I began to notice on my Audi A6 C7 that at a cold start the revs jump, the engine is twitching, while there are no errors codes, although usually with such a problem, P0171 fuel trim system too lean appears / the mixture is too poor. 5-10 minutes pass and the engine becomes smooth. The engine was running, but nothing appeared in the ignition pass counters. With a hot start - no problems.
As it turned out, with the help of diagnostics, the reason was a lean fuel mixture at idle due to air leakage in the intake, the values reached up to +25% correction of the fuel trim mixture. I sprayed "quick start" aerosol on various engine parts and at the same time looked at the graph of short-term fuel adaptation, when the aerosol gets to the place of air leakage, the graph drops to negative values (rich mixture).
In this way, I found air suction through the gasket of the temperature sensor and in the connection of the ventilation pipes of the EVAP fuel tank. After compacting these problem areas, the cold engine now does not shake and the adaptation of the mixture after starting has become -2%.
In VCDS Vag-Com, you need to look for Short / Long term adaptation of mixture parameters.
It is necessary to start checking the engine for air leakage after the oxygen sensors (lambdas) and catalysts have warmed up. With a cold temperature sensor, the short-term adaptation will be 0%, when the values appear, you can spray on the intake collector and follow the schedule.
Popular places of air suction on 1.8/2.0 TFSI engines: timing chain cover, crankshaft seals, crankcase ventilation valve PCV, oil separator gasket, intake manifold geometry control axis, oil dipstick and tube, vacuum pump fitting.
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Timecodes:
0:00 Start
0:50 Symptoms of the problem, lean mixture
1:15 Ways to search for air leakage
1:53 About fuel adaptation
5:20 Example of finding an air leak with correction graphs
6:59 Popular places of air leakage
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