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Discussion Starter #1
The last month or so, when the temps started getting really cold, i noticed that when i drive around with the heater on, the temp gauge climbs, well into the H end of the gauge. It hasn't overheated, but it gets close. It'll drop down to the mid range, then climb up again. If I turn off the heater, the gauge settles down to a more normal reading, med. to med-low.

That's behavior just the opposite of what I expect, so I'm confused. I replaced my t-stat about a year ago and the fluid is all at normal levels and looks pretty good. Last spring I backflushed the radiator until it flowed pretty clear.

Since it doesn't actually overheat, I'm not panicking yet, but I'm a fan of catching potential problems before they become big ones.
 

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Where is your temp sensor mounted? Was it moved from the stock location by any chance?
 

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Whoa, I was just going to type up the same issue, that's weird haha. 100% stock 96 5.0, having similar issue with heater on. Not climbing as far up as yours but moving a letter or two on "temperature" in the gauge. I just bought mine last week and haven't done a thing to it yet other than cleaning. I do have a slight leak from radiator somewhere so that will get replaced and since the heater core is so cheap and easy to change may just throw that in too. New t-stat, upper and lower along with heater hoses. Interested though in why when heater on, the temp climbs.
 

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Discussion Starter #4

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Discussion Starter #5
What brand & temperature t'stat did you install? Have you ever checked the dash gauge against anything (like an IR thermometer)?
I don't honestly recall the brand. I picked it up from O'Reilly Auto so it's probably a Murray. I do have a IR thermometer, but I haven't taken any readings yet. I assume I want the temp of the hose as it comes out of the water pump?
 

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As the gauge climbs do you actually feel the heater getting hotter?
 

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I get the opposite affect. I drove over 100 miles to go camping. It was cool weather with a little rain. Cruising 70mph with the heater on. My temp gage went to cold and lost heat from heater. It also happens when I go up the mountain. I am running an electric fan with a controller. Going to try adjusting the controller but not sure if that will fix it.
Any ideas?
 

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I get the opposite affect. I drove over 100 miles to go camping. It was cool weather with a little rain. Cruising 70mph with the heater on. My temp gage went to cold and lost heat from heater. It also happens when I go up the mountain. I am running an electric fan with a controller. Going to try adjusting the controller but not sure if that will fix it.
Any ideas?
Bad T-stat
 
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