One of our longtime customers brought in his 2007 Audi S6 Quattro 5.2-liter V10, saying that the check engine light was on and that the battery was discharging after two weeks. Besides the customer’s statement that he had to jump-start the car, he did not notice any other problems. He dropped the car off and we got to work.
The first thing we did was test drive the car to confirm customer complaints. The test drive did not last long because the car was misfiring badly. After getting the car back to the shop, we connected it to the scanner. The first code we pulled was a P0306 misfire in cylinder No. 6. We swapped the No. 6 coil with the No. 7 coil. After starting the engine, the misfire stayed in cylinder No. 6. We then replaced the No. 6 spark plug. The misfire stayed in cylinder No. 6. We checked the compression in cylinder No. 6. Compression in Cylinder No. 6 is 205 pounds per square inch (PSI). We knew the problem was not with the coils or spark plugs (secondary ignition). We knew that we had good compression, so the mechanical side of the engine was OK. The problem had to be with the fuel system. The next thing we had to do was check the No. 6 injector. The problem was that the injectors are under the intake, and replacing them is extremely expensive.
We still had to solve the battery discharge problem. It did not make sense to repair the misfire problem before solving the parasitic draw issues. We ran multiple tests to see if there were any codes pertaining to a parasitic draw. The only issue we found was that the battery was at the end of its life. Before testing the battery, we made sure it was fully charged. After charging the battery, we performed a carbon pile test. The battery tested at 9.2 volts at 300 mps. Minimum voltage is 9.4v to 9.6v, so we had to replace the battery.
After battery replacement, we performed a parasitic draw test. We found that the car had an intermittent 300 milliamp draw. Acceptable draw is just 80 mil amps. We then began consulting the wiring diagrams and started disconnecting power supplies from three different fuse blocks. Since the three different fuse blocks converge at multiple power grids (bus), we could not see a drop in power.
We then started disconnecting modules one by one to try to isolate the problem. One thing we found was that if the car was locked, it had a draw all the time, but if it was unlocked, the draw was intermittent. Eventually, we got to the power seat module under the driver’s seat. Once we disconnected it, the draw was gone. We made sure that the power, ground and signal to the seat module were OK. We then replaced the seat module and the draw was gone.
The interesting thing was that after replacing the battery, seat module, program module, the clear codes, and test driving the car, the misfire was gone. We then drove the car over 100 miles to confirm it. We think the engine control module was resetting every time the battery was going dead and causing a problem with the injectors. The other interesting correlation was the door locks and the seat module. On a car with keyless entry, multiple components activate as soon as the car sees the key getting close to the car (seat position, steering wheel position, fuel pump activation, etc.). Either way, the customer understands that the misfire could return, but hopefully it won’t.
Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call (650) 960- 6988, email [email protected] or visit sunnyvaleforeigncar.com.
