Low Boost Pressure

Low boost pressure


Fault symptom

- Weak engine.

- The turbo/APC meter shows a low reading, compare with "Turbocharger, Specifications". Specifications


Conditions

No diagnostic trouble codes.


Diagnostic procedure




1 Investigate the cause of the fault
The following checking procedure will determine whether the fault is a control fault or a basic engine / turbo fault:
- Disconnect the air hose marked "W" from the boost pressure control valve and check that it is connected to the wastegate.
- Drive the car. When accelerating at wide open throttle from a low engine speed the pressure switch should open before 3000 rpm.

Does the pressure switch open?
YES Continue with point 2.
NO Check and if necessary rectify the following points:
- leakage in the intake system between turbo and throttle body (e.g. in the charge air bypass valve).
- clogged air cleaner
- turbo unit fault
- incorrectly adjusted basic boost pressure.
- fault in basic engine
- clogged exhaust system
- clogged three way catalytic converter.
Then continue with point 17.




2 Check the brake light switch
- Ignition switch in the ON position
- Depress the brake pedal.

Do the brake lights work?
YES Continue with point 3.
NO Rectify the fault.

3 Check the Cruise Control
- Check as described in "Cruise Control, turbo".

Result OK?
YES Continue with point 4.
NO Rectify the fault.




4 Check the knock sensor's signal level
- Connect the ISAT Scan Tool.
- Select command code 329 (-M90) or 229 (M91-) with the ISAT scan tool.
- With the engine switched off, the ISAT scan tool display should show 80000 (sensor voltage is 0 V).
- With the engine idling, the ISAT scan tool should show 80001 (sensor voltage is 0.1 V).
- When the engine is rewed up the ISAT scan tool should show a slightly higher reading, such as "80003" (0.3 V).
- Drive the car and obtain a signal level reading from the knock sensor when boost pressure is adjusted downwards (reduced).
If the signal level increases violently at the moment boost pressure is adjusted downwards, knocking will occur.

Does the signal level Increase violently when boost pressure is adjusted downwards?
YES Continue with point 4.
NO Continue with point 6. If the signal level does not change appreciably, no knocking will occurs.




Note:
-M90 cars
Using a BOB it is possible to ascertain whether or not knocking occurs.
With the BOB connected, connect the test lamp to pin 3 and ground (such as pin 13). If knocking occurs the lamp will start to flash.

Cars with LH 2.4.2
Using the ISAT scan tool it is possible to ascertain whether or not knocking occurs.
Connect the ISAT scan tool and contact LH, system number 1. Select command code 206 with the scan tool and check whether the display shows the following:
8B106 enrichment-knocking
8B006 enrichment-pre-ignition
Normally the signal alternates between 8B106 and 8B006 when there is no knocking or pre-ignition.




5 Check for possible causes of the fault
Check the following:
- Noise from the tappet.
- Loose or cracked brackets on the engine.
- Poor grade of fuel.
- Poor fuel flow, see "Fuel Pump". Testing and Inspection
- Oil coating from the crankcase. Check the non-return valve in the hose between the camshaft cover and intake manifold. Oil in the cylinder can give rise to knocking.
- Noise from the oil reducing valve.
- Noise from the timing chain.

Any of the above faults manifested?
YES Rectify any faults and continue with point 17.
NO Continue with point 6.

6 Check the knock sensor
- Check the knock sensor's connector for corrosion, etc..
- Check that the knock sensor is correctly mounted and securely tightened.
- Check that the knock sensor's screened lead is run correctly and not pinched or damaged.

Are the knock sensor and leads OK?
YES Continue with point 7.
NO Rectify any faults and then continue with point 17.




7 Check fuse 5

Is the fuse OK?
YES Continue with point 8.
NO Change the fuse and continue with point 10.

8 Check the air hoses
- Check the valve's three control hoses, making sure that they are in good condition and correctly connected.

Are the air hoses OK?
YES Continue with point 9.
NO Rectify any faults and then continue with point 17.

9 Check the power supply
- Ignition switch in the ON position.
- Check that the valve is supplied with power by connecting test lamp 86 11 857 to pin 2 of the valve's connector and a good grounding point.

Did the test lamp light up?
YES Continue with point 11.
NO Check and if necessary repair or replace the lead between pin 2 of the connector and fuse 5. The continue with point 17.




10 Check the resistance of the valve
- Take resistance readings in the valve's connector across:
- pins 1 and 2
- pins 2 and 3.
- Nominal resistance is 3±1 Ohm in both cases.

Are the readings OK?
YES Continue with point 11.
NO Change the boost pressure control valve and then continue with point 17.

Note: If there is a short circuit in any winding the control module is probably also damaged. If a new valve is then fitted, it will be automatically damaged beyond repair. In the event of a short circuit the control module must therefore also be changed. If necessary, also change fuse 5.

11 Check the lead connected to pin 1 of the valve
- Ignition switch in the ON position.
- Plug in the valve's connector. A weak buzzing sound should be heard.

Can a sound be heard?
YES Continue with point 12.
NO Check the lead between pin 1 of the valve's connector and pin 10 of the control module. If the lead is OK, continue with point 13.




12 Check the lead connected to pin 3 of the valve
- Unplug the valve's 3-pin connector.
- Ignition switch in the ON position.
- Connect test lamp 86 11 857 to pins 2 and 3 of the connector.
The test lamp should not light up.
- Connect the ISAT scan tool and select command code 451.
The test lamp should light up.

Did the test lamp light up?
YES Proceed to point 13.
NO Check the lead between pin 3 of the valve's connector and pin 33 of the control module.




13 Check functionality
- Try fitting a replacement valve.

Result OK?
YES Change the boost pressure control valve and then continue with point 17.
NO Continue with point 14.

Note:
- If the boost pressure control valve sticks or if it is suspected that the valve or connecting nipples, hose fittings or throttle body are coated with oil, this may come from the crankcase breather.
- If the coating of oil is fairly thick, the camshaft cover can be changed for a later version incorporating an oil trap of larger capacity (introduced in production M93).
- Check that the non-return valve in the thin hose between the camshaft cover's crankcase breather nipple and the intake manifold is in perfect working order.
- Check that the constrictions in the crankcase nipple are open.
- If no fault can be found or the amount of oil is extremely small, then in connection with replacement of the boost pressure control valve it can be rotated through 90 degrees so that the valve's nipples and hoses point downwards.
Proceed as follows:
- Remove the retaining bracket's rivets and rotate the bracket through 90 degrees. Bend the bracket slightly outwards so that the hoses are clear of the fan cowl.
- Insert a sheet of metal between the cowl and the radiator to avoid damaging the radiator core when drilling holes. Then drill a new hole in the fan cowl and secure the bracket in its new position by means of pop rivets.




14 Check the wide open throttle position of the throttle position switch (not LH 2.4.2 or ETS)
- Engine running at idling speed.
- Select command code 304 (-M90) or 204 (M91-) with the ISAT scan tool.
- Check the wide open throttle input signal on control module pin 8.
- On selection of command code 304 (-M90) the ISAT scan tool display should show:
- in the idling/half-throttle position 0 V = 8B100
- in the wide open throttle position 12 V = 8B000.
- On selection of command code 204 (M91-) the ISAT scan tool display should show:
- in the idling/halt-throttle position 12 V = 80000
- in the wide open throttle position 0 V = 80100.

Important: If trouble is experienced in obtaining the wide open throttle position, check that the floor carpet is not obstructing the accelerator. Press hard down on the pedal.

Is the signal OK?
YES Continue with point 15.
NO Check and if necessary rectify or adjust the lead in question and the wide open throttle switch. Then continue with point 17.







15 Check pressure sensor operation
If the pressure sensor does not provide the right voltage across the entire pressure range it may give rise to incorrect boost pressure. Check the performance of the pressure sensor.
- Connect the ISAT Scan Tool.
- Ignition switch in the ON position.
- Select command 320 with the ISAT scan tool.
- The ISAT scan tool display should show 80022. The value is dependent on the pressure.
- First build up pressure and then a vacuum according to the table. Use a pressure gauge and a pressure/vacuum pump.
- The ISAT scan tool reading should be as indicated in the table Note that the scan tool may show lower readings at high altitudes.

Are the readings OK?
YES Continue with point 16.
NO Change the pressure sensor and then continue with point 17.

16 Check/adjust basic boost pressure

Check and if necessary adjust the basic boost pressure, see "Check basic boost pressure on the road". Testing and Inspection

Continue with point 17.

17 Carry out basic boost pressure adaptation

When boost pressure has been rectified, adaptation of the system should be carried out, see "Adaptation of the Turbocharger System". Service and Repair