EEC Technical Trainer Stuart Still investigates a VW Passat 2.0D – Black Smoke Emitted and Misfire on Idle

I was recently presented with a VW Passat 2.0 D that had 225K miles on the clock and was fitted with a Bosch EDC 1.5 system. The main symptoms were hesitation and misfire on idle and I was informed that both the turbo and EGR valve had been replaced six months previous.

Armed with those facts my first course of action was to check things over with the scan tool and look for any DTCs. I found none that were of any interest so I then took the car for a test drive. In this instance I decided to leave my scan tool connected, and I didn’t have to drive too far to notice that the performance was very poor with a lot of black smoke emitted at the same time.
Back in the workshop I started my investigation, with a few things to take into account. Firstly the engine was misfiring on idle, but when on a normal running temperature it appeared to have few issues.

The clouds of black smoke and hesitation under load pointed to a potential injector issue so I opted to carry out a quick back leak test. The results of the test concluded that the injectors weren’t the cause of the problem as the return flow was well within the specified limit on both low and fast idle.

At this point the engine had been running for around 30 minutes and no misfires had taken place. I did, however, notice a smell around the engine bay and, with all other options exhausted, it was time to consider a
different approach.

The systems on this particular car were very similar to a Vauxhall Corsa 1.3 D that had previously visited my workshop. Using this experience I removed the inlet pipes and found that the EGR valve had small passages
(these engines clog up very easily at low mileage (below 20,000) so be on your guard). This helped me to finally work out what the problem was: the intake system was contaminated with carbon and debris.