hot the thermostat temp is set at since you didn't mention what theromostat you installed. Also a radiator can be recored cheaper than a replace if you want to go that route.

  3. #3


    Thanks for the reply Don. This forum looks like it can be very helpful to me once I fully understand how it works!
    I apologize for not including more information. When I replaced the thermostat, I put in a 180 Stant Superstat and plan to put back the 192 in the fall.
    It's interesting that today the outdoor temp was only in the high 80's and my temp gauge never reached 210 even with the AC on. I still have trouble believing the radiator is bad and still think the temp sender for the gauge is in a terrible place. May have had an air bubble somewhere too that has now worked its way out. The infrared thermometer I borrowed was a big help in measuring the engine temps in different places.
    With a hot AC condenser in front of the radiator on a 100 degree day and sitting at a stoplight, it's asking a lot of a radiator to cool like it can when the AC is off and it's 60 degrees outside. However my friend's Maxima, my brother's Stratus and even my old Alliance engine temp gauges went up and never moved past halfway whether it was zero or 100 outside.
    Guess I'll just keep an eye on it. I'll check a local radiator shop about recoring. A new one is $105 which isn't bad. Thanks again.

  4. #4


    I'm kinda new at some of this stuff. but.......

    Your jeep should never go over 210. Something is not right. Right now this week it has been 100 degrees in my area and my jeep doesn't go over 210. In fact I just replace the thermostat with a 195 degree stat and it is running cooler now. I also replaced the sensor which tells me the temp inside the dash. Did you replace that sensor?

    You must have a bubble or maybe not the correct mixture of fluid or the thermostat in backwards? not trying to be insulting. Have you noticed any leaks on the floor.

    Try starting the jeep with the radiator cap off and then maybe it can work some air out, when the fluid looks to start coming out then quickly put the radiator cap back on.

    Any cracks in the thermostat housing?

    When was the last time you replace your water pump and fan clutch which I am going to do this week.

    Does your fan turn on at all, sometimes they break and don't turn on, if that is the case then it is only getting air flow from outside and that is not enough.

    You need to stop driving this vehicle until you figure it out or your going to fudge up the head gasket. 225 is HOOOOT

    Please respond I want to know if you fixed this!!!


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