Well, after a lot of messing about I think the problem is caused by a leak in the air intake system.
I've got a CDA fitted, but it's not an easy fit
The BMC site shows it running straight from the throttle body over the radiator and down to the cold air.
This doesn't seem possible to me. If I try and follow this set-up I can't close my bonnet. Their diagram doesn't seem to include the metal section which contains the mass air sensor.
The guy who fitted it for me used one of the original parts to direct the system towards the ground (he had to cut a piece of hard plastic pipe with a small resonance chamber on it). The down section then contains the mass air sensor section and the CDA.
The problem with this is two-fold:
1. Once you add the corrugated metal tubing to the bottom of the CDA and point it towards the cold air the clearance of the car isn't very good. So, after a week I end up with a metal tube which is no longer round!
2. The grounding of the metal pipe seems to knock the system and cause leaks. I think the leak comes from where the hard plastic right angle pipe joins the rubber section connected to the metal body of the mas air sensor ..... if you see what I mean!
So, I've removed all the sections from the throttle body forward and re-fitted them, except the corrugated metal pipe. Last night and this morning, the car seems to run a lot better, but of course I don't think I'll get such a good cold air intake with it pointing directly at the ground.
My next ideas are to:
1. Try and mount the CDA a couple of centimetres higher to get the ground clearance I want. I'm hoping I can do this without breaking the seal around the hard plastic pipe higher up.
2. I could replace the section which comes off the throttle body and is also connected to the breather pipe. If I can find a rubber:silicon pipe which starts at 70mm for the throttle body and ends at 80mm for the mass air sensor body, plus bending at about 45 degrees I can probably mount it the way BMC indicate and get rid of the hard plastic join.
Any ideas where I can get a piece of pipe like that? I've tried my local car spairs shop, but no joy? Any thoughts regarding if I'm on the right track?
Thanks in advance,
Matt.