Every few years, a customer brings in a car that drives me crazy. We worked on such a car in May – a 2007 Mini Cooper S.
The car failed an emission test because the onboard monitors were not completed. During a repair to the cooling system in April, we had to disconnect the battery. If the battery is disconnected or if the Digital Motor Electronics (DME) memory has to be cleared, all of the DME onboard monitors will reset.
In the 1970s, car manufacturers started controlling the powertrain with computers not only to improve fuel economy, but also to control excess emissions. As time went on, these onboard control systems became more sophisticated. By January 1996, all U.S. vehicles had to switch to the On-Board Diagnostic II (OBD-II) monitoring system. There were many reasons why OBD-II was implemented, such as uniformity across all brands, fuel control, complete engine data streaming, fault code detection, freeze frame data, powertrain self-testing and onboard monitoring.
Each of these systems is a candidate for its own article, but I want to focus on on-board monitoring. Every time you start a car, the engine control module begins to run a group of self-tests, also called monitors. These monitors test the ignition system, fuel system, fuel evaporative system, secondary air system, engine, timing system and exhaust system. These tests will run while the car is idling, driving and after it’s turned off. Certain criteria must be met for the engine control module either to pass or fail the monitor. If the monitor fails, it triggers a code in the computer.
For example, the catalytic converter monitor and oxygen sensor monitor cannot run completely until the engine and exhaust systems are at full operating temperature. This test also must run for a certain duration and usually while the car is moving between 55-65 mph for at least 10 minutes. If any of the criteria (temperature, speed, engine data or duration) are not met, the monitor will not complete.
Checking the monitors
Now back to that Mini Cooper. After connecting to the MINI factory scanner, we checked for diagnostic codes. There weren’t any. We then checked the monitors and found that the evaporative monitor and oxygen sensor monitor had not passed.
However, it is not uncommon for a monitor to fail the first time. The amount of fuel in the tank plays a part in this: It must be more than a quarter full but not more than three-quarters full. Evaporative monitors are notoriously more difficult to run because the test does not finish until the car is turned off and sits for several hours. The oxygen sensor monitor usually runs and completes during the drive cycles.
It took us approximately three days of drive cycles before the evaporative monitor passed, yet the oxygen sensor monitor did not. So we went back and checked all the basics. During an earlier repair, we updated the DME software. We tested each oxygen sensor and found information that revealed that the oxygen sensor monitor will stall if the DME locks up. We then performed a complete battery reset on the entire system and reset adaptations on the oxygen sensors. After running several more drive cycles, the oxygen sensor would not reset.
As I mentioned earlier, certain criteria must be met before a monitor can be completed – including engine timing. These days, we rarely check physical engine timing because it’s part of the data stream. This Mini doesn’t even have any timing marks. The only way to check engine time is to remove the valve cover and look at the cam shaft alignment.
We looked at the data stream, and the timing was correct. So I called the customer and got authorization to pull the valve cover. Once the valve cover was off, the problem was right in front of us. The main upper-timing chain guide had broken, causing the timing chain to be off by one-half of a tooth. If the timing on this Mini is even one-half tooth off, the computer cannot calculate idle correctly. Even though the idle did not have a problem or the data stream did not see a problem in time, the monitor test was so exact that it kept stopping the test.
We replaced the the upper-timing chain guide and ran the monitor test. It passed.
The one thing I love about this business is that we are always learning. As cars become more complex, we must do our best to stay ahead of the technology – but we can never forget the basics.