Idle Problem Troubleshooting
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
- 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
- attach vacuum tester to vacuum line for Vacuum Valve (F
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
- 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
24-17 for testing details
- O2 Sensor
- Ground points - can cause various intermittent issues
- 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
- On LH, check position 2 to ground. Should be closed when
throttle is at idle, and open (infinite resistance) when
throttle is engaged.
- On EZK, check position 8 to ground, Same as LH - closed at
idle, open when throttle engaged
- 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.
- 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