Idle problems on the 928 can be traced to many sources.   This is my attempt to provide a comprehensive troubleshooting guide based on my own experiences tracking down an intermittent idle problem on my '87 928 S4.   The basics should be applicable to any model year, but the information  (WSM pages, links) are specific to the S4, and for the most part later models.  

Currently a work in progress.


 

• Vacuum systems
  • Off brake booster
    • See John Pirtle's excellent writeup on troubleshooting this vacuum system
  • Diagrams for '87 and up air/fuel vacuum diagrams: diagram, WSM 24-216 , index, WSM 24-215
     
    • other model years, check WSM starting at page 24-22
       
  • ported vacuum - increases with throttle -  feeds vapor recovery system
    • attach vacuum tester to vacuum line for  Vacuum Valve (F in diagram, WSM 24-216) , and you'll need to have somebody rev the engine to see if you get vacuum.  
       
  • manifold vacuum -  decreases with throttle - feeds 5 way off throttle to AT,FPR,2 fuel dampeners
    • check the FPR and fuel dampeners:
      • pull vacuum tube and touch a finger to the connector and see if there is any gasoline.  If there is, you will need to replace the part
         
      • you should also check to see if it holds vacuum with tester.
         
  • General issues - are your vacuum hoses routed properly?
• Temp II
  • cold resistance - 2K Ohms
  • warm resistance - 200-300 Ohms
  • be sure to check each prong to ground - do not test across the 2 prongs!
  • If sensor (s) is/are bad, LH/EZK default to hot settings
  • See WSM 24-17 for testing details
     
• O2 Sensor
  • .1-.9 volts measured on pin 1 at connector by fuse panel
  • See WSM 24-219 for testing details
     
• Ground points - can cause various intermittent issues
  • Wally Plumleys' annual electrical maintenance
  • Ground contact diagram
• Emissions test ports
  • ever wonder what those two plugs sticking out from under the intake box are?   For testing emissions at the dealer with some fancy tool.  These are definitely on the 87 and 88, may be on the earlier models as well.   These go down to the manifolds, and the often rust out, allowing air in before the O2 sensor,   Check, and remove if rusted and use a BB and the stock fitting to plug.  Crap, I think it was 5/8", but just go to the ACE and find the biggest one that will fit.
  • There is also an OEM part they put on the '89 and later models -  930 113 153 00
     
• Check to make sure Throttle positioning switch is working correctly
  • On LH, check position 2 to ground.  Should be closed when throttle is at idle, and open (infinite resistance) when throttle is engaged.
    • LH pin out
  • On EZK, check position 8 to ground,  Same as LH - closed at idle, open when throttle engaged
    • EZK pin out
  • This may not fix a gross RPM problem, but if the TPS isn't working right, ISV does not get engaged, so LH is trying to handle all idle control
  • Above pics from Electronik Repair, Inc
     
• Losing Electrons?
   
  •  plugs, cables, caps, rotors, coils.
• Relays
  • Fuel  Pump Relay
  • EZK Relay
  • LH Relay
     
• MAF and LH brain
  • cause variety of maladies, best way to diagnose is swap with known good.  Yeah, I know, lots of help here!

 

Questions?  You can PM me on Rennlist, "fabric", or,
fabric at gmail dot com