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Abundant sunshine. High 88F. Winds NNW at 10 to 15 mph..

Clear skies. Low 61F. Winds NE at 5 to 10 mph.

We have to tackle difficult problems at the shop all the time. Due to our experience and expertise, we usually take a logical approach to every test plan.

There are times, however, when even the most logical approach doesn’t solve the problem.

One of my longtime customers recently brought in his 2012 Audi Q5 2.0 liter with the check-engine light on. The saga with this car began in 2021, when we pulled code P2177 – lean off idle intermittent. This code indicates that when the vehicle accelerates from a stop, there is not enough fuel getting into the combustion chamber.

Most commonly, a lean code on a VW or Audi means there is an air leak somewhere in the intake system. These air leaks can be external or internal. An example of an external air leak is a broken vacuum hose, broken intake air boot or even an intake gasket. An example of an internal air leak is a broken breather diagram or a stuck open-air control valve.

The first thing we did was make sure all the intake air hoses were attached, and then we smoke-tested the intake. After the test, we could not find an external air leak. With the engine running, we closely monitored the fuel trims.

During our test-drives, the car was driving normally and not exhibiting any type of problems, and the fuel trims were within specification.

We know that VW 2.0-liter direct injected engines have a major carbon buildup problem, yet that usually causes a misfire code. So, we recommended to the customer that we add a fuel additive, clear the codes and test-drive the car farther. We drove it 50 miles with no problems. We explained that because we could not find the reason for the lean condition, it might be better for the customer to pick up the car and drive it until the code came back or there was a drivability issue.

We then did not see the car again until July. The customer had driven the car approximately 7,000 miles before the check-engine light went on again with the same code (P2177).

Again, we checked for external and internal vacuum leaks. This time, we found that the engine oil separator (breather valve) had a bad internal vacuum leak. We checked the breather vacuum at the oil filler cap; it was off the charts.

We replaced the oil separator, cleared codes and test-drove the car. After approximately 45 miles of driving, the lean code came back – P2177. We double checked the breather, yet it tested fine and the breather vacuum at the oil filler cap was now normal. We triple checked the intake system work, and for sure the intake was sealed.

Next, we tested the fuel pressure from the electric fuel pump and from the mechanical fuel pump. Both were good and in specification. We had to test the fuel pressure because if it were too low, it would fool the computer into thinking the fuel mixture was lean. We then moved onto the fuel pressure sensor and oxygen sensors. All tested within specification and were operating normally.

At that point, the most logical culprit was the fuel injectors. It was possible that the fuel injectors were not atomizing the fuel properly and causing the car to go lean. We got the OK to remove the intake manifold and clean or replace the injectors. Once the intake was off, we found a massive amount of carbon on the valves and injectors. We used a walnut shell bead blaster to clean the carbon and replaced all four fuel injectors.

We test-drove the car close to 100 miles and everything was perfect.

Now we know that these VW and Audi 2.0 engines are prone to carbon buildup, even if they rarely set off this type of lean code. The direct-injected motors were introduced by VW/Audi in 2006. We have seen carbon affect these motors in many ways, but never this way. It is exciting to solve these types of problems and in turn we learn so much.

Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call (650) 960-6988, email sfcsmv@gmail.com or visit sunnyvaleforeigncar.com.

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