How long to wait - coolant bleed

“Notanumbrr”,that’s interesting .
If yours is a 2003 car I think that it will be on the early version of position for the stat. In itself that should not make too much difference but your observations are of interested an noted.
When I look at my engine “someone” in the past has had the hoses off exactly where you suggest. I am assuming this as the clips are not original and all the others are.
It is a. Low mileage car. All was well be fore I drained the system to renew the underfloor pipes and cam belt/water-pump.
I’m a bit puzzled as to why the engine bay fan is coming on but the rad fan remains off.

Can anyone help with exact position of the sensors for those two fans please? In the 2003 version of the TF I thought the engine bay sensor was mounted external to the engine and off the beam that crosses the engine but that bracket is empty in this 2004/2005 car.

I may need to be more patient and wait longer before expecting flows through the pipes etc i.e. until the stat should open and hopefully pump water around the system.

That was on a 1998 Mk1 MGF but my 2003 Mk2 TF appears to have the same or at least similar cooling system configuration.

If you have the engine cover off it wont take you any time to remove that hose and check.

The engine bay sensor was on the beam above the engine bay (top of the boot appeture) in all cases on my 2003 Mk2 135 TF, a Mk1 2002 115 TF and a 1998 Mk1 F. It senses air temperature rather than coolant.

Id read somewhere that some of the later Mk2 TFs didnt have this and instead the engine bay fan was controlled by the ECU based on coolant temperature. Im not sure when this changed if so but it wasnt at the Mk1/ Mk2 change over point ,

I think the radiator fan is enabled by one of the 2 coolant temperature sensors screwed into the engine outlet, one being on top and visible, the other underneath. They are different spec sensors, one is for the ECU and the other runs the water temperature guage on the dash. Ive worked on other types of cars more recently so my memory may be hazy but I think the ECU one indirectly controls the radiator fan via an underbonnet relay.

Just a note to be careful on the radiator plug. Aftermarket rads often have a different thread from standard MGR.
I bought a Mike Satur version, and then found my rad had a different thread!🤬

Thank you David. I’m reasonably certain that my car is original. Only done 19k and I have good history. There is no mention of a radiator replacement and the matrix (externally) looks as though a replacement in the past is unlikely.

However, I can categorically state that the port is smaller than a spark plug (either size).
I took a gamble on a BWW part, and another ebay part said to be the right part (and after dropping both and uncertain as to which one is which) one fits and one doesnt!

Looking at what fits and bearing in mind my eyesight isn’t what it was and the fact that I can’t find my thread gauges at the moment I think it is M12 x1mm pitch.

I’m just trying to concoct a means to bleed into a 1L bottle to avoid both water everywhere and too much loss of coolant. Looks like I’ll miss the MOT date.

As mine is a late 2004 car and the bracket for the sensor is empty but the engine bay fan switches on and off I think it must be controlled by the ECU.
So here is the puzzle. The temp gauge in the dash was responding I.e. going up as the engine warmed up. So that suggests both the sensor for the gauge and the ECU are “operational”. If that is correct there must be two settings for the temperature in the ECU, one for rad fan, and one for engine bay. Logically they would have different trigger points. In the past I’m virtually certain that the rad fan came on before the engine bay. 
At present the rad fan doesn’t seem to switch on. The car has been garaged since Oct and on SORN and not run since whilst I have been working on changing water pipes and cam belt and water pump.
That seems to be cause for concern with no obvious explanation.
Can anyone clarify the settings and which comes first please?

In case of MG TF starting from VIN RD 634869 engine bay ambient sensor is deleted, so there is none, and engine bay fan is tied with engine ECU coolant temp sensor. So when that sensor reaches temperature and engine is runing both fans should start working. 

What is happening to you tells me your rad fan is not activated for some reason. You can test that with unpluging connector from engine ECU temp sensor and both fans should come on immediately. If rad fan does not come on, there is a problem. Could be fuse, could be relay, could be fan itself, or wiring. All of that can be tested.

Great, thank you for that information and the prompt reply.

Worth testing the radiator fan motor with 12 volts and if its fine then check the operation of the fan's relay which is under the bonnet on the left side.

Please check here where the rad fan is:  https://www.the-t-bar.com/forum/roverlike-s-guides/97846-mg-f-tf-fuse-relay-listing
For Pektron equipped cars rad fan relay is within fusebox under steering wheel.

Thank you all for continuing to help.Ill test the circuit later today.
I m struggling to get an accurate measurement for the thread pitch for the rad bleed but now consider it to be M12 x 1.5 rather than M12 x 1. I’ll confirm this in a day or so.
My aim is to set up a hose from the rad bleed point back to the tank to avoid loss of coolant and big puddles on the floor. Then do the same for the heater bleed.
Clearly that will not clear the return loop of either rad or heater instantly but once the stat opens it should achieve that although slowly.
Sadly when the system was empty I totally forgot about the jiggle valve in the head but did notice the inaccessibility of the associated hose and clips. I’m definitely not in any hurry to check that at this point. As water is seen to hit the side of the expansion tank and increase in ferocity with increase in rpm It is a safe assumption that that route is clear. 
The restrictor in the smaller water pipe under the inlet manifold is a different matter and again not one I want to explore. So here is a question.

Has anyone partially crimped that hose that feeds to the expansion tank because in doing so it should force more water through the restrictor in the smaller water pipe under the inlet manifold. In doing that the pipe should perhaps get hotter or at least feel a difference as the system warms up.

I seriously think you are over complicating things.
That's not criticism but sometimes we get bogged down and forget the simple things.
Fill the system with water. Open the heater valve. Undo the 3 airing points, make sure the heater point is clear. I had to poke mine with welding rod.
Let it settle for a few minutes.
If it misbehaves BLOW thru the rad airing point with a hose and you will hear air bubble out along the underfloor hoses up in to the header tank.
Nothing has changed with your fan, PRT or hoses since you last used it.
It is airlocked.

First some feedback on the fan issue.
Managed to disconnect the terminal block from the transducer at the engine (easier said than done)
Switched ignition on and the engine bay fan worked but not the rad fan (some of you will know that is normal).
Checked fuses in the front bonnet area and all ok.
Removed the 9 bolts that hold the panel in place that secures the radiator and “tease” it forward just enough to see the hello fan blades. Got a long stick and just pushed the fan round a few degrees. As it moved there was a definite initial resistance. But once that was overcome it moved freely. (I have had that experience with the engine bay fan when I picked it up on purchase- and in my earlier car too).
So try the power to the fan with the key - nothing. But with the engine switched on the fan immediately switched on.
So all is now well with that now and the contact has been remade with the transducer.
So thank you Roverlike for the technical input.
PS never did find the relay under the steering wheel. That can wait for another day.