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Discussion Starter · #1 ·
Took my 99 tj sport on a country ride today (411) miles. On the back roads of upstate ny she continued to run hot and forced my check gauges to come on during one climb. I can't figure out why she's running hot so here's what I did so far.

Thermostat replaced
Lower and upper hose replaced
Coolant change
Radiator 3 yrs ago


Checked fan clutch ( cold then warm ) still has resistance

Serpentine belt looks good

Fan runs straight - no wobble etc.

When she was hot I pulled over and shut her down and I could hear the coolant boiling in the radiator. Any help to fix this is much appreciated, cause damn I'm frustrated ! Thanks in advance !
 

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From my research on similar issues, if the coolant is boiling then there is a leak somewhere in the cooling system. The first and easiest place to start checking is the radiator cap.
 

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Three things to check,

1. check your radiator cap. Heck they are so cheap, just replace it. A cap that wont hold pressure can cause a boil-over.

2. With the cap off (cold engine) and then bring the engine up to operating temp, look into the filler neck and make sure you can see fluid movement. Especially when you gently rev the engine. There have been cases where the pump impellers have all but corroded off... so even though the pump did not leak, it did not circulate coolant.

3. Lastly, check your ignition timing. Timing that has been advanced too far can cause overheating.
 

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If you suspect a leak, your local parts store will have a pump that goes where the radiator cap would go, and allows you to pressurize the system. It should be able to hold 14-15 psi no problem. If it doesn't hold that, look around for coolant, it'll be pissing out at that point. Good luck!
 

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For future reference when an engine is overheating it's best to pull over and take all loads off the engine ( turn AC off ) then allow it to idle till it cools down. Shutting it off while it's overheated causes the coolant inside the engine to just sit and superheat/boil potentially cracking the plastic radiator tanks, cylinder head or engine block.

Make sure you have good airflow through the radiator and have that radiator cap pressure tested.
 

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When you changed the coolant was it dirty? Did you do a back flush or just drain the rad? Did you drain the block?
I was in a similar situation last week and it seemed on the steep decline and then incline I rocked the bad coolant from the bottom of the block into the top of the cooling system and it overheated. All my parts have less than 100 miles on them. I drained the block and it was pretty bad. Seems to be fine now but I will test a little harder this weekend.
 

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I had an issue with my TJ some time back. I ended up replacing the water pump. The circulation gizmo in the pump was worn away and the coolant was no longer circulating through the system.
If you end up replacing your's, make sure to flush the system out well before fitting the new one.
Hope this helps.
 
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