Piston Slap: A simple vacuum leak for a complex Audi?

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Erin writes:

My 2010 Audi A5 2.0t (108,000 miles) is having some issues; my boyfriend is a car guy and has been working on this for a while and cannot figure it out. We would appreciate any help on this! It intermittently misfires (especially under load); there is a ticking coming from the VVT sensors, followed by multiple misfire. The car will also intermittently run perfectly, and the turbo seems to have plenty of pull. I can drive through misfire in sport mode (once the turbo builds boost). The catalyst air temp is averaging 1500 degrees during driving. Any ideas?

We have replaced:

  1. Evap canister purge valve
  2. PCV Valve
  3. Spark Plugs (NGK double platinum)
  4. Coil Packs
  5. Air filter
  6. MAF sensor (used but tested)
  7. CAM position sensor
  8. Fuel Lines

Here are the codes triggered:

  1. P0102 Mass or volume air flow circuit low input
  2. P0101 Mass or volume air flow circuit range/performance
  3. P0108 Manifold absolute pressure/barometric pressure cut it high input
  4. P0104 Mass or volume air flow circuit intermittent
  5. P0110 Intake Air Temperature sensor circuit 1

Sajeev answers: 

As much I’m terrified of repairing depreciated German vehicles from the current millennium, I reckon this is an easy diagnosis: check for vacuum leaks and replace anything that fails the test.

A vacuum leak is the likely culprit only because such a leak allows extra air (i.e. unmetered/unaccounted for by the MAF sensor) into the intake manifold. That air is then sucked into the engine, and the subsequent dilution on all cylinders (or bank of cylinders on a “V” shaped engine) confuses the heck outta the computer. That said, the easiest way to check for vacuum leaks is the aforementioned smoke test, but you might get lucky and only need a smartphone-enabled endoscope to find an errant hole in the system.

Then again, this is a 10-year-old Volkswagen Audi Gesellschaft product, so complexity reigns supreme. I reckon getting a smoke tester is the easiest way to find the source of your vacuum leak. Here’s a great video showing how it’d work on a 1.8T Volkswagen, but the same principles apply to your Audi.

I’m keeping my fingers crossed that the smoke will show up quickly from an easy-to-replace location. Best of luck!

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