Bought it a month ago with 97xxx miles on it. The coolant reservoir was pretty empty so I added the coolant.
The only problem is the check engine light on with the codes P0171 and P0174 (lean codes for cylinder's) Replaced the breather hose which had a large crack. But after check engine reset comes back on after 20-30 miles.
In the last 2 days the temperature started to get lower (60-70's) and rains. While driving back from work, the engine temperature went up in about 2 minutes. I stopped and checked the coolant level which was just below the minimum level, also the engine oil was pretty low. The fans are working properly until the temperature goes below 1/2 of temperature. I waited the engine temp to go down and drove it to the nearest shop and changed the oil.
After that the check engine went off (though the codes are still visible on the obd checker), but the engine temperature still goes up.
When the car is idling the temperature is at 1/2, but as soon as I raise the rpm's the engine temp starts to go up.
Added the coolant to more than minimum, no leaks seen.
Could it be the thermostat?

 

Yes, it could be the thermostat, but also could be the coolant sensor.

On a more remote path, what coolant formula are you using to top off? I would be additionally concerned if mixed formulas were used with an OAT derivative as this can possibly cause clogging in the coolant system, probably first in the heater coil.

 

The one recommended by AutoZone for the Jaguar, Dex Cool, orange/reddish, visually the same that was there before...
Could the coolant cause it after almost a month after adding it?

 

Freeze, since you are loosing coolant, I will be betting you have a bad water pump. The seal on the front of the pump is starting to go, which is a sign that the impeller is normally fairly gone too. Granted, replacing the thermostat isn't a problem at this point either as you will have the system drained and pulling the thermostat is 1 clamp and 3 bolts to get out.

If you need more info, let me know.

 

Ok, some news.
Ok, so I checked the temperature of the coolant hoses and the ones located on the lower side of the engine are cold, so it's definitely the thermostat.
I bought the new one, and I want to replace it BUT... I made a big mistake...
I locked my battery in the trunk, so when I connected another battery, I accidentally connected the + and - the other way and of course I had a shortage =((((
Now, after I connected back my battery, I put the keys in, and turn them in the run position, the whole electric system is turning on and off every second, and the car is not starting....
I checked the fuses from under the hood so the small ones look ok....
Where should I search the shortcut now?

p.s. really appreciate your answer...

 

Freeze, to me, it is sounding like you have damaged your Generic Electronic Module (GEM). This is what controls the battery saver relay, and therefore a lot of the electrical stuff inside of the car.

If you want to start troubleshooting this, let me know and I will see what I can do. But, this is more than likely going to require a lot of time with a multimeter and being able to understand what the multimeter is telling you based on the voltage that you are seeing.

 

I'll have to start troubleshooting this, so your help is really needed =(

 

I need to troubleshoot it so please help if you can.... Downloaded the JTIS but doesn't work on my laptop =(

 

Thermo, thanks for the help... I went to the car today, and just checked the connection again, cleaned the plus and minus on the battery and on the cables and the car started!
Now I can start changing the thermostat... hopefully with no more problems