Coolant System Fiesta Mk5

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Hi guys I'm new here so hope that I'm posting in the correct forum.

I have a Ford Courier (MK3) and that is based on the Fiesta MK5 1.8 TDi diesel engine.

I've been having a lot of trouble with it as the engine seems to run hot. The dash light doesn't come on, but the radiator fan comes on quite a bit. Also the engine has been smelling hot.

I tried to bleed the system because you can clearly hear air gurgling away in the engine... but have not been able to get the air out. I also noticed that half of the radiator was hot and the other half cold. The coolant in the expansion tank was also cold even though the engine was running at temp.

I decided to replace the thermo... just in case that was stuck open/closed and then I replaced the radiator.

These helped a bit but the engine was still running hot.

It was then I realised that the hoses to and from the expansion tank were cold. I removed the hosed that led from the top of the radiator to the expansion tank and found a restrictor in the hose. A peice of plastic with a tiny hole drilled through it. I discovered the same in the expansion tank overflow hose. I have since removed them and the coolant is now moving through the system unrestricted as it should.

Why would anyone do this... what benefit did it have?