Okay, we've visited on this temperature thing before, but this is weird!! I was driving home this afternoon with the AC on. Back roads, about 55 & 70ish degrees for 30 miles. All was well till I got in town & drove about 5 minutes with a couple of red lights, but no traffic. The temp shot up from where it normally rides to about 3/4 in a matter of seconds & then slowly rose till I got to the house (less than a mile) but never got to the red. parked the truck, checked the electric fans & they were running just fine & would hold a piece of copy paper to the grill. Got back in the truck to see where the guage read & it was right at the red. Turned the AC off & it dropped immediately back to normal & the electric fans were off. Let it sit for about 10 minutes, and started it again & the guage was reading normal operating temperature, but it acted like it was starving for gas. revved the motor to about 2000rpm's and it an better, but not great. Dropped it in gear, stepped on the gas & it ran normal. Drove about 2 miles, shut the truck off & let it sit for about another 10 minutes. When I started it again, it acted like it was running out of gas, but when I started driving it again, it ran fine. I hit the AC switch when I got home & the guage jumped again. All seems to be associated with the AC to me, but have no idea how. Ideas??

 

I'm on my fourth Disco '94, '95 ,'98,'01. I have the '95 now and having a real "picnic" with it in Central America. I've learned more about these cars in the last month than ever before. I also have temp problems and thought you might like to know a way to eliminate the radiator as a cause just in case you really are overheating, but don't have a laser. Start cold and just let the motor heat to the point where the thermostat just starts passing coolant and heating the hose. Give it just another minute or so and switch off. You can now feel the radiator top vs. bottom and because you have not allowed it to get totally heat soaked, you will notice right away if there is a difference in temp. Any difference in temp would of course be telling you that your due to rod the radiator, because the water should be passing across at the exact same rate. Cool below and warm or hot above would indicate a problem. I just rodded mine for 80 bucks in Costa Rica, it was 60% blocked! Hope that is help to someone. Bill

 

Let me be frank - it is overheating and you may have damaged truck.

1. Gauge can not be trusted. It will read about 50% at what would be considered high temp anyway. You must have a scanner or OBDII connected thing, like an Ultra Gauge.

2. Radiator has enough wind when driving to stay cool. In town, you depend on the viscous clutch fan. When cold, clutch should spin by hand and when released coast to a stop quickly, less than one revolution. Feels like peanut butter inside. Repeat test with engine fully warmed up and off. Still should not spin more than 1 revolution, perhaps even a little less than when cold. If it freewheels, the silicone fluid inside has been lost and fan has no power coupling when warm. Replacement clutch is Chevy 2000 Express van, 4.3 liter, no AC. You'll need to make fan holes slightly larger, Chevy fan uses larger bolts.

3. You can unplug AC compressor and turn on AC, both electric fans should run. This provides about 1/3 the cooling of the main fan.

4. Don't run truck hot or you will be replacing head gaskets or the engine.

5. Radiator can be partially blocked, needs maximum air to stay cool. With truck warmed up, and off, measure temp up/down on radiator fins. If more that 10F cooler at bottom then blockage has begun. Enough blocakage at slow speed the fan won't get enough hot air on faceplate to re-engage. D1 rad is copper and can have side tank unsoldered and calcium rodded out, usually under $75. New copper rad is high, aluminum one is $235.

6. With a 180F stat I make 183-187 F in 90+ temps. Not 212, 217, etc.

 

I've noticed my factory temp gauge responding to electrical accessories. Turn on headlights, the gauge goes up. Headlights + turn signals, gauge actually flicks up and down quite hilariously. Yet another reason why we all need an ultragauge. (Which remains rock steady through all of this.)
Like Savannah said, it definitely got way too hot, and you probably have a clogged radiator. Other possibilities: thermostat, expansion tank cap, water pump, fan clutch. Bigger problems like cracked blocks usually have other symptoms. (Water in oil, white smoke out the exhaust, etc.)

 

Ultraguage like this one would work?

UltraGauge EM v1.2 [UltraGauge_EM_v1.2] - $69.95 : UltraGauge, OBD II Scan Tool & Information Center

 

That's the one!