Hot/cold idle issue

[Laird Scooby]

Originally Posted by AllHailKingVolvo

Just a thought-having been through this very issue with my wife’s 740 with B200E, have you checked that the microswitch that sits above the throttle butterfly lever is activated when off throttle?

Ours would idle horribly when cold, lumpy and uneven, then when hot would idle at 1400rpm. If I set the hot idle correctly, it wouldn’t run without throttle when cold.

I did some sleuthing and found that the little cap head bolts that hold the microswitch in place were loose, and it had worked it’s way into a position where the throttle lever wasn’t activating the switch. I refitted it correctly, which brought the hot idle down to around 500rpm when activated. I adjusted that up to 900rpm hot, and it now starts and runs very well whether hot or cold.

All great points Phil and very valid but the OP has alredy proved the AAV isn't closing as it warms up. If it had been i'd definitely have either already suggested those points you made or seconded your suggestion.

[lightyears]

Originally Posted by AllHailKingVolvo

Just a thought-having been through this very issue with my wife’s 740 with B200E, have you checked that the microswitch that sits above the throttle butterfly lever is activated when off throttle?

Ours would idle horribly when cold, lumpy and uneven, then when hot would idle at 1400rpm. If I set the hot idle correctly, it wouldn’t run without throttle when cold.

I did some sleuthing and found that the little cap head bolts that hold the microswitch in place were loose, and it had worked it’s way into a position where the throttle lever wasn’t activating the switch. I refitted it correctly, which brought the hot idle down to around 500rpm when activated. I adjusted that up to 900rpm hot, and it now starts and runs very well whether hot or cold.

Thanks for the pointers, it did cross my mind the other day about the switch, having cleaned that area up it appears to be working very well. On a slight update to, I managed to source a working AAV with my part number from a German seller on eBay, should be with me shortly.

[AllHailKingVolvo]

Originally Posted by Laird Scooby

All great points Phil and very valid but the OP has alredy proved the AAV isn't closing as it warms up. If it had been i'd definitely have either already suggested those points you made or seconded your suggestion.

Ah apologies-I’d missed the memo on that! Thankfully the AAV on ours is working as it should for the time being, but am following this thread with interest as I’m keen to learn as much about the K-Jet as possible!

[Laird Scooby]

Originally Posted by AllHailKingVolvo

Ah apologies-I’d missed the memo on that! Thankfully the AAV on ours is working as it should for the time being, but am following this thread with interest as I’m keen to learn as much about the K-Jet as possible!

No worries Phil, the relevant bits about confirming the old AAV had siezed were on the page before somewhere.

The K-Jet is a very simple (and if correctly set up, very efficient) system with few inherent faults. The magic comes in identifying whether it's a fuel or ignition fault and the common point there is the microswitch on the throttle body.

[lightyears]

Well the replacement part came today, put it all on and it’s like a different car, had to reset the idle slightly but other than that it works well, thanks for the help guys. For reference if anyone finds this post in the future, this part comes off a BMW E21 from the mid 80s which happens to share the same part number from Bosch.

[Laird Scooby]

Originally Posted by lightyears

Well the replacement part came today, put it all on and it’s like a different car, had to reset the idle slightly but other than that it works well, thanks for the help guys. For reference if anyone finds this post in the future, this part comes off a BMW E21 from the mid 80s which happens to share the same part number from Bosch.

Glad to hear it's sorted but when you say you had to reset the idle slightly, how exactly did you do that?

[lightyears]

The previous keeper had wound the black air mix screw right in, I suppose to compensate for the valve being stuck the whole time. I followed a guide on getting the idle correct and stable at 800-900rpm when warm. It has made a lot of difference and the car runs so much better as a result.

Edit: Used a CO meter to get it as close to the Haines recommended value as possible.

[Laird Scooby]

Originally Posted by lightyears

The previous keeper had wound the black air mix screw right in, I suppose to compensate for the valve being stuck the whole time. I followed a guide on getting the idle correct and stable at 800-900rpm when warm. It has made a lot of difference and the car runs so much better as a result.

Edit: Used a CO meter to get it as close to the Haines recommended value as possible.

All sounds good to me! Was worried you'd adjusted the throttle butterfly which is factory set - yes it can be adjusted afterwards but it's a long drawn out procedure to type (although not much in pratice) so i'm glad you used the idle bypass screw.

Every once in a while it's worth screwing that all the way in, counting the number of turns to completely closed from where it was then removing completely and squirting some carb cleaner into the hole and air passages that join it and also on the tip of the screw itself and giving it a clean. Then screw it all the way back in and back it out the right number of turns, giving it a tweak once warm to correct the idle speed to 900rpm again.