Thermostats open and close a lot is it possible that a thermostat malfunctions and stays in closed position overheating the engine?
I bought an aftermarket MotoRad thermostat I am wondering If I should've bought an original from Jaguar

 

A thermostat sticking shut is always a possibility. You can do a simple check and pull the thermostat from the car. You can then put it inside a pan of hot water and heat it up some more. You should see the thermostat start to open at probably around 180-185 and be full open at the set temperature of 195.

During normal driving, the thermostat does not necessarily go full closed and back to full open to maintain temp. It really drifts towards closed and towards open, restricting flow as necessary to force the needed amount of water through the radiator to maintain the temp. If you have a random overheat, then I would say to look at the thermostat. But, if it is fairly consistent, then I would be looking elsewhere since you have a newer thermostat from the sounds of it. Normally a thermostat will get some gunk on it and stick every now and then or it will start drifting in where it will maintain temp (ie, over the course of a few months, start controlling higher and higher until you get the periodic overheat from after say coming off of the highway, but it would only last for a few minutes, then it will return to normal).

 

Chris, what are your views on the aluminum (e.g. Euro Amp) thermostats and if it is a very important upgrade like the Aluminum coolant parts? Also when I recently left my car sitting in the garage for 2 months, when I returned my mpg had dropped very significantly (say 35 - 40%) and some of the very shrewd listers have kindly suggested the thermostat may be stuck causing the engine to run rich. (seems that leaving an X351 sitting is an excellent way to expose its weak points!! - but battery was perfect, so thank you very much for all your great help on that prior issue)
I also have a potential rough running feeling btw 3 - 4,000 rpm in neutral and between 4 - 5,000 rpm when in gear and driving... Thought I would post this here due to your very informative post above. Thank you !

 

QP7, running a steel or aluminum thermostat is really insignificant provided you are using distilled water as your base for coolant if you are using concentrate or flushing your coolant system. If you are using the pre-mix 50/50 stuff, then this is not an issue. The reason being, if you use tap water, it has very small parts of various elements in it (zinc, chlorine, fluorides, etc), these can cause two different kinds of metal to interact with each other that are in physical contact (the steel thermostat in an aluminum housing). This will cause one of the metals to corrode and lead to problems. Using distilled water or the 50/50 pre-mix, these errant minute elements have been removed. This also saves your radiator and other key parts and helps them last longer.

Your drop in mileage I would be more suspecting of bad fuel than coolant. Especially since you are having a rough feeling at those higher RPMs. If you want to rule out the thermostat, I would say to have fun with the car and drive it aggressively for a few minutes (or do a 5 mile highway run) and then immediately go through a drive through (for a coffee or the like). If the car handles this, then the thermostat is doing what it should. In short, you are building up a bunch of heat in the engine from the hard driving and then slowing the car down to an idle to let it deal with the heat where it has minimal coolant flow and minimal air flow.

If you have let your car sit for 2 months, I would say to toss in a bottle of fuel injector cleaner and a bottle of dry gas. and also fill up the tank. The dry gas should get rid of any water in the gas, filling up the tank is going to dilute any gas that has broken down from sitting (especially a problem for places that use ethanol, even more for those using 15%+), and the fuel injector cleaner to help clean out anything in the injectors.

 

Thermo, you are a great help as always. Given my location you can imagine this was stressful as I was heading below +/- 8 mpg and the fuel cleaner in the tank, along with an Italian tune up, did not do it. Tech is flying back in as planned next week, but ironically too soon for me to send him some parts - upshot I was going to be without an effective car for two weeks + . So then I read your very kind post and saw "dry gas". Huh? I had ("had") no idea what that is. So after some searching realized it was a good idea given the overall circumstances. Hence, I just drove to the gas station watching the tank literally disappear - $20 was gone in 40 miles or so...!!!; anyway they had some "STP Water Remover", per the following link: -
https://www.stp.com/product/all-season-water-remover/

I put it in with $30 of gas, and after one blast of acceleration for about 0.5 miles, suddenly the miracle occurred - back to excellent fuel consumption.

THANK YOU !!! - I then learned how to make my trip buttons work, and after same fairly / hilariously stupid attempts by pressing the wrong stalk and repeatedly washing my windscreen, I finally used the left one and at a steady 50 mph it was reading 6 litres per 100km, or about 47 mpg (Imperial gallon) or 39 mpg (USA gallon) and my trip average after leaving the super market (which was my stop after putting in the STP) was 9 litres per 100 km or 31 mpg (imperial) or 26.1 mpg (USA).

Now I was driving slow, but still this was an astonishing difference - I have my car back! Thank you very much, Thermo. You saved me from huge heartache.

Thank you very much. Now driving endlessly in AC'd comfort, enjoying the ocean view.

 

QP7, suggest you keep running fresh gas through your tank. I think Thermo has the right solution too. I suspect you haven't gotten all the water out yet and that's why you're running rough. Also, you could have experienced fuel separation from the ethanol blended fuel and that leaves gunk to be sucked up into your fuel system. The next time you leave for a lengthy period, put a fuel stabilizer/fuel preservative in your tank and fill the tank completely full to prevent condensation as I mentioned in your other thread.

 

All agreed - many thanks again, Freddy. I wonder what else I should do in case I have damaged my fuel system? The underground tanks at the fuel stations here are likely not the best either... so I'm going to do some research on which ones are most recently built, and have least water / contamination in the rainy season.

 

QP7, when it comes to maintaining the fuel system, here is what I would tell you to do:

-ALWAYS!!!!!!!!! use a brand name gas, avoid the corner store stations unless you know where they get their gas from and they have a good through put of vehicles (ie, keeps the gas flowing through the tanks and does not get to sit for long periods of time).
-If you use the same station all the time, every oil change, run a tank or two of another major brand. Each manufacturer uses a different blend of detergents and by switching between brands, you push different detergents and what brand A misses, Brand B should get, keep everything clean.
-If going on a long trip, fuel stabilizer, full tank of gas, and maybe a little bit of dry gas when you return to ensure the amount of moisture you get in the system is minimized.

That should pretty much do it. All simple things to do.

 

Thanks again, Thermo - all good advice and appreciated.