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Rodgebone 02-03-2011 10:43 AM

4.0l XJ overheating? Start here - the basics!


So your 4.0L XJ is running hot or overheating. What to do? It’s the most commonly asked question on these pages and you will find many interpretations, causes and solutions. These are some basic guidelines to follow that should answer some questions before they get asked. While some of the information provided here comes straight from the FSM, most of it is my own opinion which is based in particular upon years of examining countless different XJs (hundreds in fact) with varying levels of system integrity, modifications and usage. Naturally these opinions are open for debate (which I encourage) as no one knows everything! For the purposes of this discussion we will be focusing ONLY on the 4.0L petrol engine and generally in stock form so as to create a benchmark for failures and modifications. So please don’t ask about diesels as that is a whole other ball of wax. The following info should give you a good indication of what you can expect from your cooling system and how to identify and address common issues.

XJs are notorious for running warm - it’s the number one issue 4.0L owners face here in Oz. This is largely due to the fact they were NEVER designed to use the big straight six 4.0L; the Cherokee came out in 1983 with a horribly underpowered little V6 until Jeep realized what a disaster it was. So they shoe-horned the big 4.0L into the already cramped engine compartment. As a result, this configuration produced an abnormal amount of ambient heat being trapped under the bonnet with nowhere to vent it (not to mention an inadequately undersized radiator). Add to this our RHD radiators run the tranny heat exchanger on the hot side of the radiator as opposed to US designs. So not only is it a poorly thought-out system to begin with, but our Ozzy imports suffer additionally from the conversion process.

So what's too hot and what's normal? Interpretation of this is highly subjective and will vary with operating conditions, mods and system integrity but the design actually allows for very hot temps within normal operating parameters (almost to 117*C which is virtually redline on the gauge). Generally, operating temps can fluctuate anywhere around 95-112*C in normal driving conditions but ideally about 108*C is max. Personally, I consider anything over 112*C on a hot day suspect but the system shouldn’t actually boil over until it hits the redline. So between 112*C and 117*C (boil over) you should watch things very closely. If possible, stop, place the tranny in neutral with the engine running and set the heater on high fan. This should let things cool off enough so you can continue on. High ambient temperatures coupled with stop and go traffic, towing, extended 4 wheeling in the sand and extended 4 LO climbs can aggravate a properly functioning system even on its best day. Outside influences can impact temps to a large degree and push the system beyond its capabilities quite easily so always proceed with caution. It shouldn’t be like this but it is so just accept it - it’s a Cherokee thing. Expecting more than that isn’t going to lower the temps so mod away and/or repair things till you get it where you want it. If this isn’t your cup of tea then don’t buy an XJ because you are likely going to deal with it sooner or later.

If you have an obvious problem such as a leaking radiator you need to consider the whole picture and not just the issue at hand. Of course you can replace the radiator but you need to know why it failed in the first place - was it just component failure or is there is a bigger issue looming in the background. The integrity of the system is very dependent on maintenance but normal wear and tear is going to happen no matter how well you take care of things. However, proper maintenance can prevent issues and extend system life greatly. If you do no other maintenance on your XJ at least maintain the cooling system - it’s a very delicate design so take care of it! I know this sounds like dribble and you've heard it a thousand times before but I can’t emphasize how critical it is. If you fob it off and have a problem you then you don’t deserve to complain - fix it and move on. Maintenance and repairs are never free no matter what you drive so don’t whine about spending money on your car's health - if this is you then maybe bicycles and trains are more your speed. And not having funds to repair stuff properly or doing a "bandaid" solution is only going to get you more grief – you can expect it.

What to look for when evaluating your cooling system:
1 - Ask yourself this: "How long have I had the car and what do I know about its history regarding repairs and maintenance?" If you bought the car recently then assume nothing has been done and start from scratch. Take service records with a grain of salt as they can be misinterpreted or even incorrect. Example: if your water pump looks brand new then it probably is but how do you know the correct pump was fitted? Did someone install a non-reverse flow impeller unit by mistake? And what concentration of coolant was used? Was the system pressure tested after the repair to evaluate other potential concerns? Is it a cheap pump with a short life span? You simply don’t know by looking so don’t assume anything. If you have owned the car for over 2 years and haven’t touched the cooling system then assume you need a complete evaluation and more than likely some form of servicing.

2 - Evaluate the system: Check the coolant level. The radiator should be completely full when hot and the overflow bottle should be at the "full” or “full hot" mark. If not, then you are losing coolant (or it wasn’t serviced properly to begin with). Check the coolant condition - it should be a 50/50 mix of alloy-friendly ethylene glycol/distilled water (DO NOT USE POLYPROYLENE GLYCOL) and the concentration should read at least +128*C to -20*C using a coolant tester. Watered down coolant will lower the boiling point and raise operating temps. Maintain at least a minimum of 44% ethylene glycol and don’t EVER run straight water (except in emergencies). Do not run without a thermostat or the engine will begin to form sludge from condensation in the crankcase which will cause long term engine damage (condensation is water and not the best thing for lubricating bearings). You should not see oil, rust or flaking in the coolant. If it doesn’t match this criteria then flush it thoroughly and/or make appropriate repairs. If you have rust in the system you have big problems. You will never get rid of it completely unless every last component (including the block) is boiled out and/or replaced. The cheaper solution is to flush it every year which should be adequate in these cases although components will have shorter life spans. Normally, the system should be flushed every 3-4 years. Make sure you use quality coolant with anti-corrosives and rust inhibitors along with distilled water (do NOT use tap water). Check the system visually for signs of leaks using a pressure tester while the engine is hot and use a bright torch so you can see any smaller seeps. You are looking for the obvious wet stuff but also look closely for past or intermittent leaks as evidenced by white or green crusty flaking, build up and dry runs. If you find a leak, fix it and then retest it again as a big leak will mask a smaller leak - never assume you fixed it all in one hit. Also make sure you pressure test the radiator cap for proper operation. It should hold at the specified pressure rating stamped into the cap (16 or 18psi depending on year) and release pressure a few psi above this (18-20psi). Caps generally should be replaced every 2-3 years as a rule of thumb. They are a common failure but play a big role and are often overlooked even by shops. A cap which doesn’t hold the specified pressure will lower the boiling point of the system which can lead to warmer running temps.

2A - Places to inspect for leaks: all hoses/connections, the radiator (especially along the driver side tank seam), both tranny cooler line connections to the radiator, the thermostat housing/gasket, the water pump/gasket and water pump weep hole located on top of the bearing housing and the welsh/freeze plugs - there are 5 plugs on the passenger side of the block plus one in the back of the cylinder head. A 7th plug is hidden between the engine and transmission and cannot be seen directly so watch for coolant at the bottom of the transmission bellhousing. Also check the heater control valve ('96 units only) and also the heater core itself. If the heater core is leaking you may note coolant coming out of the condensation drain tube located on the bottom passenger side of the firewall in the engine bay and/or a wet carpet on the passenger floor (make sure you peel the carpet back and check for moisture directly on the floorboard). Check that the heater is actually putting out heat; if not you may have a restriction in the heater core, a water pump impeller that is spinning on the shaft (especially cheap pumps with plastic impellers), corroded/damaged water pump impeller fins or air in the system. Another way to check this is to feel the heater hoses near the firewall when the engine is hot - if they are only warm (not hot) then the water pump is failing and not providing the required circulation. When pressure testing and noting a loss of pressure but you don’t see any leaks then coolant is probably going out the exhaust which usually means a cracked head. Inspect the cylinder head gasket line on the block for evidence of leaks and remove the spark plugs and check them for coolant contamination. You can also shine a light into the cylinders through the spark plug holes and see if any of the piston tops are shiny and clean (in comparison to the rest of the pistons). If you see this then you are getting coolant into the combustion chamber and out the tailpipe. This will be accompanied by an unexplained loss of coolant while driving without any visual leaks and the engine may be misfiring or running rough. A compression and block test can further verify this although sometimes you won’t find the problem until the head is actually removed and pressure tested/magna-fluxed at a machine shop. Water on the dipstick or oil in the coolant is a dead giveaway of a cracked head or blown head gasket so get it fixed pronto and don’t drive it until you do!

3 - Other things to check: Inspect the components of the system. Check the fan belt for slipping, glazing and improper tension. Check the condition of all the hoses - they should not be brittle, swollen or extremely soft and should not exhibit rot, rust or scaling inside. If they are over 5-6 years old then they are probably due for replacement as normal preventative maintenance. Also replace the thermostat every 2-3 years as preventative maintenance and make sure you ONLY install a 195*F unit - DO NOT USE ANY OTHER TEMP-RATED T-STAT! Check to make sure the electric auxiliary fan comes on when fully hot and that it comes on automatically with the AC or defroster switched on. Check the fan clutch for proper operation: with the motor stone cold and turned off it should freewheel with some light resistance but not more than five revolutions. If it doesn’t spin at all or is locked up replace it. Start the engine when stone cold (best done after sitting overnight) and listen for fan noise to increase as rpms increase (a normal condition). If fan pitch noise does not increase with rpms then the fan clutch is bad and needs replacement. As the engine warms up fan noise will gradually decrease (again, a normal condition). You will also hear the fan roar on initial start up then die down within 15 seconds or so (also normal). Once the engine is fully warm use a rolled up newspaper and insert it into the base of the moving fan blades. You should not be able to easily stop the fan from turning - if you can stop it then the fan clutch is defective and should be replaced. Also check that the fan clutch is not leaking oil - if it is, replace it. Make sure your fan shrouds are in place and intact - removal or trimming of the shrouds can be detrimental to airflow output from the fans. Check all the grounds for your aftermarket accessories (spotty, winches, radios etc) to ensure they are making good connections to the chassis, engine and battery. Poor grounds between the chassis, engine and battery can promote excessive electrical flow through the cooling system causing pitting and erosion (electrolysis) of metal components like welsh plugs, water pump impellers and thermostat housings. Make sure your engine and transmission oils are full, fresh and able to dissipate heat properly (oils don’t just lubricate, they also remove heat). Check that your spark plugs are the correct heat range and not producing excessive firing temps in the combustion chamber. Check the computer for stored codes - a malfunctioning system, sensor or wrong application/stuck thermostat can sometimes set a code or check engine light.

4 - Airflow and coolant flow: Check the radiator for restrictions in the core (a very common problem often overlooked). Check for airflow restrictions through the radiator. Blow out the fins in the radiator, AC condenser and tranny cooler from the engine side outwards row by row with compressed air (pay particular attention to the bottom and sides where debris build up accumulates). Make sure the fins are straight and not bent over preventing airflow through the core (use a small flat tip screwdriver to straighten them – a time consuming process but essential). Also ensure you haven’t compromised airflow by adding too many accessories directly in the path of the radiator (nudge bars, spotties, winches etc). While some amount of airflow disruption probably won’t affect things a lot there is great debate about what is too much. Use your discretion here but know you will affect airflow by adding too many accessories. There is also some debate about whether removal of the air dam and/or skid plate affects temps. Personally, I’ve found this unproven but again, be aware of potential effects. Rule of thumb: if you modify something and notice a negative impact which is causing you concern REMOVE IT!

When time allows I will post a follow-up to this info in the next section: "4.0L XJ OVERHEATING Part 2: Cooling System Failures and Repairs" and later we will have a third discussion: "4.0L XJ OVERHEATING Part 3: Cooling System Modifications".

Nakkas 02-03-2011 03:24 PM


Originally Posted by carvesdodo (Post 1170616)
Or run a lower temp thermostat that provides a lower, constant, engine running temp that doesnt interfere with the vehicles normal running requirements.

Why?? If the car was designed to run at 195F, why lower it? It won't stop it from overheating if the cooling system is not up to scratch.. and the car will not run at the PCM programmed temp.

carvesdodo 02-03-2011 04:23 PM


Originally Posted by Nakkas (Post 1170621)
Why?? If the car was designed to run at 195F, why lower it? It won't stop it from overheating if the cooling system is not up to scratch.. and the car will not run at the PCM programmed temp.

Who says it was ONLY designed to run at 195" ??? ..... Cerainly not the little fella that listed the 180" thermostat in an XJ parts manual.

Why lower it .... ??? ......... Why use a bit of thicker wire than nescessary for the intended amperage load ???

And which PCM programmed temp are we talking about .....
The P0125 -6*c one that is too cold ?,
The -5*c one that wouldnt be too cold ?,
The 70*c warm up cycle trip definition temp in the FSM ?,
The temp everyone thinks is the operating temp just because its labelled on the hit & miss dashboard gauge ?,
Or maybe the 20*c that just happens to be the fluid temp - when the heated O2 sensors have reached their operating temp and trigger closed loop as long as you're in idle or cruise modes ?,
Cant imagine it would be a 92*c temp coz that temp reference doesnt exist in the FSM .... as a running temp.

Its all old hat Nakkas .... Been done to death before. Have a real good read of sections 14 & 25 in your FSM ... Even tho most temp data seems to have been censored out in the later editions - theres enough info in there to grasp the Fuel Injection system operation .... If you take the time ...... altho you should be swotting up on the V8 manual now.

dazza96 02-03-2011 04:38 PM

i am with carves on this one i run a 160 thermostate and all good
cheers dazza
(also to acording to the book 160 thermostate is fine or they would not make it and sell it in aus)

Pantsly 02-03-2011 05:40 PM

I run a cooler thermostat and the truck seems to still sit on the exactly the same temps as it did with the standard thermo just takes a little longer to get there. Cant say I have seen any detrimental effects.

je33pn 02-03-2011 06:19 PM

Got yer stroker in yet!!

junglejuice 02-03-2011 06:37 PM

Excellent write up there Rodge, look forward ot the next installments....

junglejuice 02-03-2011 06:48 PM

Interestingly in the 94-96 parts manual (my model) it lists only 2 thermostats, an unrated unit and a 180* (severe use) unit. Another interesting thing is my '95 model had a radiator in it specified for a '94 model and I thought it was alloy/plastic when in fact when I removed it recently I found that it was actually copper/plastic and had been there at least 8yrs as that is how long I have owned it and only once did I have an issue of overheating!!!!!

ozjeepster 02-03-2011 06:49 PM

I drove my xj into the workshop at lunch & hooked her up to the jeep Techconnec just to run her numbers & check for any ecm pcm updates & low & behold what popped up HIGH AMBIENT OR HOT SOAK TEMPERATURES 2000 - 2001 XJ its a software update that lowers the temp threshold used to determine when the engine fans are turned on , Cooling fans will cycle more often .

Service Bulletin 18-031-03 18-039-03

Ran the PCM update & my thermos used to come on at 104 c to 105 c now they come on at about 98c or by the temp gage a needle width before 100c & thats where it stays , im pretty happy with that , but i have also just fully replaced all the cooling system & removed the AC gear so i have greater air flow through the front so it may vary a little from Jeep TO Jeep ;)

PCM update & re-program is for 4.0 ltr Auto's only

Top work there Rodger , cant wait to read the followups & sorry for the little Hijack but i thinkk it may also help the update xjer's ;)

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