Rough idle, idle speed and exhaust test

Step 1: Is ignition system in proper condition?

Check battery voltage, ignition distributor, spark plugs, ignition coil, ignition timing, etc.

  • Yes: Go to Step 2

  • No: Repair or replace damaged ignition components

Step 2: Is engine in proper mechanical condition?

If engine still does not start or starts only poorly after the electrical test, check for mechanical problems: Compression, basic adjustment of valves and engine oil pressure.

  • Yes: Go to Step 3

  • No: Repair or replace damaged components

Step 3: Is the throttle valve closed?

  • Yes: Go to Step 4

  • No: Go to step 3-A

Step 3-A: Can the throttle valve be closed further?

Determine whether the throttle valve can be closed farther and whether the engine speed decreases when this is done.

  • Yes: Remove the throttle valve clamps: if the throttle linkage is bent, straighten it; reset the adjusting screw at the throttle valve

Step 4: Is the throttle valve switch set correctly?

  • Yes: Go to Step 5

  • No: Adjust throttle valve switch

Step 5: Can the idle speed not be corrected with the idle screw?

Set the idle speed with the idle screw. Was test value not reached or engine idle uneven?

  • Yes: Go to Step 6

  • No: Go to Step 5-A

Step 5-A: Can the correct CO value be reached?

Perform exhaust gas test using CO tester with engine at normal operating temperature. Can the value be reached?

  • No: Go to Step 6

Step 6: No leaks in air intake system, system in proper condition?

See Engine does not start Step 8.

  • Yes: Go to Step 7

  • No: Repair leaks

Step 7: Does the auxiliary air valve close?

Is the auxiliary air device closed when the engine is at normal operating temperature?

Using a clip, clamp one hose leading to the auxiliary air device closed. The engine speed may drop slightly. If engine speed drops, auxiliary air device is open. Or: visual check with the hose detached: when the engine is at normal operating temperature the auxiliary air device must be closed.

  • Yes: Go to Step 8

  • No: Replace auxiliary air device

Step 8: Is the start valve in proper condition (no leaks)?

  • Yes: Go to Step 9

  • No: Go to Step 8-A

Step 8-A: Does the cold start valve leak fuel?

Test the start valve for leaks. For this purpose clamp the fuel intake line closed with a clip. Does engine then run evenly?

  • Yes: Replace start valve

Step 9: Are the fuel injectors in proper mechanical condition?

With the engine operating, detach the valve plugs individually one after the other from the fuel injection valves. With a good injection valve, the engine speed must drop as the plug is detached.

  • Yes: Go to Step 10

  • No: Replace defective valves

Step 10: Is the fuel pressure correct?

Test the fuel pressure.

  • Yes: Go to Step 11

Step 11: Is the air flow meter in proper condition?

For testing, see under Engine does not start under Step 9.

  • Yes: Go to Step 12

Step 12: Adjust exhaust CO level

Exhaust gas test (repeat) using CO tester with the engine at normal operating temperature. See Test Chart for value.

If the concentration of CO is too high, turn the bypass screw in the air flow meter 1/2 turn counterclockwise. Measure the speed and the concentration of CO again. Make corrections in several steps.