Changed Cylinder Head, Normal Temps at idle, temp drops when driving, w/ coolant loss

Hey,
New to the forum, as well as working on my truck. I have a 2002 2.7L Tacoma 3RZ. I'm to the point where I don't know what's going on.

I recently replaced the cylinder head, head gasket, and valve cover, after I noticed rough starts, and the temp gauge dropping after climbing to normal operating temperatures.

Now, after replacing the top end, I noticed the temp gauge dropping on the third test run, after reaching normal operating temps. I replaced the radiator cap, and it seemed to run normally for approx. 2hrs, but when I got home I noticed the reserve tank at empty.

Now when driving, the engine will reach normal operating temps (151 degrees) than drop, staying at approx. 1/3rd on temp gauge, and throw code P128, bad thermostat. I'm on the way now to go purchase a compression tester to check my work, as well as a new thermostat. Any help here would be hugely appreciated. I don't know what the hell to do.

Here's a timeline from the start of the first problem.
1) Rough startups noticed
2) Coolant losses noticed
3) Engine temp will go up to normal, then drops to 1/3rd

I take it into the mechanic for pressure test
4) Compression test comes back from mechanic with 1st and 4th cylinders losing compression
5) Removed the original cylinder head, and have it tested for leaks/level, not even worth saving
6) Install a remaned cylinder head
7) When installing valve cover, it cracks on the front left side torquing to spec (15ft/lbs )
8) New valve cover installed, first two tests seem to operate normally, truck comes up to normal operating temps
9) Third, longer test, approx. 20mins, the engine temp goes to a third and holds. After I stop to grab a beer from the gas station, when starting the temp jumps to normal, and then almost immediately starts dropping.
10) I drive it into the mechanic to have the exhaust fixed, watching the temp gauge and it swings approx. 30 degrees total over a 35min drive, never above 160F, or below 130F.
11) Change the radiator cap, and go on a 1hr drive, temp sits normal
12) Let the truck sit for a few hours, drive back home, and the temp goes to about a 3rd, and slowly climbs to normal temps by the time I get home.
13) Check coolant levels and notice that the entire reserve tank is almost tapped.
14) Refill, and I only commute approx. 10mins to work (uphill the whole way), and I start getting PID 128, bad thermostat, with the temp climbing to normal operating temps and then dropping after I reach 154F, or normal operating temps
15) I clear the code and on the drive home (all downhill), no code
16) A few more days of this, I'll sometimes pull a code, sometimes not, with small coolant loss throughout.
17) I purchased UV dye last night, let the truck idle (with me sometimes pushing on accelerator), and it comes up to normal operating temps, and holds there. I can't find any dye.

A couple other things:
The manual states that the 2.7L takes 8.5quarts of coolant, by now I've just put 8 in, though I've tried burping the engine (running it with radiator cap off, then filling as air bubbles up, though I barely noticed any drop while doing this).

I purchased another thermostat and will be replacing tonight along with running compression test on each cylinder.

While driving, engine temp is between 134-151 degrees. After a half hour drive, I'll pull-in with the engine at 138F, then when I come out, temp spikes to 154, then slowly (10mins to drop 10 degrees) the engine will drop back to 138F.

While driving slowly uphill, I'll see the engine temp slowly climb back up to 150F, then as I drive drop again, and hover around 140F.

 

I don't have any input for ya really but normal operating temp is no where near 151°F. 186-192 is where it should be.

 

It was a bad thermostat. Engine is running smoothly at 186F.

 

Wow!
Good work!

 

What's up with your compression test numbers though on 1 & 4??

 

If the head was cracked on those cylinders.... It would read low on a compression test.