So I've noticed a LOT of overheating threads in the past month and have learned a bit about what causes them. I thought it would be nice to have A thread dedicated to overheating issues.
Before I start listing causes, if you're new to jeeps or cars in general, don't slack on maintenance. Trust me, it cost me a jeep. Before you have overheating issues, have your system flushed every 36k miles or three years, whichever is sooner. If it's a renix XJ (87-90 4.0L models) you call a DD then maintenance should be done religiously. 50/50 mixtures are very common, but if you live somewhere that sees temperatures below -25F it will freeze, so you lucky guys and girls get to run 70/30
. If you're a DIYer use pure distilled water as minerals from well water like to deposit in things like the radiator, heater core, everywhere. Running pure water in the system is fine in a pinch, but drain some out and refill it with coolant later. Water just doesn't transfer heat like antifreeze does.
Always check your coolant level and condition before throwing money at your vehicle. Never open a system that is under pressure.
Alright, now we're onto the good stuff. Situation One: your jeep runs hot or cold all the time regardless of ambient temperature. This is a telltale sign of a bad thermostat. People will fight to the death over what temp stat to use, but the 4.0 came with a 195F from the factory. Thermostats can be faulty right out of the box so maybe buy two or three and boil them in water to see which ones work and which don't.
Situation Two: it's always warm or hot. This can be a little trickier as leaks in the system will cause a rise in temperature. For every psi of pressure applied to water the boiling point is raised 4F. The first thing you want to do is remove the serpentine belt and check for play in the water pump pulley. If there is slop, replace it. If not, start looking and listening for leaks. When you get done driving it pop the hood and listen hard for hissing. Both my renixes have hissed at the pressure bottle overflow hose. In my experience this doesn't affect it much but tighten the hose clamp anyways. You might as well tighten everything while you're in there, it'll take all of five minutes. Another cause for always running hot is a bad radiator cap; it could be stuck open or the spring is weak. Most caps are 16psi (16psi x 4F/psi = and extra 64F of boiling temperature). They're cheap and easy to replace. But don't be that guy who opens it right after shutting down, you will be severely burned.
Situation Three: overheats in traffic but is fine on the highway: most likely the mechanical fan clutch. Lots of guys upgrade it with a ZJ clutch. The weight of that fan spinning saps power, so some guys opt for an electric fan instead. Many write ups can be found on that subject. Another thing to check out is the electric fan. It turns on at 220F I believe via sensor and relay, both of which can go bad. A good test for that system is to turn the AC on and listen for the e-fan. Replace the relay if nothing happens. If it passes that test, but still doesn't turn on periodically without AC try a new switch, located in the radiator on the driver's side.
It's 2:10am right now, I'll be back later to add and edit.
Finally, the best way to solve a problem is to find others with the same problem and learn how they fixed it. Share some stories and pass some knowledge.
DISCLAIMER: I only know what I have read often. I am no cooling system guru.
To save a little money, buy straight coolant. $12 for a gallon vs $8 for half water and half coolant.