This is starting to annoy me now, so I will hope to have a look this weekend, but thought I'd voice it on here first to see if any one has experienced similar issues.
When I first bought my S it suffered a little with a poor cold idle, it would often try to stall at idle (and sometimes did) and overrun felt excessive (i.e. felt like there was excessive engine braking). I removed all inlet components, cleaned, checked and adjusted the ISV, TB/plenum & AFM and replaced a few of the vac elbows that could not be got to once the plenum was back in place - also cleaned out and replaced the seals for the AOS. After that, the stalling issue was gone, but the idle was still not quite right. Maybe it was like this before, but I can't remember as keeping it from stalling was the immediate issue rather than a rock steady idle.
Now, fast forwarding a few months to the warmer weather, and it feels like it is gradually running a little worse. Feels and smells rich. Plugs have been replaced (old ones looked normal colour - no excessive sooting) and leads swapped, but still the same.
Here are my symptoms:
Idle - on the rev counter it can be seen to be hunting a tiny bit, but more noticeable watching the engine, it looks like it has a tiny miss every now & then.
Low revs & part throttle it often bogs down a bit. It feels better on a cool evening than a warm day.
Exhaust fumes smell rich, but the exhaust is not getting noticeably sooty.
It has a very distinct stumble at 2250rpm at constant throttle, causes car to surge. Not noticed as you accelerate through this rev range though.
At full throttle the car is a bit flat until you hit 4k (normal for an S?), then pulls well, but then bogs again at around 5k, before picking up again (I don't remember it doing this when I first got it)
Last tank of part dual carriageway, and part town driving gave 21.5 mpg. When I first got the car I was getting more like high 20's and that was in the winter.
I have checked the AFM operation, and that is nice and linear, inlet air temp sensor was also within spec. I checked the ECU temp sensor and when cold it was reading the correct resistance, although I have not yet checked it when hot.
I plan to double check the ECU temp sensor this weekend, check fuel pressure and grounds. The battery meter only ever shows as high as 13V, but should it not show charging voltage when running?
Is there anything else anyone can suggest checking out? Can any other S owners concur with how my car sounds/feels like it is driving? I can deal with it being a bit flat 'off cam' but at the moment, just pulling out a junction will cause it to bog down and stutter, which can't be right!
I plan to get it onto a CO meter as soon as I can, at the last MOT (December), CO was very low , HC low also.
Any suggestions would be very helpful
