This has happened last two nights when driving home....
1999 D1, I drive about a mile on side roads (30 mph) and when I get onto the highway (60 mph) after about a mile, the temp gauge quickly climbs to the top end. Freaking out, I pull over and slow down to get to an exit, (about 1/4 mile) and as I am slowing down, the gauge goes back to just under 1/2 way, where it normally sit. I then exit and re-enter the highway. I drive the 40 miles home with no problems (gauge wise) Driving the same roads to work, I never see this happen. Any ideas??

two nights in a row now....

 

Are you smelling anything unusual?
Could this be the result of a stuck thermostat?

 

You know that gauge isn't worth a darn, right?

If you don't already have an obd dongle and smart phone app, get one. It will be the best and most cost efficient trouble shooting tool you have. $30-60 well spent.

I use a cheap dongle with wifi (iphone) and an app called "OBD Fusion" (at the time I purchased it iphone had issues with bluetooth). I'm pretty sure just about any bluetooth dongle now works with both Android and iOS. If you have an Android device, TorquePro seems to be the leading app.

 

I have a Scangauge II from another car that I should put in there. I have noticed the rear view mirror I installed with the compass is not even close to being right. truck runs fine and only happens those 2 times. did not notice any hot smell. Had the heater on both times so maybe that had something to do with it?? When I got home, I found out the brake lights are not working. I can shift out of park, and hear the lock solinoid click when I press the pedal so maybe I knocked a connector off when I put the new ignition switch in.

Must be the full moon or something.

 

Drove to work today, outside temp was in the 80's, as soon as I got on the highway and up to 65-70 MPH temp gauge climbed up again. I had my scanner plugged in and temps went up to the 235 range when I pulled off the road.
With the gauge in middle area, temp read 207 -212. If I stay under 50-55, it sticks right around 210-195. I get slower and it drops to 195. I am thinking it is a thermostat issue. running the heater doesn't seem to make a difference, A/C is off.

driving home temp was around 77 and it never went over 212... Temp gauge is no where close to being linear.212 is about 1/2 way, top end is at 235...

My other truck (1997 D1) sits at 190 pretty much all the time and outside temp has no affect on it. Supposed to be warm this week so I will be driving the old beast until parts arrive.

 

And your cooling fluid cap has a good seal?
You can test your thermostat by putting it in a pot of boiling water to see it open/close.

Exactly how are you bleeding the cooling system?

 

new cooling cap 2 months ago. If I pull the thermostat, I will just replace it once it is apart. Truck parked on a 2 ft high hill, front higher than the rear, Filled the upper hose after filling the radiator, heater on. It has been working fine since I replaced the upper hose, which had a bulge in it and looked to be original. This only started a week ago and this was the first I have had a gauge where I can read temperatures. Today I will be driving my '97 the same route with even higher outside temps to see what I get for readings.

Other cars I have had did not have temps go above 208 or so . My question is if it is a thermostat, radiator, or fan. it has the fan on the motor, with the clutch. This is not my DD yet so I can play with it to figure it out.

 

if temp goes up that high at highway speed I would look at the radiator. An inexpensive IR thermometer can be helpful.

 

Ok, a bit warmer outside 91 degrees, driving the same route in my '97, I got temps 191-200 at 55-60 mph (no A/C) and at 65-70 mph 200-207. Never got above 210, and gauge sat right in the middle range, never going over 1/2 way.

the 235 temp on the '99 was with a scanner in the OBDII port, readings on both vehicles were at the port.

 

Engine Temp Sensor?