I wanted to get everyone's thoughts on this.

This past weekend I took the Rover, dog, and wife up to Yosemite to drive through the park. Recent snow fall made for an absolutely gorgeous afternoon.

I was keeping an eye on the temp gauge because it had been running a little cold there in the mountains but not below the "normal" range. On the way back though it ended up basically pegging all the way cold and the check engine light came on.

Its important to note that when this happened the outside air was well below freezing, we were traveling at around 50-60 MPH downhill essentially at idle. Once we got back down into the valley and got on the freeway the temp gauge climbed back up to its standard 8 o'clock position.

Fast forward to today and I got the code read and P0116 is what I got which is "Engine Coolant Temperature Sensor Falling Temp Fault". Kinda obvious.

My thoughts are I must have a thermostat stuck open. I should say that I haven't had any problems with normal every day driving as far as engine temp, either hot or cold and operating temp happens within 10 minutes from a cold start.

According to paperwork I have thermostat was replaced by previous owner within the year, although I don't know if its a 180*.

Basically, what do y'all think I need to do about this if anything?

 

A stuck thermometer could be the problem and is clearly the first move based on cost and ease of replacement.

But if I'm reading the GEMS manual correctly the threshold for a MIL illumination are a reading of -40 deg C or a 25 deg C fall in temp over 4secs in two successive trips, it is entirely possible your ECTS is on its way out.

From the GEMS manual....

 

I'll keep an eye on it for now since I haven't seen any other indications except for the downhill experience I mentioned above.

 

I get the same error. I changed out the entire thermostat housing and coolant sensor with a new part and after a week, get the same error again.
I also changed the coolant expansion tank as I was getting incorrect 'Low coolant' messages on the dashboard.

Any suggestions to check next would be welcomed.

I also get poor fuel economy (10-11 MPG), intermittent P0171 and P0174 Bank 1 and Bank 2 'system running lean' codes. Could these be related?

I have run a bunch of Seafoam and 44K through the oil and fuel systems, changed all the fluids, smoke checked the EVAP for leaks, changed the EGR valve and am about to change the leaking valve solenoid gasket on the drivers side.

I have heard it could be a O2 sensor, but there is nothing popping up in my ancient Snap On Modis 14.2 system.

 

I have a coolant temp gauge connected to my 97 and when I coast from Donner Pass down to my area West of there my thermostat does not open (160*) and will sometimes drop the dash needle down below normal operating temperature. When I start up hill or flatten out then temp will climb to 160-180 and the dash needle goes back to the 'wavy' line on the temp gauge. I have not thrown a code even though I have done this a few times since the start of our mild CA winter this year. Well I haven't thrown a code for that anyway...

 

Never changed out the thermostat while in there? These Rovers use a "jiggle pin" and it must be at the top of the thermostat otherwise you'll trap air. Air is not a good thing in a cooling system. EGR? Rover V8's don't use one unless you have a 300TDi. PCV valve yes, EGR no. Running lean will overheat an engine. Poor fuel economy is normal, do a basic tune up, plugs/wires/filters. If you don't know how old the upper O2's are replace them. Instead of Seafoam use Techron fuel injector cleaner through the fuel system, it works much better at removing fuel deposits. The other thing you may want to look at/replace is the fuel pump filter sock and the inline fuel filter. Tire air pressure is another overlooked factor. Keep em about 32psi all the way around to reduce the rolling resistance. The book says 26 front, 34 rear and I call bs on it. That should do it for now.. start a new thread if your still having problems after you've tackled my suggestions. It'll get more looks and suggestions.