The difference between an efficient/profitable driveability diagnostician versus one who may struggle a bit can take a few different turns. It is typically a matter of data misinterpretation, lack of proper tooling, or (most likely) not testing under the correct operating conditions, that separate one from being successful and confident in their abilities.
The first two differences are not too difficult to rectify but if the diagnostician isn’t gathering the data under the correct operating conditions, he or she is throwing away a tremendous amount of valuable diagnostic information.
Driveability complaints from the gasoline internal combustion engine stem from one of three generalized areas:
- Engine mechanical integrity/breathability issues
- Improper air/fuel ratio
- Inadequate or improperly timed ignition discharge
In order for proper engine performance to occur the engine must breathe air efficiently, and the engine’s computer must measure that incoming air accurately to deliver the appropriate amount of fuel (to match the incoming air at the proper ratio). With that, different faults will show up under different engine operating conditions.
A successful driveability diagnostician will possess the tools and skill set to swiftly and easily capture the appropriate data under the correct operating conditions. This will help rule out what cannot be wrong with the vehicle, leaving only the true potential faults to further investigate. Placing the vehicle under different operating conditions will help flush the fault to the surface.
In this video, Motor Age Technical Editor, Brandon Steckler creates a simple vacuum leak fault on a V8 engine to cause a fuel trim correction for a lean condition. Using an OBD2 scan tool to capture graphed global OBD2 data parameter identifiers (PIDs), Brandon demonstrates that a simple change in throttle angle/engine rpm yields a demand for fuel and eliminates the need for fuel trim.
The engine’s pistons carry out their induction strokes to generate manifold vacuum. As the pistons pump air across the restriction of the almost-closed throttle plate they displace more air than the throttle plate will allow. Because an engine vacuum leak is only truly a problem when engine vacuum is present, eliminating the intake manifold vacuum (by opening the throttle) will also eliminate the effect the vacuum leak had when the throttle plate was closed, and intake manifold vacuum was strong. Fuel trim (the corrective factor) will reflect the correction at idle (closed throttle/strong intake manifold vacuum) and eliminate the correction under loaded conditions (open throttle/minimum intake manifold vacuum).
Once a technician is comfortable with capturing the data using the proper tools and understands what story the data is telling, a preliminary diagnosis of many driveability problems becomes simplified and efficient. This, of course, leads to confidence and an enjoyable day in the workshop.
Tune in to this episode of The Trainer and learn what the Analytical Road Test can do for you and just how simple this process can be!
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