I replaced my t-stat a few weeks ago.
After the first initial drive, i noticed my temp gauge (after 15 mins) would almost peg out, then dive down to the N or below in NORMAL. Other times it creeps, then drops. It hardly ever stays still for more than 5-10 mins. It does this various times then may settle down only to start over again.

I dont seem to be losing any fluid, yet the overflow tank gets filled up from time to time. In fact, ive even seen the overflow tank.....overlow! It filled up so much that the rad fluid came up through the cap.

I checked oil at temps and saw no "mud" so i hopefully can rule out a head gasket. I am getting "normal" heat as it relates to the guage. If its high, i get kick *** heat. If its low.....not so much.

Here is what i've done: I have replaced the cap. I replaced the thermostat with ANOTHER one, still doing it. I also tried NUMEROUS times to "burp" the system including letting it run without cap, leaving cap off overnight, running heater full blast for hours.etc.. It doesnt matter.

I have actually heard and seen the stat make loud clicking noise(s) and then the upper hose cavitate on itself. It sounds like air is inside/getting in or there is a pocket of air, but i cannot get rid of it no matter what. When the cap is off and it runs, it starts out by pulsing and eventually flows over the top, but once the the stat opens, it drops way down and i can see it flowing.


I was thinking about having a shop do a complete flush, but they told me they dont remove the thermostat when they flush so i dont know how good of a complete flush it would be.

I really dont know what else to do........any ideas?

-Glade

 

Update to other findings:

I let the truck sit overnight again after changing the T-stat again. Level dropped (burped) about 1/2 gallon. Filled back to top, took for a ride. Still doing same thing.

I have since noticed the upper hose is very firm and HOT, the lower hose is not as firm and not as hot.
Same thing for the heater core hoses, yet they are closer to temps. At times the heater core hoses are not that close in temp, but they eventually get there.

After filling up the radiator, the overflow reservoir was at the correct level. After driving around and having the needle go up / down / steady, i check overlow .... the level was UP from before. Fluid had vented into the overflow and filled it up about 1 inch. When letting truck sit for awhile, reservoir drops down to COLD level, but will not go lower than that. Radiator is again low (possibly more air burping?) yet reservoir wont drop below cold level to fill it up. Is that normal??

Another thing i have noticed that is consistent. When im driving and the temp starts climbing up, if i give it more gas, it drops down considerably. In fact, while driving the temps generally stay low (Near the "N" most times). When i stop, it climbs up.

Just checked again for film in oil or coolant .....none. Also, absolutely no coolant leaking (that i can see).

Thanks for helping..
-Glade

 

Sounds like you got it. Just keep adding AF to raditor, might be just small amounts of air.

 

Fixed it.

One of the heater core lines was evidently vapor locked.


Thanks for ALL the great help and ideas!!!

-Glade