TFLCar's Jetta Dyno Test Doesn't Show Us "Test Mode"

Volkswagen is working on a fix to bring their “defeat device” equipped cars back into EPA compliance. The SCR upgrade option is very costly and another possibility is a software fix. The latter option would likely come in the form of an ECM calibration that would work similar to “test mode” at all times, possibly robbing power and fuel economy.

The crew over at TFLCar attempted to emulate this test mode on a dyno but fell short of collecting reliable results.

While I applaud the idea of finding an affected car and trying to generate real-world test results, the tests and analysis in the video do not correlate to an actual EPA test.

The first red flag in the video was the conversation with what appears to be an emissions station technician. He makes a point of mentioning that they have to disengage traction control and that is likely what puts the car into test mode. He throws around a bunch of acronyms including ESP and ASR and then tells us that they disengaged it on the Jetta to do their test. This is not possible as the Mk VI Jetta does not have a button to disable traction control, so they would not have a way to disable it short of retrofitting a button to every Jetta that rolled in.

There is a big focus on running the car on a loaded, 4-wheel drive dyno and how they are able to simulate on-road driving before then disengaging the valve for the rear dyno roller to simulate an EPA test with just the front wheels rolling. The first test completed successfully without any errors and they share the horsepower and torque results along with a set of corrected results. The correction factor is usually put in place to correct for barometric pressure and temperature. As Denver is much higher than sea level, their correction factor causes the figures to go up by a small margin.

They discuss how the corrected results are very close to the numbers stated from the factory but fail to understand or explain that factory figures are measured at the crankshaft while theirs were measured at the wheels. If they were measuring on the exact same device with the exact same variables put into the system, their results should have actually been 10 to 15-percent lower due the loss of power through the drivetrain. Since they do not know what device was used to measure the power at the factory or what factors were involved the comparison to factory, figures would be meaningless anyway.

Once the first run was completed successfully, the technician disengages the rear roller and tells us that he is going into two-wheel-drive mode. The technician was likely trying to communicate the change on the dyno, but the message comes across as if he has made a change to the Jetta which is only available in front-wheel drive. The test initially fails as the car goes into limp mode due to a missing signal from the rear speed sensors. Eventually, errors pop up on dash showing that ABS, stability, and traction control have issues.

The technician makes the incorrect assumption that this must mean that the car has switched into the “test mode” and proceeds to run the test in a single gear. It is very unlikely that the car switched into test mode as it requires a strict set of variables to be set, such as steering angle and barometric pressure, which cannot be met easily in such a setting.

The test mode also requires running one of the EPA test cycles, such as the FTP-75 test, in order to engage the test mode. This tests requires starting from a cold engine and then running the car for 30 minutes in various gears and speeds to simulate an urban driving cycle. Since TFLCar’s test was ran in a single gear, it never approached anything mimicking the EPA test.

The second test did show a drop in horsepower and torque, but that can easily be explained by the ECM programming and dyno configuration. Since the second test was ran with errors for ABS, stability, and traction control, it is very likely that a calibration exists within the ECM to limit torque in such a condition in order to make the car safer.

The dyno can also play a role in skewing the results since the load on the dyno is changed when the rear roller is disengaged. Once the load factor is changed, torque will drop, especially as the car starts moving in the lower end of the RPM range.

The best way to think about the load change is to think about a car in neutral and how the engine is able to rev and seemingly not produce a lot of torque since there is no load on it. Once the engine is in gear and loaded, it produces more torque. These changes can be measured by running a car in various gears and seeing the change in the output numbers. The load on the engine will change as the gear ratios change and produce varying torque results.

Power and torque are going to drop if the fix from Volkswagen involves a reflash that is similar to the test mode calibration, but the only way to get an accurate result of exactly how steep those drops will be is to perform the test on a car before it is reflashed and then run the test again with the same exact variables and settings once the new calibration is in place.