PRT Conversion / Rad fan problem?

[Roobert]

Hi all
My car is a 2002 1.8tf with OEM thermostat fitted as standard.
After replacing head gasket, timing belt & water pump and reading many posts on the subject I decided it made good sense to fit a prt in front of the radiator replacing the original with the metal ring and gasket whilst it was all apart.

The car runs fine, heater gets hot even though warm up time is a little longer as expected now having a bigger bypass system.
The problem is that when the temperature rises sitting in traffic that the radiator fan does not cut in to cool it down..
I have tested the fan by removing the connector from the fan switch whilst engine running and it kicks in no problem.
Also tried another fan switch even though it was working before the conversion, but still boils over...
I then remembered reading a post www.MGFcar.de where one of the steps to this conversion was to take the old by pass hose of the engine outlet
(The one with the 2 temp sender units on the branch) and block the outlet so that there is only a 2mm hole in it!
It says the reason for leaving a 2mm hole in the old bypass is just to ensure that the sender units (one being for the radiator fan) get a good look at the water exiting the head......

Having pressure bled the system numerous times to no avail in getting the fan to cut in I decided to restrict the old by pass as described above.
It was only then that the rad fan started to kick in as expected after ticking over for 30 or so minutes !!
My question is has anyone else had this problem as it isn't mentioned in any of your up front conversions??

There also seemed to be a lot of contradiction as to the direction of flow.
On my car the hot water is sent from the head along the left hand pipe to the bottom of the radiator, and returns from the top right as per early mgf pre prt.
So I have plumbed my pressure relief thermostat into the n/s return rad hose with the t-piece on the right hand radiator feed pipe which keeps the flow the same way as OEM. So thermostat on the return as most of you have done.
But I thought having a thermostat on the inlet was half of the original design fault, but I do understand that now there is less shock or fluctuation in temp at the head as we now have a bigger bypass, and cooler water from the rad has hotter water to mix with before it gets there..
It seems to me that when the prt is fitted behind the engine as standard, it is on the outlet side and also changes direction of flow through the radiator
(Hot in to the top right hand hose and cold out of the bottom left hand hose)
Question 2
Should I or is there any benefit in swapping over the 2 pipes that feed the under car steel pipes to achieve this more conventional way of hot in to top of rad and cold out through the bottom hose, and hence swap the prt position to the then n/s return???
Would just mean 2 new rad hoses to chop about again..

Hope that was all digestible and that someone can shed some light
Thanks
Rob