4.0 Engine Overheating

[Steel City 06]

My 5 best guesses:

1. Bad thermostat. A bad thermostat will generally react poorly to changing temperatures.
2. Bad water pump. If the pump isn’t flowing enough through the block , hot and cold spots will develop.
3. Insufficient bypass flow through the heater core. If the heater core is partially (or completely) blocked, extreme hot and cold spots will develop in the engine until the thermostat opens to a significant degree.
4. Trapped air in system. Running the engine up to redline a few times once the engine has fully warmed up will generally fix this without opening anything.
5. Insufficient airflow through radiator. Bad fan, bad fan clutch, or similar. Or simply too much blockage from winches, bumpers, lights, etc.

 

[Ericict]

All kidding aside, I wouldn't rule out the ambient temp messing you up. I've have some one-off overheating instances that I could never attribute to anything except hot weather

I dunno….we run hotter than that down here in Ks with no issues and the guys in Az are much hotter than most.

 

[Jerry Bransford]

My 5 best guesses:

1. Bad thermostat. A bad thermostat will generally react poorly to changing temperatures.
2. Bad water pump. If the pump isn’t flowing enough through the block , hot and cold spots will develop.
3. Insufficient bypass flow through the heater core. If the heater core is partially (or completely) blocked, extreme hot and cold spots will develop in the engine until the thermostat opens to a significant degree.
4. Trapped air in system. Running the engine up to redline a few times once the engine has fully warmed up will generally fix this without opening anything.
5. Insufficient airflow through radiator. Bad fan, bad fan clutch, or similar. Or simply too much blockage from winches, bumpers, lights, etc.

I agree with all but the last part of #5. A properly functioning OE cooling system has way more reserve cooling capacity to be affected by those items.

Not to mention the same properly functioning OE cooling system laughs at hot ambient temperature conditions. It has no problems coping with even the heat in the deserts of SoCal and Arizona.

 

[Rob L.]

I dunno….we run hotter than that down here in Ks with no issues and the guys in Az are much hotter than most.

It's all relative. We hardly bust 100 in TN but at 100F and high humidity, you're not transferring much heat out of the system without a lot of air flow.

I would take a considerably hotter temp over a lower humid temp any day

anyways, just a thought to consider

 

[Steel City 06]

I agree with all but the last part of #5. A properly functioning OE cooling system has way more reserve cooling capacity to be affected by those items.

Not to mention the same properly functioning OE cooling system laughs at hot ambient temperature conditions. It has no problems coping with even the heat in the deserts of SoCal and Arizona.

I have actually overheated a cooling system (though not on a TJ) from excessive airflow blockage. I had a cargo basket on my front hitch receiver of my F150 (4.2 Essex engine). When that was combined with several minutes of continuous WOT at redline going up a freeway hill it overheated. (I also stupidly shut the engine off immediately which made the overheating problem worse.)

That said, the TJs are probably a bit more resilient than that. However I suspect if you block enough flow you will overheat at extended WOT near redline.

 

[Rob L.]

I have actually overheated a cooling system (though not on a TJ) from excessive airflow blockage. I had a cargo basket on my front hitch receiver of my F150 (4.2 Essex engine). When that was combined with several minutes of continuous WOT at redline going up a freeway hill it overheated. (I also stupidly shut the engine off immediately which made the overheating problem worse.)

That said, the TJs are probably a bit more resilient than that. However I suspect if you block enough flow you will overheat at extended WOT near redline.

They had to offset the front license plate on the F150 Ecoboost because it wasn't allowing the Charge-Air-Cooler enough airflow. Crazy how such small things can have a big impact

 

[Mike_H]

Don't know if you've ever got your answer on the fan clutch operating when cold. That is pretty normal. Mine does it too. The fluid pools up in the bottom of the clutch and it takes a little bit for it to distribute once you fire the engine up. It should go away in less than a minute on a cold start though.

regarding the rest...If I had to guess, you have an air bubble. If you can pull the cap and squeeze the upper radiator tube (when cold!) that will help speed the process. Then start the engine and watch for coolant circulating through the radiator. Let it come up to temp with the cap off. You can tell when the thermostat opens, because the upper hose will go from cold to hot pretty quickly. Let it run for 5 minutes or so after the thermostat opens. It doesn't take long. Watch out for bubbling hot fluid though! You'll get bubbles out the opening in the radiator.

Shut it down, let it cool off and check your levels. Put the cap on and test drive. If the rest of the system is in good shape, you should be good to go.

I generally don't suspect a water pump unless its weeping. I know the impellers can corrode off, but I've never seen one do that. Could be a bit climate dependent though. No one runs straight water in the midwest because of the chance of freezing at any time. Anyway, I'd say if you can see water circulating through the radiator while bleeding the air, the water pump will be OK.

 

[eSkilliam]

At one point my radiator cap was bad and the system wasn't holding pressure, causing it to overheat(and boil the coolant which is kind of scary to see coolant bubbling in your reservoir). $7 for a new cap from the auto parts store and my problem went away. It's worth trying before you start tearing apart the vehicle gain.

 

[IAmTodd]

I've been suffering from similar issues. Idling with A/C on the temp will creep up. I had the boiling which was solved with a new Mopar cap (and coolant flush) but still the high temps - 225-230. I was running the Explorer fan but swapped back to stock with a new clutch.

The radiator was replaced prior to my ownership with a non-OE replacement and I do have the PSC cooler setup behind the grill with a power steering and transmission cooler (possibly blocking airflow?). I plan to go through the entire system and refresh everything in hopes the aged parts are the culprit.

I remember XJ folks would complain of overheating but I could wheel all day with the AC pumping with no problem. The electric fan would be screaming the entire day but never an issue.

 

[Jezza]

I would replace that thermostat. Most aftermarket t-stats are complete garbage. Get a Mopar or Gates premium. Those are the only ones I would trust. The fact that they sell that other junk as useable is borderline criminal.

 

[JMT]

@Jerry Bransford @Mike_H . I removed the thermostat housing and the thermostat did not have the fitting allowing air to pass, so I drilled a 1/8” one near the lip and re-installed it at the top. Re-installed The housing and remove the radiator cap to allow it to burp. I squeezed the upper tube to help move air out. I also added some 50-50 G05 coolant to the radiator (at the cap) and started the engine. It ran for a while heating up slowly and the fan seemed to keep the same speed the entire time. I could not see fluid moving through the radiator. When it got to 210 it kept heating up to my great dismay. I also noticed that the housing is still not sealed and coolant was leaking very slowly but it seemed like it was dripping in a steady pattern. I don’t know if that could indicate the pump is working or not. The bigger radiator hose was really hot. Thoughts?

 

[JMT]

I would replace that thermostat. Most aftermarket t-stats are complete garbage. Get a Mopar or Gates premium. Those are the only ones I would trust. The fact that they sell that other junk as useable is borderline criminal.

I replaced the thermostat with a Stant 195

 

[JKP]

With the cap off, you can see the flow in the radiator if the pump is working. Also, does the heater work? If there is no flow, the heater will not be hot.

If the speed and sound of the fan never changed, your clutch is shot.

 

[Darsmith]

My '03 Sahara usually runs around 195 but this week it's been running upwards of 210-220 around town with the ambient temps of 110 here in eastern WA.

 

[Jerry Bransford]

@Jerry Bransford @Mike_H I also noticed that the housing is still not sealed and coolant was leaking very slowly but it seemed like it was dripping in a steady pattern. I don’t know if that could indicate the pump is working or not. The bigger radiator hose was really hot. Thoughts?

What housing is leaking? Thermostat housing or water pump housing behind the main pulley via its weep hole?

 

[Jerry Bransford]

My '03 Sahara usually runs around 195 but this week it's been running upwards of 210-220 around town with the ambient temps of 110 here in eastern WA.

That indicates someone installed the wrong thermostat, it should always run very close to 210. A 195 degree thermostat is what should be in there, not a 180 or 165 degree model.

 

[Darsmith]

That indicates someone installed the wrong thermostat, it should always run very close to 210. A 195 degree thermostat is what should be in there, not a 180 or 165 degree model.

I'm probably due for a cooling system flush so I will change out the thermostat when I do and check the current one when I pull it. I've also been working the A/C pretty hard.

 

[Trevlaw]

That indicates someone installed the wrong thermostat, it should always run very close to 210. A 195 degree thermostat is what should be in there, not a 180 or 165 degree model.

Not necessarily, mine did the same thing last week with the heat wave. 3k rpm for 45 minutes on the highway in 115* and I was creeping up around 225-230. I just replaced the thermostat not too long ago with a 195. Though there's probably something else wrong causing the issue. I'm gonna check my fan clutch after my drive home tonight

 

[JMT]

With the cap off, you can see the flow in the radiator if the pump is working. Also, does the heater work? If there is no flow, the heater will not be hot.

If the speed and sound of the fan never changed, your clutch is shot.

Super helpful! Looks like I’m in for a clutch and water pump. Thank you!!!!

 

[Steel City 06]

If you're going to drain the cooling system it might be worth running a cycle of thermocure or another descaler first. Corrosion products can build up in the block and effectively insulate the block from the coolant.

Be aware it can make previously unknown problems known, as it can also unplug leaks that have clogged up. But those were going to fail anyways and it's better to discover them when you are already planning on working on it, as opposed to the side of the freeway.