Baystate Jeepers

For the last year and a half my heep has always runs 210 degrees. Sometimes it jussssst creeps over the 210 line, but barely

Today on the highway I saw it more than half way to the red line on the temp gauge. I was cruising at 65 - 70, 4th or 5th gear (depending on hills). Outside was hot and humid (felt like 90). So I stopped and checked the coolant, radiator was full but coolant jug was empty so I put 1/2 gallon of coolant in the jug. Back on the hway, same thing

When I got stuck in traffic something unexpected happened. The temp dropped to 210. Not bad, but not expected! Not moving and idling, it stayed at 210. Anything over 20 MPH, temp went up.

So I'm thinking the fan clutch is bad. It seems like anything over an idle, the fan can't keep up, even though the radiator is seeing more airflow at highway speed.

What do you think?

Head gasket

Usually a bad fan clutch causes problems at low speeds and not at cruising speeds.
Possibly you t-stat is failing to open all the way. Don't use a lower temp one. Use the stock temp one.

When the engine is well up to temp see if the radiator hose has pressure in it. If not then you radiator cap is toast.

Possibly your lower radiator hose is collapsing at cruising RPM's. There is normally a spring in the stock hose to prevent this from happening. If the hose was changed with one without the spring it could be collapsing.
If the radiator cap is failing to hold pressure it can lead to the lower hose collapse issue since there isn't any pressure to keep it balooned out.

Possibly your radiator is gunked up with deposits and can't provide the required cooling.
Pop the cap and look for mineral deposits in the openings of the core tube openings.

I went nuts with my ex-97 TJ. Ran too hot at highway speeds. Did the stat, pump, fan clutch, radiator cap, checked for the spring.. no improvement.
Dropped in a new radiator - presto all better.

Kurt wrote:Head gasket

oh, wait

You may have already done this but check to see if the exterior of the radiator is clogged with mud or other debris. If you have a/c check between the condenser and radiator.

I wash out the radiator with a garden hose to clean out all the dirt and crap in there Do it fromthe inside out so your pushing out the dirt Ido a few times a year The bottom of it comes out brown all the dirt and dust Be carefull onlt usea garden hose any more pressure will bend the cooling fins over that reminds me I have not done that yet this year Thanks

Kurt, Andy and Brian, thanks. Paul..... you're a funny bastid!!

One other thing I found out, makes me think it's dirt in the fins / condenser or restricted internal flow in the rad. When the ambient temp drops below 80, the problem just about goes away. Seems like my rad can handle its chores at lower temps but when it's 90 degs outside, it can't keep up. Maybe. Still seems the additional air flow should help but maybe not, the engine is working harder to get up to speed. I'm starting with the cheap and easy stuff first - garden hose rinse and repeat, and a $7 rad cap