2006 VW Passat VR6
Problem presented to the AECS Equipment technical support team.
Client complaining of intermittent low engine power at times. Our lead technician has driven and scanned the vehicle. No fault codes were found in the engine control unit. The technician noted a momentary hesitation/misfire on one of the road tests.
Intermittent problems are our most difficult to resolve. It is always uncertain how much time we spend before even getting any opportunity to do some meaningful diagnostics.
With no fault codes present in our engine control module we really had a difficult problem to get solved. What this will mean is the fault is of course intermittent but also only occurring for such a brief period of time that it is not setting any fault codes in our engine controller. This is often several engine revolutions before fault code setting.
This is a job where a diagnostic scope will be best suited. The workshop had an ATS 2 channel scope. They also had a G force sensor which we suggested they use in this case. We can mount the G force sensor on top of the vehicles engine and monitor the rocking motions from compression and power strokes produced , every and each one of them.
If we also record a reference point such as an ignition coil trigger pulse, we can then identify our firing order and the relationship between the rocking motions from the G force sensor.
Very simply, compression of a cylinder will produce a rocking motion, a power stroke will create the opposite motion, no power stroke ( ie misfire ) , no motion produced, follow your firing order and you can know which cylinder is not operating correctly. With knowing where your misfire is you can then test coils, injectors, EGR valves or other components which could be causing the misfire for correct functioning and zero in on fault.
Launch scan tool health report of VW Passat
The workshop mounted the ATS scope G force sensor to the top of the engine securely.
Often you can wedge this sufficiently well under wiring conduit or use some duct tape.
Next step is to set up your measurement on laptop, utilising the remote trigger button. Then drive the vehicle and hold this in one hand and when the driver detects a misfire, press button to pause the scopes recording and capture the misfire onscreen.
Run the scope leads out of engine bay and into the front passengers seat through window to scope and laptop.
ATS G force sensor with scope laptop after road test and capture of misfire event.
Scope recording showing misfiring captured. The missing upward spike of blue trace in the middle of recording is the misfire. This is random and intermittent making diagnostics difficult, slow and time consuming without a scope. Also expensive for vehicle owner.
After several road tests our technician was able to capture the misfire event. In the above recording there is actually only one lost power stroke of the engine. Blink and you would miss this as it milliseconds of time.
After analysis of the scope recording and following the firing order of the engine, the misfire was traced to cylinder 6. The next step is to measure cylinder 6 ignition coil spark line and injector 6 activation during a misfire to locate which unit is failing.
Short spark duration causing misfire from not all fuel igniting.
Correct ignition event.
One of our two channel scopes leads will measure our injector and the other to the ignition coil 6. After another series of road tests the misfire occured again and was captured. The spark line duration was almost not present and not long enough to burn all the fuel in the cylinder.
Most likely cause for this issue in igntion coils is enamelled insulation breaking down on coil windings causing a short circuit. .
A new ignition coil was installed and vehicle returned to the owner. This difficult to find problem had been clearly located in a reasonable time with a high degree of confidence in the solution.
