Adjusting Idle And Co
Last Updated on Thu, 19 Sep 2019 | Porsche 928 Repair

Engine in perfect mechanical condition. Ignition timing set correctly.
Adjustments must be made as quickly as possible, to prevent excessive heat in the intake ports.

1. Detach right air intake hose.

2. Remove air cleaner upper section.

3. Detach air hose at diverter valve and insert a suitable plug. (Disconnect auxiliary air pump hose on air pipe and insert a suitable plug in open air pipe (e. g. rubber grommet for doors, Part No. 999.703.163.40).)

4. Install air cleaner upper section and air intake hose.

5. Run engine to operating temperature (80 to 90 C/ 176 to 194 F oil temperature). Use Special Tool 9122 or US 8025 to measure oil temperature.


6. Connect CO level tester and separate tachometer according to instructions of manufacturer.

7. Adjust control screw on throttle housing until specified speed is reached. Turn control screw or bypass screw on throttle housing, until speed of 700 + 50 rpm is reached.

Adjust mixture. Insert Special Tool 9134 through funnel-shaped opening into spring-loaded driver in mixture control unit. Press down special tool approx. 18 mm to engage spring-loaded drive in mixture control screw.

It is essential to conform with the following points.
a) Always make CO level adjustments from lean to rich. Example: when mixture is too rich, first turn mixture control screw counterclockwise further than necessary and then clockwise to adjust to specifications.

b) Just turn control slightly, since the smallest adjustment will change CO level considerably.

8. Adjust fuel/air mixture. Guide Special Tool 9187 into adjusting bore of heated wire air flow sensor and turn idle speed potentiometer accordingly.

9. Turn clockwise for richer or counterclockwise for leaner mixture. CO specifications: 0.5 - 1.5 % R. o. W. 0.5 - 1.0 % Australia, Switzerland, Sweden

10. Remove special tool.

11. Accelerate engine briefly.

12. Wait until CO tester shows exhaust gas concentration at idle speed. See page 25-6 for adjusting data. If necessary, repeat adjusting procedures.

13. Recheck idle speed and correct, if necessary.

14. Remove plug after completion of adjustments. Attach air hose to diverter valve.