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1996 zj 4.0
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Discussion Starter · #1 ·
So I looked across all the forums and tried to diagnose my particular problem but couldn’t seem to find it.

my 96 zj with the 4L runs at a perfect 195 most of the time until I turn on the ac then. It climbs into the 240+ range (I don’t leave it on to find out if it hits the red). This happens mostly at speed so there is plenty of wind flow to cool things. It’s happened in hot weather (85degrees ) and cool weather (65 degrees)

What’s really bizarre is it started just all of a sudden. I was on a road trip and it was fine. Camped and then two days later on the way back it started. I thought it might be dust on the Ac condenser so I washed it with a brush and hose to no avail.

I have a newer radiator 2 year old
new exhaust
new fan clutch
new water pump
Full coolant
New thermostat and thermostat housing with the sensor replaced with a Delphi one

I am hoping I do not need to change the AC Ccondensor (which if I did I would just replace theevaporator and compressor too)

any thoughts on this particular problem ?
Thanks in advance 🤙🏻
 

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Check to see if it is really that hot. Maybe there is some electrical glitch that is messing with the temperature reading when the AC is switched on. You might want to clean some wiring harness ground locations.

For it to all of a sudden begin to get hot, a piece of rust or other debris inside the AC system could have become lodged somewhere and is restricting the flow, causing the compressor to work very hard and draw a lot of power, so maybe a system flush would resolve that. Is the AC working the same as before? The compressor clutch I guess would be something that is more likely to suddenly go bad, but I guess the compressor itself can too. If the clutch is not kicking off from time to time, the constant running of the compressor would drive up engine temp I assume. The usual issue is that the engine gets hotter in traffic with the AC on and often that's fan clutch related, but an old, over-working AC system can do it too. Maybe having the system properly charged can help.
 

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Unless the condenser is externally clogged, it cannot cause the engine to overheat. If the engine is overheating when on the road, that sounds like a capacity issue. Like Redriver said, the cooling system could be clogged somewhere.
 
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1996 zj 4.0
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Discussion Starter · #5 ·
It did happen all of a sudden after some bumpy off-roading so maybe a piece of rust or something did dislodge in ac system ?
The air is still blowing cold at speed and at a stop.

it could be the clutch that’s the only bearing I haven’t replaced at the front of the engine and it has 230k on the clock

I’d definitely prefer to replace that over the whole ac system
 

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have you added say larger tires? that would create more work unless it was geared. work =heat. Just a guess. assuming the cooling system is good and the real temp your reading is correct. Knowing the trans temps would help but I dont think you can read them easy. Tran over temping would cause engine over heat if the two share cooling. shroud in place?all guesses.....

at 240 should have been blowing coolant in to the over flow i would think.

when it was new it did work i bet even here in AZ where it hotter then most any place in the USA. So something is wrong.
heck it gets over 115 here and everyone runs AC. One car i have the stock and even a very good rad did not do enough here in 115+. But this car has a old school Ls7 and never had air was not an option even. to keep it cool with a air system i had to use two pushers in front. unless its at speed and high speed you needed them as well. never over heated. and it had like a 195 T stat.
 

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Discussion Starter · #8 ·
have you added say larger tires? that would create more work unless it was geared. work =heat. Just a guess. assuming the cooling system is good and the real temp your reading is correct. Knowing the trans temps would help but I dont think you can read them easy. Tran over temping would cause engine over heat if the two share cooling. shroud in place?all guesses.....

at 240 should have been blowing coolant in to the over flow i would think.

when it was new it did work i bet even here in AZ where it hotter then most any place in the USA. So something is wrong.
heck it gets over 115 here and everyone runs AC. One car i have the stock and even a very good rad did not do enough here in 115+. But this car has a old school Ls7 and never had air was not an option even. to keep it cool with a air system i had to use two pushers in front. unless its at speed and high speed you needed them as well. never over heated. and it had like a 195 T stat.
it does have 31’s and a re gear to 4.11 with an auxiliary trans cooler with a rebuilt transmission (which was a whole other nightmare)

I think I might start with a AC system flush and work my way up instead of the usual mindset I have of replacing everything
 

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Discussion Starter · #9 ·
I think my next plan is to actually take the engine temperature to see if the gage is accurate. Then move on from there. I’ll post the results once I do that test
 

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Not sure about the 4.0L ZJ but the 5.2 liter has a 230'f fan clutch so I'm going to say what is happening is a design feature. You have a 230"f fan clutch on the front of your motor. You need a 205'F fan clutch. What this means is the fan is not pulling air but is free wheeling until the radiator temp reaches 230'F. Then the fan clutch engages. So you have no fan until you get to 230"F !!!! This is for fuel economy. Unfortunately 230'F is too ****ing hot. The motor doesn't like it.

There is a thread by DesertRider that covers replacing the Fan Clutch and the fan on the 5.2L V8 so that you get more airflow sooner across the radiators on the front of your ZJ. I bet the 4.0 Liter has the same clutch and ****ty metal fan with just 4 blades(?)

Doing this solved all my cooling issues with my V8. BTW I did all of what you are doing. All the temp sensors turned out to be working correctly. The fact that most of the time yours indicated 195'F indicates that your sensors are also working correctly as that is the normal open of the thermostat. Do not try a 160'F thermostat. Your fuel mileage will suck humongously as the fuel computer with think your engine is perpetually still warming up to a as yet not achieved normal operating temp (195'F) and will richen the mixture. Ask me how I know.
 

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I think my next plan is to actually take the engine temperature to see if the gage is accurate. Then move on from there. I’ll post the results once I do that test
That's the thing to do. Since you said the AC still works fine, as before, I wouldn't think it is anything to do with the system, such as an internal blockage I pondered. Because the high temp readings started after a bumpy trail, it makes me suspect an electrical connection or the gauge. I guess something mechanical could have gotten jarred around and is not working properly now, such as the compressor clutch or water pump impeller. Maybe some rust/gunk in the coolant passages or pump came loose and is now blocking the flow partially.
 

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Not sure about the 4.0L ZJ but the 5.2 liter has a 230'f fan clutch so I'm going to say what is happening is a design feature. You have a 230"f fan clutch on the front of your motor. You need a 205'F fan clutch.

There is a thread by DesertRider that covers replacing the Fan Clutch and the fan on the 5.2L V8 so that you get more airflow sooner across the radiators on the front of your ZJ. I bet the 4.0 Liter has the same clutch and ****ty metal fan with just 4 blades(?)

Doing this solved all my cooling issues with my V8. BTW I did all of what you are doing. All the temp sensors turned out to be working correctly.
Yeah, but did your cooling issues start all of a sudden after a drive on a bumpy trail, and only with the AC on while driving on the highway? His issue doesn't sound like an airflow problem.

The 4.0Ls had 2 options for fan clutches (and maybe the 5.2Ls did too), standard and "extra cooling", or "heavy duty" or something, that would have been on vehicles with towing set-up. There are threads on here about that and I think it was the final word that the correct heavy duty clutch is now no longer available. There are some that are labelled HD but aren't HD to the same level, or something like that. Somebody posted about how to modify the spring anchor point to make the clutch stiffer.
 

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Yeah, but did your cooling issues start all of a sudden after a drive on a bumpy trail, and only with the AC on while driving on the highway? His issue doesn't sound like an airflow problem.
My cooling issues started when I started towing stuff (Boat Uhauls etc). I have no cooling issues now with a 205'F fan clutch and an 11 blade plastic fan and a 205'F thermostat and my fuel mileage is back to 21MPG up from 16MPG when I had the 160'F thermostat. Just trying to save the lad some time.
 

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Once you check actual temp to see if the Guage is correct, the sensor for the Guage is on the driver's side rear of the engine just behind the valve cover. Looks like a bolt sticking up with a single wire connector (if you need to replace it) the sensor at the thermostat is for the computer.
195 degrees should be correct thermostat.
Does this have factory tow and a thermal clutch fan and another electric fan or just the clutch fan on the motor? Do you have the factory fan shroud still?
If it idols ok and gets hot driving, leave in park and turn on the ac then Rev and hold at 3k rpm for 30-60 seconds. Does it get hot? Have someone up front as well to see if the fan is working when you try that.
Does your heat work fine? No antifreeze smell in the cabin? Heater core could be bad if so causing bad coolant flow, bad pressure.
Also, nothing is infront of your radiator causing interference with air flow? And not caked with dirt and mud on the front side?
Oil looks like oil? Coolant looks like coolant? Not milky.
How is the oil pressure when it gets hot? Tranny temp?
 

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So I looked across all the forums and tried to diagnose my particular problem but couldn’t seem to find it.

my 96 zj with the 4L runs at a perfect 195 most of the time until I turn on the ac then. It climbs into the 240+ range (I don’t leave it on to find out if it hits the red). This happens mostly at speed so there is plenty of wind flow to cool things. It’s happened in hot weather (85degrees ) and cool weather (65 degrees)

What’s really bizarre is it started just all of a sudden. I was on a road trip and it was fine. Camped and then two days later on the way back it started. I thought it might be dust on the Ac condenser so I washed it with a brush and hose to no avail.

I have a newer radiator 2 year old
new exhaust
new fan clutch
new water pump
Full coolant
New thermostat and thermostat housing with the sensor replaced with a Delphi one

I am hoping I do not need to change the AC Ccondensor (which if I did I would just replace theevaporator and compressor too)

any thoughts on this particular problem ?
Thanks in advance 🤙🏻
Double check your fan clutch. And your temperature sensor.
 

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1996 zj 4.0
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Discussion Starter · #18 ·
Once you check actual temp to see if the Guage is correct, the sensor for the Guage is on the driver's side rear of the engine just behind the valve cover. Looks like a bolt sticking up with a single wire connector (if you need to replace it) the sensor at the thermostat is for the computer.
195 degrees should be correct thermostat.
Does this have factory tow and a thermal clutch fan and another electric fan or just the clutch fan on the motor? Do you have the factory fan shroud still?
If it idols ok and gets hot driving, leave in park and turn on the ac then Rev and hold at 3k rpm for 30-60 seconds. Does it get hot? Have someone up front as well to see if the fan is working when you try that.
Does your heat work fine? No antifreeze smell in the cabin? Heater core could be bad if so causing bad coolant flow, bad pressure.
Also, nothing is infront of your radiator causing interference with air flow? And not caked with dirt and mud on the front side?
Oil looks like oil? Coolant looks like coolant? Not milky.
How is the oil pressure when it gets hot? Tranny temp?
so I havent checked the actual temp because I need to get one of those infrared thermometers.

it just has the clutch fan, relatively new with the shroud. coolant is clean and only about a year old. Oil was changed a few weeks ago so it is good. Theres a slight coolant smell in the cabin with the heat on but that has been there for years, I dont think the heater core leaks because my coolant levels dont really drop and theres no puddle on my floor.

I will try the revving while somebody watches the fan, maybe it isn't kicking on under load which would explain this problem. Annoying because I replaced it like 2 years ago.

I have an auxiliary trans cooler in front of the radiator and condenser. But I have driven in very hot climate with that in front for years and have not this problem. This particular overheating problem started all of a sudden.
 

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I wouldnt be surprised at all if a 2 year old fan clutch went bad, stuff happens. I currently have the hayden 2791 severe duty fan clutch in my 5.2, it seems to be working well. It will roar for a good bit on startup but it quietens down after a minute or so of driving. It will slightly roar when stopped at traffic lights, which I think is good. I wouldn't want anything less on my Jeep.
 
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Once you check actual temp to see if the Guage is correct, the sensor for the Guage is on the driver's side rear of the engine just behind the valve cover. Looks like a bolt sticking up with a single wire connector (if you need to replace it) the sensor at the thermostat is for the computer.
That's only for the early (93-95) ZJs. The later (96-98) ZJ's only have one coolant sensor, in the thermostat housing. OP's is a 96.
 
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