Hey y'all:
I'm not convinced I've seen the correct symptoms for the problem to be the fan clutch, and before I plunk the $50, I'd like to be.

I drove the Disco all over town today. Never went on the freeway, probably never over 50mph. Over 90d ambient, and A/C running full blast. the temp gauge never got horizontal. It was rock steady.

But 2 nights ago, it was fine on the way home 'til I got off the freeway and was siting at a stop-light. I watched the temp climbing and couldn't get it to go back down, even once the light turned and I was cruising at 45mph again. I turned-off the A/C and it slowly came down, but the moment I turned it back on, BAM, it rose again.

SO this is a little inconsistent for my understanding.

Waddy'all think?

 

how well it works at 40mph is not depending on fan, it is mostly rad and water pump. so when slow or idle the change is less air flow due to clutch, electric fans, blocked fins, or all of them. the engine can not see your foot on brake

 

I change my answer to radiator.

 

I'm with ya.
so: if it truly IS a fan clutch problem, how come I can go out now, start it up and leave it idling with the A/C on full blast and it won't overheat?
But if I took it out on the freeway, and THEN brought it home and left it sitting in the drive idling with the A/C on, it'd overheat in minutes?

It's not all adding-up in my head. there's still something I'm missing. some other factor.

 

you only have half a radiator, the other half is sludge. can crank and idle. run down the road and you can not transfer enough heat when slow. could be combo of clutch and radiator

 

Don't know how to ask this without sounding argumentative. I'm not!

But why wouldn't it overheat sitting in my driveway idling with the A/C on? And I ran a flush: it runs clear and clean.

I am confused with it. I'm going to keep driving it as the DD and see if a more consistent pattern emerges. I can't turn it over to the wife / son until I've solved this. neither one of them would ever watch that gauge.

 

it wont overheat at idle. when you run engine at 50 - 70% power, with 5280 feet per minute of air times surface area of radiator you have 15 -20 times the airflow at idle in cubic feet per minute. so if overheat at idle after speed run, and fan clutch tests good then replace radiator

 

The water pumps in these are notorious for failing due to either corrosion internally and also the impeller is staked on and can loosen and slip rather than push the water thru as needed.

If it is original wp, you may be wise to change it!

 

Sorry, a little late to the party but these are $$$ facts
2001 P38
Drove with a POSSIBLE over heating
Bought Radiator, Water Pump, Fan Clutch, Hoses, Belts, Expansion tank so that FIXED it WRONG
/
The car had been run TO LONG at the BORDERLINE of overheating and it dropped a sleeve, NEW MOTOR!

If you like the car, Radiator is 232 on EBAY CHEAPER than a motor by far.
I am replacing everything on MJ because of what I learned on the P38 which I still have and it runs great now.

 

You can check radiator with infrared thermometer or Mark I palm reader. Warm up truck. Turn off. Touch or read lower fins in radiator, and upper ones. If lower ones are more that a little cooler, like more than 10F cooler, then water flow is restricted or blocked in bottom rows.

Danny and I agree on most things. The water pump might be an issue, but IMHO if it was the sole problem, then you would overheat at idle and under speed. Also, poorly maintained cooling systems will have a good deal of scale buildup, which tends to prevent freewheeling pump impeller.

Pump is rated to move 2.64 gallons per minute at 1000 rpm. So a purist can hook up some water from a jug and measure how long it takes to move it out.

If you are basing measurements on the dash temp gauge, it reads very broadly, and will allow you to damage engine before taking action. Get a scanner or Ultra Gauge.