Another part of trying to sort the idle is to replace the idle control valve, which has been a bit suspect from the beginning.
I don’t particularly enjoy taking the intake off, but its the only way to get to the idle valve.
I cleaned the idle valve a while back, but obviously it was still suspect. Even after cleaning I was still getting a reading of zero in DIS for idle air flow (I haven’t had a chance to test since replacement).
After some research it turned out that the S50B30 idle valve is shared with the M60/M62 BMW V8 engines, so although its bigger and not shared with the other I6 engines, it’s not too hard to source one from an E34/E39/E38.
I got a good used replacement, and got to work pulling the intake out again. Thankfully last time I had it all off I chose to fit a pod filter instead of the standard airbox, as that was one part that pissed me off a lot when reassembling; refitting the airbox. Taking the plenum off doesn’t take too long, it’s just a bit fiddly.
It became obvious that despite me cleaning it, the original valve was still sticking, and it moved nowhere near as freely as the used replacement. An easy test of how freely they operate is to twist/shake the unit side to side and see how easily the shutter inside it moves.
The old one needs quite a firm shake to move the shutter, whilst the replacement moves very freely. I suspect this is how the original one moved in DIS but may not be working correctly. With a full whack of voltage to fully open or close it (as the DIS test does), it works OK, but you can’t finesse the movement and tweak it just a little.
With one of my previous orders, I had ordered a replacement mount for the valve, as the old one was brittle and broke last time I removed it (hence the zip tie in the above photos). On went the new mount.
I fit the replacement valve, reassembled, and tested. It seems the idle acts a bit better than it did, with noticeable changes when load is applied at idle, but it’s still rough and misfiring at idle, so obviously the idle valve was an issue, but not related to my original fault.