High idle

[morriss]

I finally decided to face the fact that my 82' Euro M28.11 CIS idles high. Maybe I messed something up a couple of years ago when I replaced all the vacuum lines and removed the smog pump and associated hoses. Or maybe the shop I took it to a couple of years ago to set my air/fuel mixture set the idle screw too far.
Before I go in and adjust the idle screw and A/F mixture screw, is there anything specific I should check?

I plugged the single output vacuum hose on the deceleration valve and the nipple on the valve itself. This only brough down the idle by 30 rpms. Hooked up the car idles at 1070-1080 rpms, and unhooked at 1040-1050 rpms.

Timing set at 28 degrees instead of 26, for more power.
Original fuel injectors, two of which weep ever-so slightly.
All new hoses, lines, and gaskets under the hood.

[morriss]

Following a later post

http://forums.pelicanparts.com/porsche-928-technical-forum/845731-high-idle-rich-running-occasional-misfire.html

I sprayed bursts of carb cleaner around the base of the throttle body, the rubber sleeves on the spider, etc. I noticed no change in the idle speed, so I'm hoping that I don't have a vacuum leak anywhere.

I went ahead and adjusted the large idle screw clockwise to bring the rpms down until they were jumping between 820-830. I then pulled the air cleaner off to get at the 3mm screw. I turned it counter clockwise until the engine started to stumble, then clockwise until it started to stuble, and then set it half way between the two, favoring it rich just a hair. I found those instructions over on the 911 forum. I took the 928 up on the highway for a a 4-5 mile drive, and all seems okay. One unexpected benefit of not havong a high idle anymore is that my syncros don't grind like they used to when shifting gears.

[MPDano]

Not only do you need to block the vac line to the diverter, but also the large hose that goes back towards your firewall. Not sure if this is your case, but when I put a US engine in my 4.5 Euro. I had a huge air leak there as the Euro heads were plumbed different than the US heads.