Visual inspection (Audi 100 C4)

A leak in the intake system area allows access to unaccounted air, i.e. air that is not taken into account in the calculations of the control unit and therefore significantly interferes with the preparation of the mixture. The mixture is uncontrollably leaned. This malfunction is most clearly manifested in an unstable rotation speed in idle mode. But this same malfunction can manifest itself in a ringing noise of the engine operating at full load.

Check vacuum hoses for tightness and cracks. All hoses on the fuel mixture preparation unit or intake manifold (brake booster, incoming air preheating, fuel evaporation system) must be verified.

Warm up the engine and let it idle, open the hood.

Spray with a spray bottle containing fuel spray (eg "Startpilot"), a block for preparing a combustible mixture near the flange to the exhaust manifold, sealing the flanges of the inlet channels and pipelines to units controlled by vacuum (with the lambda probe connector disconnected).

If the engine speed changes when spraying a certain area, there is a leak there.

Is it possible to recognize a fuel line leak?

Have the wire ends been disconnected and reconnected multiple times? Corrosion or poor handling?

Carefully examine the plugs of the individual elements of the injection system. Do not bend the contacts, but only treat them with spray.

(Content source: the specified website «AudiManual»)