Exhaust gas recirculation (EGR)

Exhaust Gas Recirculation (EGR)

The Exhaust Gas Recirculation (EGR) system redirects a small portion of exhaust gases from the exhaust manifold back into the intake manifold (see the diagram) to lower the combustion temperature. The engine computer regulates the flow of the EGR gases by opening or closing the EGR valve. The EGR flow is at its maximum during steady highway cruising. The engine computer periodically tests the operation of the EGR system. If the engine computer detects that the EGR flow is less or greater than expected, it sets the fault code P0400 - Exhaust Gas Recirculation Flow Malfunction.

Function

89 - Exhaust gas recirculation valve B2/5 - Hot film mass air flow sensor B11/4 - Coolant temperature sensor B37 - Accelerator pedal sensor L5 - Crankshaft position sensor M16/6 - Throttle valve actuator N3/10 - ME-SFI [ME] control unit Y31/1 - EGR [ARF] vacuum transducer a - Vacuum supply from intake manifold b - Controlled vacuum from EGR [ARF] vacuum transducer to exhaust gas recirculation valve

The exhaust gas recirculation valve (89) is combined with an electrical EGR pressure transducer (Y31/1).

The EGR pressure transducer (Y31/1) stabilizes the sharp vacuum fluctuations in the intake, and is actuated by the ME-SFI control unit with a PWM signal, which has a frequency of about 10 Hz and a duty cycle of about 30 to 100%.

The EGR valve is actuated with a vacuum of about 80 to 200 mbar, and is opened to varying widths in order to control the quantity of exhaust gases that are recirculated.

The exhaust gases are recirculated in line with the map stored in the ME control unit as soon as the following conditions are met:

• Coolant temperature between 60°C and 110°C
• Engine speed <3500 rpm
• Partial load.

The performance map is designed to optimize fuel consumption. In this case as much exhaust gas as possible is recirculated without impairing engine running or exhaust emissions.

Exhaust gas low in oxygen content is drawn in through the exhaust gas recirculation valve, depending on the cross-section of the valve opening. Consequently, the fuel-air mixture has a lower oxygen concentration. The combustion temperature drops and the formation of NO_X is reduced.

The inducted air mass is reduced by the quantity of exhaust gases recycled. In line with this, the ME-SFI [ME] control unit meters less fuel.

89 - Exhaust gas recirculation valve Y31/1 - EGR [ARF] vacuum transducer a - Vacuum supply from intake manifold b - Controlled vacuum from EGR [ARF] vacuum transducer to exhaust gas recirculation valve c - Air admission

89/1 - Compression spring 89/2 - Vacuum reservoir 89/3 - Diaphragm 89/4 - Valve seat 89/5 - Housing

Position

The EGR [ARF] vacuum transducer is mounted behind the right cylinder head.

M16/6 - Throttle valve actuator Y31/1 - EGR vacuum transducer

Function

A - Compression spring B - Valve plate with diaphragm c - Housing, pressure regulator D - Hose line (guides controlled vacuum of approx. -200 mbar to the electrical unit) e - Coil F - Electromagnetically operated valve unit ATM - Atmospheric pressure, air admission via cylinder head OUT - Controlled vacuum to EGR valve VAC - Vacuum supply from intake manifold

From connection VAC the intake manifold vacuum reaches the pressure regulator, which stabilizes the intake manifold vacuum in line with engine speed and throttle valve position via a control valve to approx. -200 mbar. The controlled vacuum flows further via a hose line to the following electric unit. This unit is actuated by the ME control unit with a pulse width modulation signal (frequency approx. 10 Hz, on/off ratio approx. 30 to 100%) at the ground end. The developing magnetic field influences via the coil anchor the valve unit as follows:

• No signal from control unit: The connections OUT and ATM are linked. The exhaust gas recirculation valve is close (ventilated).
• Signal from control unit: The valve unit opens proportional to the current flow in the coil. The vacuum acts partially or fully at connection OUT and operates the ERF valve accordingly.

Removing exhaust gas recirculation valve

19 - Intake manifold 89 - Exhaust gas recirculation valve 89/1 - Exhaust gas recirculation line 89/2 - Vacuum reservoir 89/3 - Union nut a - Vacuum line from intake pipe b - Vacuum line Y31/1 - ARF vacuum transducer

1 - Oil dipstick guide tube 2 - Fuel rail 3 - Plug connection 89 - Exhaust gas recirculation valve 89/3 - Union nut Y31/1 - ARF vacuum transducer

1. Remove cover on front side of engine
2. Remove air filter housing
3. Remove engine cover with integrated air filter
4. Unplug connector on EGR pressure transducer (Y31/1).
5. Detach vacuum line (b) to vacuum unit (89/2) and EGR pressure transducer (Y31/1)
6. Unscrew union nut (89/3) on exhaust gas recirculation valve (89)
7. Dismount EGR line (89/1) from intake manifold (19)
8. Unscrew exhaust gas recirculation pipe (89) from cylinder head
9. Remove exhaust gas recirculation valve (89). Installation: Replace gaskets. Note length and routing of vacuum line from intake manifold to EGR [ARF] vacuum transducer (Y31/1). Incorrect routing may cause throttle lever to jam.
10. Install in the reverse order

Fault codes

P0400 Exhaust Gas Recirculation Flow Malfunction