L322 RR 4.4 V8 2002 - Overheating - Any thoughts?

Hi,

My RR 4.4 V8 2002 Vogue started overheating.
I was driving when I got a warning on the dashboard and I've stopped until cooler and made my way back home.
Took to the mechanic and the expansion tank was leaking.
He replaced the expansion tank, I took the car but it was still overheating.
He then replaced the Viscous Fan, as he said the original wasn't working as efficiently as it should.
I took the car, and it was still overheating. It doesn't happen immediately it takes at least 30 minutes driving or more for that to happen.
By then I had spend £450 and the car still wasn't good. I've complained and took the car back.
He's checking and says he's got a fault coming up on the Electric Fan, but that fan as far as I know is only needed for the air conditioning and doesn't actually cool the engine.
It's not leaking coolant any longer now, but still overheats. He says he can't reproduce the overheating, which I understand as I suppose he doesn't have the time to drive long enough for that to happen.

Any thoughts as to what could be happening?
- Does the Electric Fan plays any role in engine cooling?
- Could it be a faulty thermostat?
- The ECU itself having a wrong temp reading?
- Some radiator blockage?

Any thoughts would be highly appreciated.

Many thanks!

My initial thoughts are an air lock, if the bleed procedure isn't carried out correctly.

Second thoughts are radiator, they silt up at the bottom reducing efficency. And even possibly the pump, the impeller can work loose on the shaft meaning you don't get good flow through the system.

Third is the thermostat - they are a hybrid type, they act as a conventional wax-stat and open and close through thermal activity in the coolant, but they are also heated when more cooling is required - this is done via the engine ECU. When crusing around, the engine is kept at around 105degC but if you start to accelerate or the engine begins to work hard, the ECU commands the stat to open wider to promote more cooling. If the stat is duff, the wiring there to is duff or the connector is corroded (common on the M62) the ECU will put a fault code up saying something like Electronic Thermostat Fault or something similar - can't remember the exact wording!

The A/C fans do come on in extreme temps as an aid to cooling.

I had curious temp readings, but mine would fluctuate up then down, this was a chaffed wire earthing out from the temp sensor.

First thing is to ensure it has been bled of air properly, a quick check is to turn the heaters on to full heat and see if you get hot air, if not there could be an air lock somewhere - but this is not a guaranteed way of checking - bleed the system properly to ensure no air locks - they are very difficult to bleed.. Then do a static pressure check to see if the system will hold pressure, if they check out, time to look at the stat or rad....maybe even the pump to ensure you are getting good flow.

Thanks so much for your reply.
I'll follow that sequence see if I can track the problem down.

The space between the Condensor & Radiator also gets full of debris.

I found raising the front of the car helped and also my auxiliary pump doesn't work which isn't helpful.

https://www.meeknet.co.uk/E38/E38_Bleeding_The_Cooling_System.htm

http://www.meeknet.co.uk/e31/BMW_M60_M62_M62TU_Engine_Cooling_System.htm