Road Testing

The Road Test is a important part of the diagnostic process and is much more that just a quick drive up the road.
The diagnostic road test is a complex procedure where the vehicle is driven in a manner to exercise the vehicle systems that are causing concerns.
A simple drive up the road will inform the driver than the vehicle can be driven and gear changes etc are possible, but unless the system under review is fully exercised it is not really possible to be sure that that system is working correctly and reliably.
So, there are a number of important questions to be considered when road testing and these will differ depending on whether you are road testing to validate a repair or road testing as part of the fault investigation process.

Fault investigation road test.
Often there are a number of theoretical causes of a vehicle problem, and a good road test procedure will narrowed down the list to 1 or 2 suspect systems. Also it is remarkable common to have more than one fault on a vehicle at the same time; it may appear from driving that there is ‘a problem’ but when the diagnostic process is complete and the vehicle rectified there were 2 issues which combined cause the problem.
A good starting point is to list down the possible causes of the fault and then plan a drive cycle that both exercised these systems, ideally individually.
For example, a diesel fuelled turbo charged vehicle (turbo with electronic boost control) has a loss of power above 60 – 70 mph, but drives well at lower speeds.
Possible systems at fault could be: Injected fuel quantity, Injection timing, Boost pressure control, Road speed input, Cruise control, EGR system.
The road test should try to eliminate as many of these possible causes as possible by changing the way the vehicle is being driven; light throttle cruising in top gear, wide open throttle is lower gears (maybe 3rd), down hill in overrun condition.

Conformation of fault rectification road test.
When fault finding and rectifying vehicle system faults regular considered road testing is important for a number of reasons:
A) Many modern vehicle systems have adaptive learning; a system where the control module ‘learns’ the characteristics of the vehicle. Adaptive learning is designed to tune out the tolerances and component wear of complex mechanical and electrical systems. Adaptive learning is a slow process designed to slowly and constantly change the ‘tune’ of the system to achieve the best operating condition. If a fault occurs then adaptive learning may well react to this condition and try to ‘tune’ to the faulty condition. If the fault is recertified the system needs to relearn the new condition, this may require numerous drive cycles or a system reset ( often but not always battery disconnect).
B) System clear out or Italian Tuning. This is hard to explain but it is when the mechanical / electrical systems are just in need of a clear out.
This was common with older sports cars when the owner complained that the car wasn’t running correctly with lumpy idle and mis-fires. A spirited drive using the full rev range would offend restore the vehicle to tip top condition.
Modern vehicle especially those with faults can get into a bit of  ‘a self fore-filling prophesy’ where measured system data causes a change in the system which then causes abnormal running conditions. Running the vehicle for pre-longed period in a faulty condition can cause it to ‘choked-up’ to a point where it is hard to clear even after the fault has been rectified.