Engine moves slowly to hot, then quickly to norm

[DLinkOZ]

Ok, so this just started on Thursday, driving home from Christmas Eve at the in-laws. As many details as I can think of:

• 2004 V6 4x4 with 80k miles
• Rock solid before this happened
• Cooling fan spins freely, a good shove and it spins about 1/2 a revolution, very smooth and no binding
• Wednesday the outside temp was 70
• Thursday the outside temp was freezing and we had ice/snow
• Seemed to begin towards the very end of a 35 mile icey drive home
• For about 5 minutes it fluctuated between 3/4 and red several times
• After that, it quickly (within 2-3 seconds) dropped back to just under half and stayed for the remaining few minutes it took to get home
• Continued today, temp in the mid to upper 40s
• Today I drove about 10 minutes to Home Depot, no problem getting there
• I was paranoid and watched the temp gauge. From the time I started the engine, it got to just under normal within 6-7 minutes but stayed steady
• About 1 mile before I got home (10 miles, sat for 10 minutes, 9 miles back), the temp slowly crept up to the top of the temp gauge then dropped quickly back to just under normal
• When it happens, it goes up slowly then QUICKLY drops back down normal
• Coolant is about 1/3 up the overflow bottle, well under the min/max lines, but no indicator lamp lights on the dash. I'll be honest and say I can't tell you if it was low before this happened (yes, I'm a bad person for not keeping closer tabs on it)

That's about all the facts I can think of off the top of my head. Some of those details come from what I've seen as questions to other postings on the forum, so wanted to get them out quickly (I couldn't find anything that closely matched my issue on a similar year model).

*EDIT*
Add one thing I just noticed - the fill level of the overflow resevoir is level with the tube running to the radiator. Just to the top of the outlet hole. And I can find no visible signs of a leak.

 

[JDraper]

Sticky thermostat is a good guess.

 

[DLinkOZ]

That's my initial plan, but I'm equally concerned about the lost coolant. Any chances that the sudden weather change could have contracted something, causing the loss? I'm not a big fan of some of the alternative reasons for the loss.

 

[Anime]

Your thermostat is on it's way out and heading towards being completely stuck closed. Replace it NOW. Just getting the temps up near the top of the range is a good way to blow a head gasket or warp a head and turn what could have been a $10-20 thermostat/coolant change into a $600+ head replacement.

When the system is overpressureized with the gasket stuck, before the higher pressures finally force it open, the coolant escapes any way it can, either through gaskets, seals, or just overflowing the tank.

I'd change the themostat, add new coolant, keep the coolant topped off for several days, then keep checking it after that to see if theres any coolant loss from a leak at normal pressures. Assuming the head gaskets haven't already blown.

 

[DLinkOZ]

Sounds like what I was hoping, and planning. I'll do that, and keep an eye on things. Thanks for the feedback, guys.

 

[DLinkOZ]

Replaced the thermostat, the old one was actually broken (the bottom portion that's spring-loaded). Only took about 3-4 cups of antifreeze/water mix to fill up the overflow, so it wasn't as low as I thought (that thing fills up quickly). There was no syrup smell (though there is now, I got a little antifreeze on the engine while swapping it out) and the coolant collected in the lower tstat housing was the normal green one would expect. Ran it and it never tried to hit even the halfway mark. So I'm going to keep tabs on it the next few days and see how it goes, hopefully I got out of it lucky after the engine getting so hot.

Again, thanks for the help all.