Cherokee sport overheating -
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post #1 of 4 Old 04-14-2017, 02:38 PM Thread Starter
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Cherokee sport overheating

Hey all, I have a bit of a head scratcher going on. I am trying to help my friend fix her 98 Cherokee sport. It has the I6 4.0L engine. A while back, the radiator cap apparently failed and caused the engine to over heat. This was accompanied by white smoke out the exhaust and a dramatic loss of power. Afterword, the engine would not start. Well another friend fixed a couple of things and got it starting. It still heats up, but only after driving for a little bit. The engine will warm up to ~210 and then jump rapidly to 260. The engine power has returned though, and not more white smoke after the first time. Even more puzzling, the engine doesn't over heat on her way to work, only on the way back. On the way to work it is fine, then after sitting for ~9 hours, it will over heat 3 times at the same spot each time on the way home. I have checked the electricals, and they are fine. The battery is new, and so is the t-stat and radiator.

I am at a loss, there is no more white smoke or loss of power, the fuses and relays are fine, and the t-stat and radiator are new. I did find quite a lot of corrosion in the radiator neck, even though it has been purportedly flushed several times. When it over heats, steam comes out from under the hood, and she pulls over to let it cool. It does not seem to be leaking coolant from the water pump, and the water pump pumps quite well when turned by hand. I could use some advice on this.

Could it be the headgasket even though there is no more power loss or smoke? Please let me know what you all think.

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post #2 of 4 Old 04-15-2017, 04:42 PM
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2000 XJ Cherokee 
Join Date: Apr 2002
Location: out in the garage - Minneapolis
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Overheating can be caused by anything that decreases the cooling system’s ability to absorb, transport, and dissipate heat, such as a low coolant level, loss of coolant (through internal or external leaks), poor heat conductivity inside the engine because of accumulated mineral deposits in the water jackets or radiator, a defective thermostat that doesn’t open, poor airflow through the radiator, a slipping mechanical fan clutch, an inoperative electric cooling fan, a collapsed lower radiator hose, an eroded or loose water pump impeller or even a defective radiator cap.

The cooling system is a group of related parts that depend on proper function from each of its component parts to keep the engine cool. Service the cooling system and replace any under-performing or suspected weak parts. Any component part of the cooling system that is not fully doing its job will stress the others, and your engine will overheat. Temperature creep on the 4x4 trails, at idle, or in stop-n-go traffic, points to a weak or failing mechanical fan clutch or worn out water pump fins.

The most important maintenance item is to flush and refill the coolant periodically. Coolant should be replaced every 36,000 miles, or every two to three years. Anti-freeze has a number of additives that are designed to prevent corrosion in the cooling system, but they have a limited life span. The corrosion causes scale that eventually builds up and begins to clog the thin flat tubes in the radiator and heater core, causing the engine to eventually overheat.

-Inspect/test or replace the mechanical fan clutch. A worn fan clutch will allow temperature creep at stoplights, in heavy traffic, and on the 4x4 trails. A fan clutch that “looks” OK is not the same as working OK.
-Inspect the electric cooling fan and the fan relay. Apply 12 volts and make sure the e-fan runs. Exchange the cooling fan relay with one of the other similar relays. Confirm that the e-fan starts when engine temps reach 215-218*. Repair or replace the fan or relay as needed.
-Inspect/test or replace the coolant temperature sensor that activates the e-fan.
-Replace the water pump. The pumping fins can deteriorate over time and the pump will not flow enough coolant to keep the temps under control.
-Inspect/replace the radiator hoses. Make sure the coiled wire is installed in the lower hose.
-Inspect all of the frost plugs in the block and the ones on the backside of the head for rust holes and coolant leakage.
-Use a chemical flushing/cleaning solution to remove mineral buildup or rust, flush with clean water, and then drain and fill the radiator with a fresh 50/50 coolant and water mix. With neglected cooling system you may have to flush several times.
-Inspect the radiator for mud/bugs/grass clogging the outside and mineral deposits clogging the inside. Clean or replace as needed.
-Replace the thermostat with a genuine Jeep 195* thermostat. Cheap thermostats are cheap for a reason.
-Replace the radiator cap if your Jeep has one. An old worn out radiator cap will allow boil overs and/or allow the coolant flash over into to steam. You will see the coolant temps suddenly jump from 210* to the Red Zone and back to 210* if your radiator cap is weak.

If you have covered all the points listed above and still have overheating issues, inspect and test the head for cracks and head gasket for leaks. Exhaust gasses entering the coolant can raise the temperature of the coolant or cause steam pockets in the coolant that will temporarily block the flow of coolant

Read more about cooling systems here –


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post #3 of 4 Old 04-19-2017, 11:56 AM
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When she is experiencing the overheat, is she moving along at highway speeds (40+) or driving slowly, in traffic, through lights, etc...? Or both?

Overheating at speed (or both situations) is typically a coolant flow problem, meaning that a bad thermostat, bad waterpump, clogged cooling system, collapsing hose etc... is keeping enough coolant from getting through the radiator to cool the engine. Yes, a head gasket can fall in this category.

If it overheats at low speeds or idle, but NOT while cruising at speed, it's usually an airflow problem, I.E. the mechanical fan clutch, the E-fan or a damaged/missing fan shroud. Those don't matter above about 30-40 because the car moving is cramming more air over the rad than they could provide, but they are common culprits of low-speed/stopped/stop and go overheats.

It can of course be caused by multiple factors, and XJ cooling systems are sensitive to maintenance (or lack thereof) to begin with. It's worth going through Tim's entire list to be sure.

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post #4 of 4 Old 04-19-2017, 04:40 PM
It's the crank sensor!
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1999 XJ Cherokee 
Join Date: Nov 2007
Location: Central Minnesota
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What were the "couple of things" that her friend did to get it started after the overheat? Always a good idea to review history for clues/guidance/strategy moving forward.

For sure, all cooling system possibilities should be explored but to answer your question, YES, it could be the head gasket. With any overheat, that is a possibility. Here are a couple of options for two simple tests that you can do to help you narrow the root cause down.

1. Perform a simple compression test. See how the cylinders match up. The compression spec. for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders.

2. Go to your local Napa, O'Reilly, etc. parts store and buy a "block test kit". About $25 or so. It sniffs the coolant for the presence of hydrocarbons (byproduct of combustion) in the coolant meaning that something is breached, like a head gasket.

Good luck and be sure to keep us updated!

99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
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