I suspect I have a stuck open thermostat or none at all as my engine temp runs about 150 to 160F. I also get poor gas Mileage so I suspect it is running rich because of low temps.

At what temp does the fuel map go normal and is a 180 degree thermostat the recommended one even though the OEM is a 190?

 

I don't know the fuel map answer, but I'd stick with 190.

 

Sounds like a stuck open T-stat. At least its better than a stuck closed one.
Siphon some coolant out of the radiator into a bucket and you'll avoid a lot of spillage when you pull the T-stat.
I'd go with the 180* one. These aging cooling systems could use a little advantage.

 

Looked on Autozone's web site and they have a 180F and 192F and some 160F.

They have a 180 "failsafe" PN 7200-180 that says,


"Patented safeguard against overheating. When overheating occurs, Fail-Safe automatically locks in the open position to permit maximum coolant flow. Manufactured from high quality components to meet the demanding conditions of the automotive cooling system. Manufactured with OEM specifications and processes. Copper case and wax compound assures rapid response to temperature changes in the coolant. Offset design, air relief valve, and OEM style seals included where applicable."

The last two sentences sound good, but locking open doesn't seem like the best idea. Makes me wonder if that's what is in my Disco now. It doesn't say at what temp it "locks open".

 

I don't know if 180 is a point where the curve sharpens, but I once saw a graph in an engine rebuild book that Ford put together based on actual experiments with 302 engines. Basically it was a j curve that illustrated that lower coolant temp = shorter engine life. Maybe 180 is at the flatter end of the curve. I personally wouldn't go any less the recommended stat. I run a 192 and never had an overheat problem. If your rig overheats, you have other problems that should be sorted out anyway like a plugged radiator, lower hose sucking in, or something else.. Either way, it's a good thing you are looking to change the t-stat. Mine crapped out once too.

 

Don't get a locking stat unless you want to replace it again. 180 vs 190 doesn't matter much IF your cooling system is working. Do get one with the little hole in the top.

 

"Locking Open" just means IF it fails, it will fail in the OPEN position.
Fail in the Open position = poor fuel economy and no heat
Fail in the Closed position = overheated truck

I'd prefer the fail Open one myself.

 

It's hot outside, you're driving up mountain, AC is on, etc and it locks open. Now you're replacing the dumb stat again. They lock too easily.

 

What exactly causes it to lock open?

 

If stat opens fully there's a one way catch that prevent the plunger from retracting. This happens when you push the cooling system for whatever reason. Coolant gets hot but not too hot. Stat opens fully, and locks open.