Temp gauge goes up then back down?

[Surrey]

Im in the middle of flushing my engine before I start installing my lift, and Im scared to go any further until I diagnose wtf is up with my temp gauge.

Before now, I dont recall having any issues with the temp gauge. (Its been my back-up vehicle for 2+ years now, doesn't see a lot of use, especially lately...)

While trying to heat the truck up a few minutes ago, the temp gauge was acting very weird. It would start to go up, then drop back down. Then start to go back up, get a tiny bit farther than last time, then drop back down. After a while of letting it idle and revving it to 1200 and 1500 RPM the needle only got as high as the N on NORMAL, and then dropped down to like 1/4 of where it was.

I read RonD's explanation of how the temp sender works, so Im going to test the gauge that way...
http://therangerstation.com/forums/showthread.php?t=134916&highlight=heat+gauge+jumps

Anyone have any idea what would be the cause of this sort of behavior??

 

[RonD]

Thermostat could be going bad, that's the only thing that can make a stock cooling system running cooler than it should.

t-stat is never open all the way unless engine is under load, it opens a bit as coolant behind it gets to 190deg, this starts coolant flowing thru the rad, which cools engine down a bit(depending on outside temp), this cooling will close the t-stat again, then it opens a bit as the new coolant warms up to 190deg, you can sometimes notice this on the gauge but usually not.
If t-stat is getting sticky it might stay open when rad coolant starts coming in, so gauge temp drops a bit, then t-stat closes so gauge goes back up, then t-stat opens again so gauge drops.

But you have just refilled the system, and air in the system can cause the same issues until purged.
I would run water in the system and see if the gauge steadies out over the next day or two, on heat up cool down cycles.
If it repeats this behavior when all air is out then I would change T-stat.

Water so you don't have to waste coolant if t-stat needs to come out.

 

[Surrey]

Okay, Thanks.

My biggest worry is overheating the thing... So your saying that I likely will not do this? And yes, I just had the hoses to/from the heater core off, as well as I just replaced the clamp on the reservoir end of the hose going to the overflow tank, so that drained completely as well.

 

[Surrey]

Took the wire off the sender, grounded it, and the gauge shot up to max... So thats working I would say.

Leaning towards the sticky thermostat... What is the best way to clear air out of the system? I dont really drive this car,(about to lift it, so it'll be sitting for at least a few more days...) so I dont think I can rely on it working itself out? Or should I fire it up and let it run for a few minutes every few hours to try and get the air out?

 

[MastuhWaffles]

It's the temp sensor going bad. My mom's expedition has this same problem. The temp randomly shoots up and there's nothing actually wrong.

 

[Surrey]

It's the temp sensor going bad. My mom's expedition has this same problem. The temp randomly shoots up and there's nothing actually wrong.

it doesn't randomly shoot up, its a very gradual up and down (the down is faster than the up) which makes me lean towards the tstat.

 

[Chris_1]

To get the air out, let it heat up - it will expand the fluid and push the air out the overflow. as it cools back down it will pull fluid back from the overflow bucket. Cycle it this way as many times as you have to - keep adding fluid to the overflow bucket as required - eventually it will stop dropping (the level in the bucket).
This is why your rad cap has two stages - pressure pushes past the big spring to allow overflow, the little piece in the middle allows fluid to come back. Make sure the little piece moves freely.

 

[ashleyroachclip]

easiest way to bleed the air from the system is to buy a flushing tee and inserted in either of the heater hose , fill the system, including the reservoir and run the engine until you have heat coming from the heater.
With the engine running , open the cap slightly until only coolant escapes, then close the cap .
Done .

 

[Surrey]

Okay cool, thanks.

I will fire it up again today, and see how it behaves/try to heat some air outta there...