mtyson

Preamble

The point of this post is to share what I've found when fixing overheating problems. The neat thing is, most stuff is in grasp of a patient do-it-yourselfer (like me).


If you are overheating, you can probably find and fix the problem yourself. Almost definitely.


There is some debate about whether the original cooling system is sufficient to the vehicle. My feeling is it pretty much is. That is to say: if everything is in working order, the XJ will keep itself cool enough for almost any application.


Good luck!!


Overview
The XJ is cooled by circulating coolant from the heated engine components to the radiator where hear is dissipated in metal fins with air passing across them.


List of Components and Failure Points
* Engine Block and Head Coolant Channels
Here is where the coolant cycles to pick up heat. Main failure here is if there are deposits or debris that blocks free movement of the coolant. To address this, a backflush of the system can be done, along with a detergent flush.


* Head Gasket
The head gasket seals the cylinder head to the block. If it fails, it can cause overheating by allowing combustion into the coolant, or by allows coolant to escape into oil (internal oil leak), or by allowing coolant to leak outside the engine (external coolant leak).


* Hoses and Clamps
The hoses carry the coolant from the different components. They are under high pressure and heat, and if the fail, or any clamps are not tight, a leak results, and can cause overheating.


* Water Pump
Pushes the coolant around. It sits at the bottom of the front of the engine on a pulley. If it fails, the Jeep will overheat because coolant isn't cycling, or is't cycling efficiently enough. Usually, coolant will escape from the WP weephole when the pump begins to fail (this is by design to indicate impending failure).


* Thermostat
The thermostat is a spring that is responsible for opening up the coolant channels to the engine when the engine is hot enough to need it. If it fails, it can not open quickly enough, it can freeze open, it can freeze closed, or any combo thereof. All of these can cause overheating.


* Radiator
The radiator takes the coolant heated by the engine, and allows it to release ("radiate") the heat through metal fins. The radiator can be a source of leaks, or can become clogged/corroded, or its fins can become blocked by debris outside.


* Mechanical Fan
The mechanical fan is driven by a pulley on the belt, just like the water pump. It is a key component in keeping air moving across the radiator. It has metal blades and engages when a metal "clutch" gets warm enough to grip the fan blades. The fan clutch wearing out is a point of failure leading to overheating.


* Electric Fan
The electric fan kicks on later than the mechanical fan. It is important for when the engine is getting too hot for just the mechanical fan. It is engaged by the computer (PCU "Power Control Unit") when it detects the coolant temperature sensor reaches high enough (210 I believe).


* Coolant Temp Sensor and Temp Sending Unit
In my Jeep ('97), there is one temperature sensor. It sits in the same housing as the thermostat, at the front top of the engine. It has a two-wire connector that goes into the fuel-rail wiring. On older jeeps, there is a second sensor, at the back of the engine, that drives the gauge in the dashboard (this one can cause the gauge to not work right, but doesn't affect cooling). In both cases, the temp sensor in the thermostat housing controls the electric fan -- if there is a problem with the sensor, the fan can fail to kick on, resulting in overheating.


Overheating Diagnosis
Alright, with the above component synopsis in mind, here's how I'd go about troubleshooting an overheat condition on the XJ.


Cooling problems can be divided into two categories: component failure and coolant issues (coolant leak/low coolant/air in coolant/bad water/coolant mix).


Low Coolant and Coolant Leak
Probably the most common issue is a coolant issue. So start there: is the coolant low? If so, you need to put coolant in it. Maybe that is your only problem (wouldn't that be nice).


Probably, you have a leak if your coolant is low. Most leaks are easy to find. Look around at all hose clamps, especially the factory pliers-style (not the worm screw) clamps: they become weak over time, and then start failing when the system is under high pressure.


Sometimes your only indication will be some splatter inside the engine compartment.


After hose clamps, the hoses themselves can split in hard to see ways and leak.


After clamps and hoses, the seals of components are suspect: the water pump and thermostat housing.


Find and fix your leak. But be sure to pay attention to the next section after filling your coolant.


Air in the System
A sneaky problem can be air in your coolant. Air can work its way to the thermostat, and cause it to not open. Same with air at the temp sensor. Air also allows boiling and changes the heat transfer characteristics of the coolant. Short story is: get the air out of the system.


Burp it/bleed it. There's no bleeder valve on the XJ coolant system, so the way to do this is: open radiator cap, turn on engine. Watch for bubbles escaping the fluid. You want to do this until no air is escaping.


To aid the process, you can park with the radiator cap corner up, or jack that side up. Also, you can use a funnel that fits snuggly into the filler neck, and fill it with water. Both of these serve to raise the height so air can escape more readily.


Also, squeeze/massage the upper radiator hose while doing this to help along air bubbles there: it is the default highest point in the system.


Wait until the thermostat opens, to allow air out of the engine itself (you'll know when the thermostat opens, the fluid will suddenly change height.)


Radiator External Obstruction
Here's another easy to find and fix problem: sometimes leaves and grit gets trapped in front of the radiator (between it and the AC condensor). That restricts the airflow. Get that crap outa there!


Bad Components
Radiator Cap
The easiest and cheapest component is the radiator cap. Replace it. Did that fix your problem?
No? Continue.


Thermostat
Also cheap, but a bit more of a job to replace. You can try to get a sense if the thermostate is opening when it should be seeing if the upper radiator hose gets hot when the temp reaches operating temp.


Even if this test is inconclusive, you can just replace the thermostat. It is fairly easy. Just make sure you get the seal where it won't leak.


Temp Gauge
The temp gauge can cause overheating by not triggering the electric fan. It is a cheap and dead simple component, so you might consider just replacing it with a good part.


Water Pump
Next up in the difficulty scale is the water pump. Usually you can tell its bad because it starts leaking. Never-the-less, if you still have a mystery overheat, and you've already done the radiator cap and thermostat, replace the WP.


Sometimes the blades get corroded or the bearing weak and it doesn't cycle efficiently.


Fans
Both fans must kick on when necessary and push air efficiently or you will overheat. The fans should turn aggressively and move a lot of air.


If they are weak or not coming on, replace. The mechanical fan is just a simple piece of metal, so its really the clutch you need to replace.


The electric fan can become weak, and need to be replace. It should really move some air when it comes on.


If your electric fan isn't coming on: make sure your temp gauge isn't screwed in to far: it should be in enough to prevent leaking, but not so far it touches the other side of the housing.


You can test everything but the sensor by doing this: unplug the sensor connector, turn on the engine. Did the electric fan come on? If no, something is bad from the connector to the fuses, to the computer or to the fan itself.


If the fan does come on, the sensor itself is bad (or bolted in too far). This works because unplugging the sensor simulates high resistance like high temp.


If the fan fails to engage with the connector unplugged, you might have a bad relay. Swap with a good relay. If that fixes, there's your problem.


If you still have an issue, you are down to wiring or the computer. You can test further by taking test leads and go direct from the battery to the slots for the relay. Then, you can follow the wires along, applying power. You can finally arrive at the computer and apply power to the pins there. If the computer fails to engage the fan, the computer it the problem. At that point, some folks will hot-wire a switch to engage the fan manually .

Finally: make sure the fan shroud is in place on the mechanical fan, otherwise it will not push air efficiently, and it'll overheat. (Thanks 4.3L XJ).


Head Gasket
Dreaded head gasket. These engines are renowned for their reliability. Nevertheless, the Hg can fail.


You can usually tell if the HG is bad by seeing milky fluid in the oil, or seeing burnt material in the coolant, or by seeing excessive air (even after burping) and churn in the radiator with the cap off.


These all indicate compression is escaping into the coolant chambers: result, overheating.


The coolant will rapidly overpressure, and overflow the reservoir, and boil there.


Replacing the HG is fairly epic job. It is doable by a do-it-yourselfer though, given time and patience.


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P.S. This is my attempt to give back something to this forum that has helped me/my family OVER AND OVER AND OVER AGAIN! THANKS GUYS!

4.3L XJ

Yoiu forgot shrouds. They must be in place so the fans can pull the air the system was designed for. Otherwise, it will overheat when moving slowly or not at all