Download NOTE : a. Checking the idle mixture adjustment will require the use of the on/off ratio tester (Bosch KDJE-P 600) or equivalent. b. All tests are to be performed after engine has reached normal operating temperature. c. All accessories must be switched off. d. The engine must be in good mechanical working order and free from electrical misfires. Fig. 72 Bosch On/Off Ratio Tester 1. Connect the on/off ratio tester to the diagnostic socket. 2. Disconnect the purge line to the throttle valve assembly at the purge valve and plug it. 3. Start the engine and run at idle speed. 4. Press the IR 100% button on the tester. 5. Run the engine at approximately 2500 RPM and record the average on/off ratio reading. NOTE: If the On/Off ratio tester has a constant value refer to DIAGNOSIS AND TESTING PROCEDURES in COMPUTERIZED ENGINE CONTROLS . 6. Let the engine return to idle speed and wait a few moments for it to stabilize. Now record the average on/off ratio reading at idle. The On/Off ratio reading should be in the 50% ±10% range, and the difference between cruise and idle RPM should not vary by more than +10. If the difference is greater than +10 refer to ADJUSTING THE AIR/FUEL MIXTURE . If the difference is not greater than +10 the fuel mixture is adjusted correctly.