Temp gauge puzzler...

[n8thgr8]

This is a wierd one... I'll describe what I'm experiencing and I could really use some help with where to go next...
98 disco 1 LSE.
After driving around for several hours (in town, but I'm in Guam, so it's hot out) my heater gauge starts to skyrocket. Turn the AC off, goes down for a bit. Then of course rises again after a while.
Here's the wierd part. If this is happening at night, I'll turn my lights off, and the temp gauge goes down. A lot, like back to normal, lights on, goes back up. WTF!?
Some other relevant info...
Was low on coolent 2 months ago, then again this week. and I thought I saw a puff of white smoke out the tail pipe last week, but haven't seen it before or sence.
I was thinking t-stat before I discovered the lights thing and saw the smoke.
Now I'm worried about headgaskets.
Thoughts?

 

[FASTTECK]

sounds like you may have 2 problems. but!
Iv have seen loose or corroded engine grounds cause the temp gauge to act up when electrical lodes are on and the return to normal after there turned off. Take you engine grounds loose check them for corrosion.

 

[range_time]

i had a problem a few weeks back where after a short in town drive on the freeway i overheated, on the freeway fine, stop and go traffic.. hotter and hotter.
I was thinking head gasket but did the cheep stuff first and found a ton of crap in the rad and the thermostat was almost blocked. $36 from the dealer and i got a new pressure tank cap also (it was also 15 years old) a quick flush with a garden hose and all is well.
Check the grounds and then do the t-stat and flush your coolent.
I also have a scan gauge so i could see the temp rise much better than on the dash gauge.

 

[range_time]

i had a problem a few weeks back where after a short in town drive on the freeway i overheated, on the freeway fine, stop and go traffic.. hotter and hotter.
I was thinking head gasket but did the cheep stuff first and found a ton of crap in the rad and the thermostat was almost blocked. $36 from the dealer and i got a new pressure tank cap also (it was also 15 years old) a quick flush with a garden hose and all is well.
Check the grounds and then do the t-stat and flush your coolent.
I also have a scan gauge so i could see the temp rise much better than on the dash gauge.

 

[n8thgr8]

Interesting, thanks guys, I'll be sure to check for corroded grounds, and I know it's due for a rad flush. I have the new tstat, so I think I'll put that in sometime this week and see what I get.

 

[n8thgr8]

I didn't realize I had posted this so long ago, and not gotten back to it...
I had dealt with this problem on and off for a long time, without finding a solution with traditional heating/ cooling system methods.
I had my mechanic drive it around for a couple of days so he could reproduce the results.
Turns out it was a corroded temp sensor.

 

[BaldEagle]

I didn't realize I had posted this so long ago, and not gotten back to it...
I had dealt with this problem on and off for a long time, without finding a solution with traditional heating/ cooling system methods.
I had my mechanic drive it around for a couple of days so he could reproduce the results.
Turns out it was a corroded temp sensor.

i've dealt with this problem for years. i'll try the temp sensor. thanks for the info.