Technical Service Bulletin Idle Adjustment and Performance

Subject: Idle Adjustment & Performance        Models: 911 Carrera ALL

Bulletin Number: 83 – 10

Idle adjustment procedure

Engine oil temperature must be approximately 90 degrees Celsius. (194 degrees Fahrenheit)

Intake air temperature must be approximately 15 to 35 degrees Celsius. (59 to 95 Fahrenheit)

CO tester connected in front of catalytic converter

  • Disconnect oxygen sensor
  • Bridge terminals B and C on the test connection receptacle (see illustration). This makes the idle stabilizer inoperative.
  • Check/adjust idle RPM
  • Specifications = 800 + 20 RPM

Adjustments are made with the throttle housing adjustment screw

  • Remove bridge from test receptacle
  • Recheck CO% ad idle RPM
  • Remove CO probe and close catalytic converter connection


For accurate adjustments, it is absolutely essential to follow the prescribed sequence

Diagnosing engine performance problems

Incorrect idle RPM

  • RPM higher than 820: The deceleration fuel shut off valve is activated, causing idle surge, jerking in stop and go traffic.
  • RPM lower than 780: May cause stalling at idle when operating A/C, hesitation upon acceleration.

Incorrect CO adjustment

  • CO valve greater than 1%: May cause idle surge after cold start
  • CO valve lower than 0.6%: Idle unstable, possible hesitation upon acceleration

Temperature Sensor II, responsible for correct warm-up enrichment (located in left cylinder head by number 3 cylinder).

Temperature                     Resistance Values

10°C(50°F)                            3.3 to 4.1 K Ohmns

20°C (68°F)                           2.2 to 2.8 K Ohmns

40°C(104°F)                          1.0 to 1.3 K Ohmns

80°C(176°F)                          20 to 250 Ohmns

100°C (212°F)                      160 to 210 Ohmns

  • If sensor is disconnected {resistance = ∞} the engine will run too rich and stall at idle
  • If sensor is shorted to ground {resistance = 0 Ohmns} the engine will experience difficulties in cold start and warm-up.

Engine speed sensor and reference mark

  • Specification: Distance for engine speed sensor to flywheel = 0.8 + 0.3 mm
  • At a distance of approximately 2 mm: Longer cranking time, more starts necessary, will not start at lower temperatures. During warm-up engine will run normally; at operating temperature will idle higher than normal. (see incorrect idle RPM)
  • At a distance of approximately 3 mm: The engine experiences ignition misses and hesitation upon acceleration. Due to heat expansion, engine may quit running for short periods at a time.

Fuel Return Hose

  • If the fuel return hose is kinked for blocked, fuel pressure will rise causing rich running and stalling at idle


If CO% cannot be adjusted to specifications or engine surges at idle when cold or under partial load, test fuel pressure

  • Specifications = 2 bar at idle (vacuum hose connected)

Spark Plugs

  • Wear limit for electrode gap = 1.3 mm

Hot Start Problems

At operating temperature, the fuel pressure drop is approximately 0.5 bar in 30 minutes If pressure drop exceeds specifications, check the following:

  • Pressure regulator for internal leak. Test by closing off fuel return hose.
  • Check valve in fuel pump for internal leak. Test by closing off fuel pressure hose.
  • Injectors for internal leaks. Test by closing off both pressure and return hoses.

Acceleration Linkage Adjustment

  • The long rod on the transmission relay lever must have a clearance of 0.5 to 1.0 mm for proper operation of the idle microswitch.

Idle Microswitch

Inoperative (open) switch will produce an increased idle (approximately 1100 RPM) and the ignition timing will be advanced. The following may also occur

  • When decelerating from over 5000 to under 4000 RPM, the engine will hesitate upon re-acceleration
  • With a cold engine, acceleration delayed by approximately 1° of throttle opening; engine surges

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