Late last night, just before bed, my wife informed me that the temperature gauge on her D1 was almost at maximum when she was driving during the day. Arrrrrgh! I asked how long it was up there and she replied "I don't know, I normally never look at it".. Apparently using some sort of logic she turned off the A/C and opened the windows. The temperature returned to normal after 2 minutes and hasn't played up since.

Plenty of good coolant in it, everything else looks fine.

Any ideas? (about the D1 I mean, not its owner, I'm working on her later).

 

With age of truck and the large copper radiator it may have a build up of scale inside that reduces the heat transfer. Add the load of the AC (summer in your half of the world) and it may be too much for the system. A couple of simple tests - open the top fitting and shine a flashlight down inside, looking at the ends of the rows of tubing, my 97 had so much scale you could see it coming out the end of the tubes. Also you can warm up the truck, and check temperature with an infrared thermometer of the fins top to bottom with truck off. More than 10F difference makes you think there may be some build up. I have tried commercial flush, white vinegar, and even removing radiator and soaking inside of it with muriatic acid up. None of those got the result of taking the radiator to a small rad shop that handles tractors and construction equipment. The Rover radiator is so small they don't charge much, they unsolder a tank, use rods to knock out the calcium, solder back up, check for leaks, and hot flush with citric acid. It works like a new one. I paid $75 US.

Your fan clutch could be bad, if she-who-must-be-obeyed does most of her driving in traffic and around town. At slow speed and stopped it provides most of the cooling. You can test by spin of the fan by hand when cold, and again when warmed up and off. Upon release the fan should stop quickly, less than a full revolution, and feel like Vegamite inside it. If it freewheels, it has lost the silcone oil inside. A new condition fan clutch makes a roar at cold start, as it is fully engaged, then it settles down to the low power setting. Mine pulls enough air to spin the electric fans slowly.

Your electric fans should have been operating, even if AC was off, once temp got above 212F. If not, suspect siezed (give them a spin), blown fuse, bad relay, etc.

I have enjoyed lower temps after changing the thermsotat (about $10 US) from the original to a 180F model. I run 179 - 183 around town, maybe 187 on the roadway at 70 mph; in 90F outdoor temps.

Hopefully you are not hearing any water rushing with bubbles sound under the dash, which could indicate exhaust gas bubbles in coolant. Head gasket.

The coolant bottle cap should be replaced, it must hold pressure when the system is operating, if not it vents and coolant is lost over and over again until too low.

There is a magic box called an Ultra Gauge that can monitor temps, reset engine codes, scan for codes. It can be programmed to alarm (she won't miss that) at high coolant temp. About $70 US. See UltraGauge Automotive Information Center and OBDII Scan Tool .

The temp you are runing is dangerous to the engine, can take out a head gasket, crack a block, make sleeves begin to slip. The gauge is not very precise. A scanner will show you exact numbers, and you can easily see if you are making progress. No need to go to 235 F.

 

Your fan clutch is bad.

 

Thanks guys. I'll test the clutch tomorrow and report back.

 

If a replacement clutch is to be fitted, here in the States a cheap replacement can be had from a 2000 Chevy Express van, 4.3 liter, without AC. Now you may not have too many of those, but the parts store computer may well have a cross for it. The fan clutch is six inch diameter, reverse rotation, and uses the same water pump threads as the D1 and D2. You have to make the mounting holes in the fan just a little larger for the larger screws.