I bought this vehicle late last year and on a short trip, the temperature red lined while on the interstate and I pulled over as soon as I could. The cap on the radiator was blown off and coolant was all over. Towed to dealership. Bottom line, the engine was blown and the tech stated that someone had put stop leak in the engine. The previous owner had immaculate records and all work was done at a Jag dealership. Fast forward to now.... Bought a used engine and after replacing the radiator, all hoses, water pump, thermostat, etc, etc. engine still overheats.
I can drive it around 50 mph for 10 miles or so and it seems fine, then it overheats. If I let it sit at idle, it overheats at 35 minutes. The cats and complete exhaust system do not show any blockage. My mechanic is a specialist in almost all foreign cars and is a master certified tech. We are both do not know where to go next !! Would like to hear what you might have to offer as a solution ?? This is a lovely classic and I want to drive and enjoy it !!

Did they replace the thermostat housing or just the thermostat? The failure of the internal plastic channels (these last only approx 50k miles) will cause the symptoms you describe.

Might need to bleed the air out of the system a few times. This happens to me any time I let a substantial amount of coolant drain and then refill it. Bleed and top it off a few times and it should level out.

Did you do your due diligence on the motor? Is the temp sensor from the donner motor or did you use your sensor or replace it with a new unit?

@kenatofc, what DTCs are stored in the system?

If you have a 2009MY the engine is a 4.2 litre unit.

+1 on replacing the water outlet, which includes the thermostat assembly.

The temp sensor only lets you know if the engine is running normal, hot or cold... how could that affect the motor ??

we replaced all hoses and thermostat assembly. my 09 is a B model engine, not supercharged.

i assume you are a mechanic has done on the due diligence pressure tested the cooling system. I assume there's also some type of flow test that can be done on the radiator. For both engines to overheat is too much coincidence to not take a hard look at the radiator especially after the dealer found some type of sealer in the system. I know this is all obvious stuff but just want to help cover all the bases. Besides if everything else has been swapped including the water pump it's the one major remaining component.

thanks. Radiator was the first to go (after the blown motor), new hoses etc. I ordered a thermostat housing to see if that might be it !!

Lets all hope you bought a good, used engine, not one with issues!!!

Replace the radiator cap if you have not. Sometimes they do go bad and leak pressure. Your symptoms would match up to this as a possibility.

Thanks for all the help.... Thought things were going ok until one of the air shocks failed and another suspension part. Had those corrected and driving to the East Coast all happy and feeling good about the car until I got within 15 miles from my destination and all hell broke loose !!!! (I mean it) bottom line was that the driveshaft broke and tore everything up underneath and could not go forward or backward. Towed to a Jag dealer and they came up with a probable cost of 12K to repair. Had a friend in the area come get me and everything from the car.... Rented a car and when I got home sold the damn car from Hell to Copart... So if anyone wants a perfect interior or wheels, Copart will be selling it soon. It is being sold by the Copart near Highlands NJ
After some searching, bought a 2012 XJ8L Portfolio and am enjoying it !! (for now)..

yep, that one was from hell, or at least somewhere really unpleasant. Glad you got into a new Jag, and the X351 is a beautiful car, if not in the mold of the previous 50 years of XJ's, lol! One feature I like is the suction oil change from up top. If I have to remove the twelve (count 'em) bolts and screws, some rusted and close to breaking again to change my oil, I may be in the market for an X351 myself! I wonder if the driveshaft failed at the transmission connection from any previous work, all a moot point now, I guess. Enjoy the new Jag!

Not that it matters now but by all accounts the thermostat is notoriously difficult to assemble correctly on the x350.

Curious about the installation of the thermostat ?
Ken

Well the thermostat consists of a number of separate parts and it's not obvious

how they fit together.

Because of this people have recommended that you buy the whole plastic tree

which includes the thermostat in situ.

Thanks, was curious... Love my 351