I discovered this failure mechanism of the PRT thermostat that leads to overheating when idling on my Freelander 1.8 and thought it would be interesting to MGRover owners as well, if they have the external PRT thermostat. Here is the problem: At idle there is little water flow through the PRT bypass hose as the water pressure is low and the PRT spring /plate is closed and limits the bypass flow. The flow is restricted to 4 small holes in the plate. This small flow of hot water has to heat the thermostat which has cold water the other side of it and the PRT is a bit of distance away from the engine. Soo.. the engine could get very hot before the thermostat eventually opens. Which is what I see happening, and its made worst by the fans coming on as the engine gets hot, as this air flow just cools the PRT and bypass flow more, which doesn't help open the thermostat. Any revs quickly opens the pressure spring/plate due to higher water pressure, this increases hot water flow to the stat and it opens, rapidly cooling the engine with colder water. Not good either = big sudden temperature change. The effect is made worse by the internal rubber seal on the PRT thermostat ageing and maybe leaking slightly. Many people say they dont have a problem, but the car temperature gauge hides it well, showing ‘normal’ from about 60C to 110C! Use an OBDII reader to show the temperature while sitting idle for 20 mins and it might show a problem. Worse in cold weather. So a better system is, I believe, with an engine water outlet thermostat. This would then regulate the outlet temperature keeping it constant, reducing engine temperature fluctuations which can be bad for the HG. This is what a lot of people do for high performance k-series engines. The good news is that with a bit of simple replumbing (swapping connections/ reversing flows) the old PRT thermostat can be transformed into an output type thermostat at zero cost. Problem fixed and the side benefit is also a better heater and less temperature fluctuation which means fewer head gasket issues. This is the video I did to describe the problem and the fix that I have been running for the last year: kzfaq.info/get/bejne/fN-hatx10buvdJs.html

Cool mate! No cheese

Thanks for this video, I've just ordered a Haynes manual to show me how to do this job! Is it not easy to get to from underneath?

dude, I need a fog and residence button, can you send it to Turkey?