Hi all,
Thought I'd better give an update because.... I've fixed it! Or at least got things running much, much better.
So, embarrassingly, it turns out the reported air leak was in fact
an air leak. Who'd have guessed? Not me, clearly. However, it was a tricky b****r so I think I can be forgiven for having missed it.
It was leaking between the two halves of throttle body assembly (i.e. between the throttle body and the other bit that the EGR and vacuum take off attach to), and possibly also between that and the manifold, though I think probably not.
Basically, at some point someone had used silicon to seal the two parts together, and I didn't really stop to question that. I guess when I was cleaning it things got disturbed, or maybe some of the gunwash I used for cleaning got into the silicon, and it started to leak.
Of course, when I pressed my hand down onto the open throttle body to test for a leak and found none, I was pressing everything together and probably helping to seal it up temporarily, and disguising the leak! Idiot.
Anyway, I only discovered this by accident when, in frustration I decided to take it all off again for another look and realised that there were strings of silicon hanging off when I pulled them apart.
I cleaned everything until it shone, and then put back the alloy gasket with a ring of fresh silicon either side. The seal with the manifold looks good to me - it's soft and springy - but I'm not sure how I can check it's fine other than to spend another £8 on a fresh one.
So, some more questions....
- Does the metal gasket between those two parts normally get installed with silicon either side, or it it some sort of single-use crushable gasket?
- Is there a torque value for the throttle body bolts?
I still have a misfire on cylinder 2 if I let it idle, but quite honestly this feels like a huge step forward as the car is quicker and nicer to drive than at any time since buying it.
Thanks so much to everyone who has offered help and advice so far!