Volkswagen Workshop Service and Repair Manuals

To reduce the amount of CO and HC in the exhaust gas, air is blown behind the exhaust valves into the cylinder head through an air pump.

At low coolant temperatures, the supply of air is interrupted by a thermopneumatic valve and at high engine speeds by the control valve.

In overrun, air is routed via the anti-backfire valve into the intake ducts. Backfiring is thus prevented.

Checking air pump

• ‒ → Pull hose -arrow- off check valve. Seal off the valve.
• ‒ Start engine and let it idle.
  The system is OK if air flows out of the hose at coolant temperatures in excess of 50 °C (testing is not possible at temperatures between 12 °C and 50 °C).

Checking control valve

• ‒ Pull vacuum hose off control valve.
• ‒ Pull vacuum hose off retard unit on distributor and connect it to the now free connection of the control valve.

• ‒ Run engine at slightly higher speed. The system is OK if air emerges from the control-valve hose leading into the open air.

Checking thermopneumatic valve

• ‒ → Remove thermopneumatic valve -arrow- and heat/cool in water bath.
• ‒ Blow throught valve

• At temperatures
• ‒ below + 12 °C it must be open
• ‒ above + 50 °C it must be closed

Checking anti-backfire valve

• ‒ → Pull off hose at anti-backfire valve -arrow-.
• ‒ Run engine at idle and briefly increase speed by opening throttle.
• ‒ Abruptly close throttle. There should be noticable suction at the connection -arrow- for some 1...3 seconds.

Note:

There is a 5.5 mm dia. restrictor in the connection for the anti-backfire valve at the intake manifold.