Peugeot 405 Manual

Idle speed and mixture check and adjustment (12000 Mile (20 000 km) / 12 Month Service)
Routine maintenance and servicing / Idle speed and mixture check and adjustment (12000 Mile (20 000 km) / 12 Month Service)


Note: On models from 1994, the makers specified interval for this procedure is 18 000 miles (30 000 km).

1 Before checking the idle speed and mixture setting, always check the following first: a) Check that (where adjustable) the ignition timing is accurate (Chapter 5B).

b) Check that the spark plugs are in good condition and correctly gapped (Section 11).

c) Check that the accelerator cable (and on carburettor models, the choke cable) is correctly adjusted (refer to the relevant Part of Chapter 4).

d) Check that the crankcase breather hoses are secure, with no leaks or kinks (Sections 7 and 29).

e) Check that the air cleaner filter element is clean (Section 21).

f) Check that the exhaust system is in good condition (refer to the relevant Part of Chapter 4).

g) If the engine is running roughly, check the compression pressures and valve clearances as described in Chapter 2.

h) On fuel injection models, check that the fuel injection/ignition system warning light is not illuminated (refer to the relevant Part of Chapter 4).

2 Take the car on a journey of sufficient length to warm it up to normal operating temperature. Note: Adjustment should ideally be completed within two minutes of return, without stopping the engine. If the radiator electric cooling fan operates, wait for the cooling fan to stop. If adjustment takes longer than stated, regularly clear any excess fuel from the inlet manifold by revving the engine two or three times to about 2000 rpm, then allow it to idle again.

Carburettor models
3 Ensure that all electrical loads are switched off, and that the choke lever is pushed fully in.

If the car does not have a tachometer, connect one following its manufacturers instructions.

Note the idle speed, and compare it with that specified. Note: Models with air conditioning have an idle compensation device, and the air conditioning compressor must be running while the idle speed is being checked and adjusted.

4 Using a suitable flat-bladed screwdriver, screw in the idle adjusting screw (to increase the speed) or out as necessary to obtain the specified speed. The screw is located on the carburettor on non-air conditioning models, and on the idle compensating device on air conditioning models (see illustrations).

10.4a Idle speed adjustment screw (arrowed) on models with idle compensation
10.4a Idle speed adjustment screw (arrowed) on models with idle compensation

10.4b Idle speed adjustment screw
10.4b Idle speed adjustment screw

5 The idle mixture (exhaust gas CO level) is set at the factory, and should require no further adjustment. If, due to a change in engine characteristics (carbon build-up, bore wear etc) or after a major carburettor overhaul, the mixture becomes incorrect, it can be reset. Note, however, that an exhaust gas analyser (CO meter) will be required to check the mixture, and to set it with the necessary standard of accuracy. If this is not available, the car must be taken to a Peugeot dealer for the work to be carried out.

6 Follow the exhaust gas analyser manufacturers instructions to check the exhaust gas CO level. If adjustment is required, it is made via mixture adjustment screw located on the carburettor. The screw is covered with a tamperproof plug to prevent unnecessary adjustment. To gain access to the screw, use a sharp instrument to hook out the plug.

7 Using a suitable flat-bladed screwdriver, turn the mixture adjustment screw by very small amounts until the level is correct (see illustration). Screwing it in (clockwise) weakens the idle mixture and reduces the CO level; screwing it out will richen the mixture and increase the CO level.

10.7 Idle mixture adjustment screw (arrowed)
10.7 Idle mixture adjustment screw (arrowed)

8 When adjustments are complete, disconnect any test equipment, and fit a new tamperproof plug to the mixture adjustment screw. Recheck the idle speed and, if necessary, readjust.

Fuel injection models
Bosch L3.1-Jetronic system 9 Ensure that all electrical loads are switched off. If the car does not have a tachometer, connect one following its manufacturers instructions. Note the idle speed, and compare it with that specified.

10 The idle speed is adjusted using the idle speed adjustment screw on the throttle housing (see illustration). Turn the screw clockwise to decrease the idle speed, or anticlockwise to increase the speed.

10.10 Adjusting the idle speed screw on the Bosch L3.1 injection system
10.10 Adjusting the idle speed screw on the Bosch L3.1 injection system

11 The idle mixture (exhaust gas CO level) is set at the factory, and should require no further adjustment. If, due to a change in engine characteristics (carbon build-up, bore wear etc) or after a major overhaul, the mixture becomes incorrect, it can be reset.

Note, however, that an exhaust gas analyser (CO meter) will be required to check the mixture, and to set it with the necessary standard of accuracy. If this is not available, the car must be taken to a Peugeot dealer for the work to be carried out.

12 Follow the exhaust gas analyser manufacturers instructions to check the exhaust gas CO level. If adjustment is required, it is made via mixture adjustment screw located on the airflow meter (see Chapter 4C). The screw may be covered with a tamperproof plug to prevent unnecessary adjustment. To gain access to the screw, use a sharp instrument to hook out the plug.

13 Using a flat-bladed screwdriver, turn the mixture adjustment screw by small amounts until the level is correct (see illustration).

10.13 Mixture (CO) adjustment screw on the Bosch L3.1 injection control unit
10.13 Mixture (CO) adjustment screw on the Bosch L3.1 injection control unit

14 When adjustments are complete, disconnect any test equipment, and fit a new tamperproof plug to the mixture adjustment screw. Recheck the idle speed and, if necessary, readjust.

Bosch ML4.1 Motronic system 15 The idle speed is non-adjustable. It is controlled by the idle speed regulator valve.

16 The idle mixture (exhaust gas CO level) is set at the factory, and should require no further adjustment. If, due to a change in engine characteristics (carbon build-up, bore wear etc) or after a major overhaul, the mixture becomes incorrect, it can be reset.

Note, however, that an exhaust gas analyser (CO meter) will be required to check the mixture, and to set it with the necessary standard of accuracy. If this is not available, the car must be taken to a Peugeot dealer for the work to be carried out.

17 Follow the exhaust gas analyser manufacturers instructions to check the exhaust gas CO level. If adjustment is required, it is made via mixture adjustment screw located on the airflow meter (see illustration). The screw may be covered with a tamperproof plug to prevent unnecessary adjustment. To gain access to the screw, use a sharp instrument to hook out the plug.

10.17 Mixture (CO) adjustment screw (arrowed) on Bosch ML4.1 Motronic system
10.17 Mixture (CO) adjustment screw (arrowed) on Bosch ML4.1 Motronic system

18 Turn the screw clockwise to increase and anti-clockwise to decrease CO content until the specified CO level is obtained.

19 When adjustments are complete, disconnect any test equipment, and fit a new tamperproof plug to the mixture adjustment screw.

Bosch LU2-Jetronic system 20 The idle mixture is not adjustable and is automatically regulated by the ECU.

21 To check the idle speed connect a tachometer to the engine, then run the engine at idle speed.

22 Turn the idle speed adjustment screw to obtain the specified idle speed (see illustration).

10.22 Idle speed adjustment screw (2) on the Bosch LU2-Jetronic injection
10.22 Idle speed adjustment screw (2) on the Bosch LU2-Jetronic injection system

23 When adjustments are complete, disconnect any test gear from the engine.

Bosch Motronic MP3.1 system 24 Ensure that all electrical loads are switched off. If the car does not have a tachometer, connect one following its manufacturers instructions. Note the idle speed, and compare it with that specified.

25 Turn the idle speed adjustment screw to obtain the specified idle speed (see illustration).

10.25 Idle speed adjustment screw (1) on the Bosch Motronic MP3.1 system
10.25 Idle speed adjustment screw (1) on the Bosch Motronic MP3.1 system

26 The idle mixture (exhaust gas CO level) is set at the factory, and should require no further adjustment. If, due to a change in engine characteristics (carbon build-up, bore wear etc) or after a major overhaul, the mixture becomes incorrect, it can be reset.

Note, however, that an exhaust gas analyser (CO meter) will be required to check the mixture, and to set it with the necessary standard of accuracy. If this is not available, the car must be taken to a Peugeot dealer for the work to be carried out.

27 Follow the exhaust gas analyser manufacturers instructions to check the exhaust gas CO level. If adjustment is required, it is made via mixture adjustment screw (see illustration). The screw may be covered with a tamperproof plug to prevent unnecessary adjustment. To gain access to the screw, use a sharp instrument to hook out the plug.

10.27 Mixture (CO) adjustment screw (2) on the Bosch MP3.1 fuel injection
10.27 Mixture (CO) adjustment screw (2) on the Bosch MP3.1 fuel injection system

28 Turn the screw clockwise to increase and anti-clockwise to decrease CO content until the specified CO level is obtained.

29 When adjustments are complete, disconnect any test equipment, and fit a new tamperproof plug to the mixture adjustment screw.

Bosch Motronic M1.3 fuel injection system
30 The idle speed is only adjustable on the XU9JA/Z engine - on other engines it is controlled by the ECU and idle speed control valve.

31 Ensure that all electrical loads are switched off. If the car does not have a tachometer, connect one following its manufacturers instructions. Note the idle speed, and compare it with that specified.

32 Turn the idle speed adjustment screw to obtain the specified idle speed (see illustration).

10.32 Idle speed adjustment screw (7) on the Bosch Motronic M1.3 injection
10.32 Idle speed adjustment screw (7) on the Bosch Motronic M1.3 injection system

33 The idle mixture (CO) is only adjustable on the XU9J4/K engine - on other engines it is controlled by the ECU.

34 The idle mixture (exhaust gas CO level) is set at the factory, and should require no further adjustment. If, due to a change in engine characteristics (carbon build-up, bore wear etc) or after a major overhaul, the mixture becomes incorrect, it can be reset.

Note, however, that an exhaust gas analyser (CO meter) will be required to check the mixture, and to set it with the necessary standard of accuracy. If this is not available, the car must be taken to a Peugeot dealer for the work to be carried out.

35 Follow the exhaust gas analyser manufacturers instructions to check the exhaust gas CO level. If adjustment is required, it is made via mixture adjustment screw located on top of the airflow meter assembly (see illustration). The screw may be covered with a tamperproof plug to prevent unnecessary adjustment. To gain access to the screw, use a sharp instrument to hook out the plug.

10.35 Mixture adjustment screw (5) on the Bosch Motronic M1.3 fuel injection
10.35 Mixture adjustment screw (5) on the Bosch Motronic M1.3 fuel injection system

36 Turn the screw clockwise to increase and anti-clockwise to decrease CO content until the specified CO level is obtained.

All other fuel injection systems 37 Experienced home mechanics, with a considerable amount of skill and equipment (including a tachometer and an accurate exhaust gas analyser) may be able to check the exhaust CO level and the idle speed.

However, if these are found to be in need of adjustment, the car must be taken to a suitably-equipped Peugeot dealer.

38 On models with a Magneti Marelli engine management (fuel injection/ignition) system, adjustment of the mixture setting (exhaust gas CO level) is possible, but adjustments can only be made by reprogramming the engine management ECU using special electronic test equipment which is connected to the diagnostic connector (see Chapter 4).

39 On all other vehicles, adjustments are not possible. If the idle speed or the exhaust gas CO level is incorrect, there must be a fault in the engine management system, and the vehicle should be taken to a Peugeot dealer for testing (see Chapter 4).


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