Novak Conversions Jeep Wrangler TJ radiator

Overheating Issue: Cooling System Overhaul

Rider620

New Member
Original poster
Joined
Sep 15, 2019
Messages
19
Location
West Coast
Back in June my 4.0 99 TJ started overheating while I was away from home several hours. Finally brought it home later that month, temps stayed around 230 on the drive. Replaced the water pump, thermostat, radiator cap, temp sensor, and fan clutch, all of those parts are OEM Mopar. Fresh G-05 coolant mix. Original radiator blew apart in 2021 and was replaced with brand new OEM Mopar along with new rad hoses. System was burped on an incline with rad cap the highest point. Radiator had good flow with a garden hose, block was flushed out, coolant system was gas tested for combustion gasses 3x and each time was negative. I'm at my wits end here, any ideas?

Temps have been verified wit heat gun and OBD scanner. The coolant will boil in the overflow bottle if left running or driving for a period of time longer than 30 minutes. It doesnt matter if its idling or driving, still overheats. No coolant in the oil, no oil in the coolant.
 
Fan shroud is in good condition, did the paper test and it holds tight against the grille. Air filter is only about 5k miles old with a non dusty off-road trip. No vac leaks that I hear. No check engine codes, only a check gauges when the temp hits 260 on the dash. Gets shut off when that happens. Don't know how I would check a lean condition but my mpg hasn't changed, sitting around 12mpg currently.
 
do you have a timing light? if so check the timing at idle before it over heats but after warm-up.
you could also hit the exhaust with a temp gun on each exhaust runner looking for a hot or cold runner.
some mech shops will do a live 02 reading on a short trip to see a lean condition or your obd scanner if it has that function. we are looking for timing being off or fuel injector failure if cooling system it all satisfactory then it would have to be one of those.

sorry if it is jumbled but a lot of what ifs are in play here it could be something as simple as a radiator cap not holding pressure.
 
Last edited:
The coolant will boil in the overflow bottle if left running or driving for a period of time longer than 30 minutes. It doesnt matter if its idling or driving, still overheats.

Try running the heat at full blast and see if that helps anything. That's always one of the first things on my list when messing with the cooling system.

BTW, I suggest shutting it down before getting above 230 if it keeps climbing, and change your oil ASAP when this is solved.
 
do you have a timing light? if so check the timing at idle before it over heats but after warm-up.
you could also hit the exhaust with a temp gun on each exhaust runner looking for a hot or cold runner.
some mech shops will do a live 02 reading on a short trip to see a lean condition or your obd scanner if it has that function. we are looking for timing being off or fuel injector failure if cooling system it all satisfactory then it would have to be one of those.

sorry if it is jumbled but a lot of what ifs are in play here it could be something as simple as a radiator cap not holding pressure.

I had a mechanic from a good reputable shop test the coolant system for exhaust gasses 3 times, each time was negative. I'll see about the o2 reading next week when I get back from work. I'll try the temp gun on the exhaust manifold and see what I get.
 
Block flush cleaner from prestone. Then took the system apart and took a hose to it. Nothing bad came out, coolant was fairly clean. It was cleaned out previously in 2021 when the rad blew. Same stuff used.

I use Thermocure or Blue Devil every 2-3 years and it always comes out dirty in mine. I run it several hours. Fixed my overheating issues in two of my Jeeps.
 
  • Like
Reactions: BlueC
Try running the heat at full blast and see if that helps anything. That's always one of the first things on my list when messing with the cooling system.

BTW, I suggest shutting it down before getting above 230 if it keeps climbing, and change your oil ASAP when this is solved.

Running the heat on high does help some but not forever. I shut down soon as it hits 240 per the OBD, but the gauge only reads about 220. However the gauge will read 240 and sometimes ping to 260, but so far temp gun and scanner have not shown 260.
 
  • Like
Reactions: BlueC
Running the heat on high does help some but not forever.

So extra capacity helps, I think you have a blockage somewhere, maybe the radiator tubes, or possibly a lot of sediment buildup in the bottom/back. When you flushed the system, did you remove the drain plug in the side?

1727479535593.jpeg
 
So extra capacity helps, I think you have a blockage somewhere, maybe the radiator tubes, or possibly a lot of sediment buildup in the bottom/back. When you flushed the system, did you remove the drain plug in the side?

View attachment 561430

The 4.0L in our TJ does not have a coolant sensor on the side of the block; it is located on the thermostat housing.
The earlier generation 4.0L has a coolant temperature sensor screwed into the rear right corner of the head.
I could be wrong, but isn't that a Renix (<'90) 4.0L ?
 
I'll take a look now that I am back home. My buddy was saying to pop one of the freeze plugs out of the back to make things a little easier on clearing out a block if there is one. If theres only the one sensor, why would the ECM be reading 240F and the dash be reading 220?
 
My buddy was saying to pop one of the freeze plugs out of the back to make things a little easier on clearing out a block if there is one.

That is an option, but I'd use that as a last resort. Find the drain bolt that I mentioned, it will be a lot easier to remove and reinstall leak-free than a core plug. Besides that, the drain bolt should be at the lowest point of the block, which is where any dirt/grime/settlement would accumulate and flush to.
 
I'll take a look now that I am back home. My buddy was saying to pop one of the freeze plugs out of the back to make things a little easier on clearing out a block if there is one. If theres only the one sensor, why would the ECM be reading 240F and the dash be reading 220?

Take the return line to the heater core and cut it in half. Stick in 2 couplers and some clear hose about 5 foot for each and take those ends off to the side. Get a big bucket and put the two hoses in the bucket. Order a submersible fountain pump from someone and hook it to the new hose in the bucket leading to the heater core. Remove the thermostat and put the housing back on. Drain the entire system and refill with distilled water. Add a couple extra gallons to the bucket and add thermocure or CLR to the bucket and run the pump for several hours. Drain it all and do it again. Add enough chemicals to be on the high side of the directions. Keep the pump suspended off the bottom of the bucket. You will also need to plug off the overflow hose. This is a controlled way to chemically flush the system. Adding chemicals and then driving it makes me very uncomfortable since there is no immediate off switch and severe damage would come swiftly.

This will work the chemical throughout the entire system and dissolve all the rust particles. It will turn the water brown and after a few times of this you will end up with clear water indicating the entire system is clean and rust free. All the rust coating the inside acts like an insulating blanket preventing the radiator from doing its job, transferring heat out of the engine.

If you just run a hose into the radiator all you are doing is packing the crap down low and creating "channeling". This is where you create openings just large enough to allow water through but do not get the crud out, just push it out of the way.
 
  • Like
Reactions: BlueC
Take the return line to the heater core and cut it in half. Stick in 2 couplers and some clear hose about 5 foot for each and take those ends off to the side. Get a big bucket and put the two hoses in the bucket. Order a submersible fountain pump from someone and hook it to the new hose in the bucket leading to the heater core. Remove the thermostat and put the housing back on. Drain the entire system and refill with distilled water. Add a couple extra gallons to the bucket and add thermocure or CLR to the bucket and run the pump for several hours. Drain it all and do it again. Add enough chemicals to be on the high side of the directions. Keep the pump suspended off the bottom of the bucket. You will also need to plug off the overflow hose. This is a controlled way to chemically flush the system. Adding chemicals and then driving it makes me very uncomfortable since there is no immediate off switch and severe damage would come swiftly.

This will work the chemical throughout the entire system and dissolve all the rust particles. It will turn the water brown and after a few times of this you will end up with clear water indicating the entire system is clean and rust free. All the rust coating the inside acts like an insulating blanket preventing the radiator from doing its job, transferring heat out of the engine.

If you just run a hose into the radiator all you are doing is packing the crap down low and creating "channeling". This is where you create openings just large enough to allow water through but do not get the crud out, just push it out of the way.

Fwiw, I’ve run Thermocure for up to a week with no issues. I usually run it for multiple days at a minimum to convert the rust inside. The heat and pressure from driving around seemed to help more than when I only idled it and barely drove it.

I was much more successful with getting crud out by running a water/air gun that uses hose water and compressed air. Raising the water pressure with pulsating air bursts helps agitate the debri much better than water under constant pressure. Junk in my heater core let loose after I had already flushed many gallons through the system using the hose. It was alarming how much junk came out using that tool.
 
Last edited:
  • Like
Reactions: Rickyd
Novak Conversions Jeep Wrangler TJ radiator