Troubleshooting temperature spike

Today I saw the temperature gauge do something strange…

I started the engine to warm it up. I've been troubleshooting a small coolant leak, so I checked the level while the engine was warming up. Sure enough it was a little low so I popped the cap on the reservoir, slowly let the air hiss out, and the poured in 3-4 cups of pre-diluted 50/50 coolant/antifreeze until it reached the fill line on the reservoir. Closed up the cap and started driving.

About 15 minutes later I noticed the temperature gauge climbed all the way to the top. Today is a warm day, but I wasn't driving particularly hard and I've never seen it do that before. The needle usually sites a bit below the middle. I was worried about overheating so I turned around to go home, driving it as gently as possible. About 10 minutes later the needle dropped back down to normal and stayed that way the rest of the drive home.

What happened? What would cause the temperature to spike abruptly like that, and then drop back down? Or does this sound more like a bad gauge? Besides today, the gauge has seemed to behave normally, but I've never had any reason to question it.

The temperature gauge goes up to 3/4 then the fan kicks in and cools the radiator back to 1/2 way but if you are moving there is usually enough air flow so the fan is not used. Check the fan is working buy bridging to 2 connection an the radiator with a bit of wire and the fan should come on.

Check the coolant level again now it's cold.

Could also be the wire/connection to the senoror the sensor is playing up?

Do you have a top fill radiator or an expansion tank near the brake master cyinder?

My temperature needle was pinned to the right for 5-10 minutes before dropping down suddenly. Does that sound normal? Maybe something got stuck? I'll look for that sensor wire. Does it connect to the radiator or the engine?

I have a little plastic jug of coolant over the driver side wheel…

wait till it is dark.
Then use a UV light on the heater hoses and block connections to see a green residual trail.

When I have had flange leaks the block evaps the stuff, so all you can do is look for the residue.

Ones that are tuff to spot.
Water Pump seal weeps. Heater Valves that are leaking.

Bad t-stat and seal.

That's a nice trick about the UV light. Are you thinking that a coolant leak could explain the behavior I saw? I would expect a leak to cause the temperature to rise and stay up (until being refilled). I wonder if the fact that it self-corrected (at least for now) is significant.

Don't want to worry you but the Diesels have an habit of cracking cylinder heads and pressurising the cooling system. Diesels run at very high compression……

This then starts to cause leaks and burst hoses and radiators. The owners chase these fixing each one in turn only for the next weakest link to go.

Are the radiator hoses rock hard when running and up to temperature?

Oooo. I hadn't considered a more serious problem like you described.

I fired it up and ran a couple local errands to get it up to temperature and then popped the hood checked both the hoses coming off the engine. They were both soft an pliable. I felt like I could pinch the sides together if I squeezed hard enough (but I didn't). Is that a good sign? I was actually surprised at how not pressurized they felt. But the temperature seemed just fine.

I had a 1981 Diesel Jetta in the late 80's. It had a cracked head and I replaced various cooling parts and it run OK tootling around but when run at 45-50+ mph for a period of time it would boil up or blow another part