Non-intrusive testing

Want to get stuck into a diagnostic investigation without sticking anything in? Karl Weaver shows you how...

Published:  21 September, 2018

As technicians we’re all expected to be able to diagnose a fault within a sensible timescale, for a reasonable price, then guarantee the fix. With correct training, information and tools this is possible. However, we are often faced with multiple faults where cause and effect may not always be straightforward. We can be in a situation where we need to rectify faults before we can move on to the next. Also, if the repair cost could outweigh the vehicle’s value or customer budget then great care must be taken explaining the situation, agreeing a starting fee and preparing and executing a successful diagnostic plan.

Recently we were presented with a BMW X3 for poor performance and a suspected DPF fault. After interrogating the customer we gathered all necessary information. Initial diagnosis confirmed multiple fault codes and a blocked DPF. Determining what caused the DPF to block is vital for the correct diagnosis and preventing reoccurrence. We created a test plan to test each fault and separated them into faults that affect the performance, faults that can cause the DPF to block or prevent regeneration and ones that don’t. In order to fully test the vehicle we would need to clean the DPF first as the exhaust back pressure was so high, the vehicle was barely drivable. As a member of the DPF Doctor network we have a very successful method of cleaning the soot from the DPF without the need for removal and access to many manufacturer-specific tips with DPF faults. The information and knowledge within the DPF Doctor network has proved to be invaluable and has given us an outstanding success rate. With our test plan ready we were able to calculate a sensible labour figure to conduct the tests required. The customer authorised the labour and the DPF clean.

Several faults were straightforward. A multimeter gave us conclusive results and made it easy to quote for replacement parts and labour time to fit them. The main fault causing poor performance required a little more thought to keep diagnosis time to a minimum. A low boost pressure fault code doesn’t tell us why the pressure is low. Driving the vehicle whilst monitoring the boost pressure showed the fault was intermittent, so an external boost leak was unlikely. A smoke test was also carried out which revealed no leaks. In this instance, the EGR valve could be a likely culprit. This engine uses a vacuum controlled EGR valve with a position sensor built into the diaphragm. As tempting as it was to unbolt it and take a look, this would all take more time then factor in the risk of rusted bolts etc. With a position sensor one would think if the valve was to stick then a fault code would be set. We had to plan a simple, conclusive, yet non-intrusive way of testing the EGR system quickly.

The conventional vacuum controlled EGR system consists of the EGR valve which includes the diaphragm with a 5 Volt position sensor and the vacuum control solenoid valve which uses vacuum from the brake servo vacuum pump and is controlled by the ECU on a duty cycle. The position sensor will typically show 0.5 to 1.2 Volts when fully closed and 3.9 to 4.5 Volts when fully open. One side of the solenoid valve has a 12 Volt (battery Voltage) supply and the ECU switches the ground path on and off at varying duties to vary the vacuum amount thus varying the EGR valve position. The ECU looks at the position of the valve and adjusts the duty to achieve the position desired similar to how an ECU uses the oxygen sensor to adjust the air/fuel ratio. With the following tests we were able to check every component in the system.

Test one
We connected the Mityvac directly to the EGR valve and the oscilloscope connected between the signal wire and battery ground. As we had already smoke tested the entire inlet system we connected the smoke machine directly to the inlet manifold in place of the intercooler hose. With the smoke machine running and the ignition on (engine off) we used the Mityvac to fully the valve to check it had no vacuum leaks (split diaphragm), then we opened and closed the valve slowly and then quickly. This confirmed the following:

  •  The diaphragm was intact and had no leaks
  •  Monitoring the Voltage showed the fully open and closed readings were both consistent and its movement was smooth. An oscilloscope would show this clearer but was unnecessary at this stage
  •  Observing the smoke machine flow and pressure gauges proved the valve was sealing when closed

Test two
With the engine running, using a T-Piece and a vacuum gauge (the gauge on the Mityvac will suffice) we checked the supply vacuum to the solenoid valve. Remember – a leak in any other vacuum controlled system can affect this and it may not always reveal itself if unless that system is activated. In our case we were suffering low boost pressure so lack of vacuum was unlikely. With this test we vent the system several times to check that it recovers quickly.

Test three
With everything reconnected, with a T-piece we connected a vacuum gauge between the solenoid valve and the EGR valve to monitor vacuum. We then connected the oscilloscope using two channels, the first monitoring the duty cycle and the second monitoring the position signal. Lastly we had the scan tool connected to monitor EGR and Mass Airflow from a serial data angle. This test shows all three vital pieces of information while road testing the vehicle: ECU command to solenoid, solenoid control of vacuum and actual EGR position. I highly recommend whenever you have the chance to connect a vacuum gauge to an EGR control or turbo control system on a good vehicle and observe how it responds. As the command changes, the needle should always move quickly.

During the road test we were cruising at a steady speed and observed the EGR commanded and open at around 50%. Under sudden acceleration we would expect the ECU to command the EGR valve closed in order to achieve maximum boost pressure and full oxygen to the cylinders. With that the duty should decrease to a minimum to release the vacuum and close the EGR valve. At this point the vehicle suffered a severe hesitation. The duty had returned to minimum but the EGR valve position Voltage showed it still half open but slowly dropping and the vacuum gauge also showed the vacuum still high and slowly dropping. That conclusively told us that at certain times the solenoid valve was struggling to release vacuum. So there we have it. An EGR valve open when it shouldn’t be fits with all the symptoms described: Loss of power, low boost pressure and excessive soot loading of the DPF.

Further inspection
Further inspection revealed that the vent pipe of the solenoid valve was fitted with a sealed filter and our one was blocked with soot. After further inspection we discovered that despite being a generally clean engine there were other traces of soot in out-of-sight areas. At some point it must have suffered a boost leak, the debris from this found its way into the filter and progressively blocked it over time.

Test three really does need an extra pair of eyes as there is a lot to monitor at the same time and it would be dangerous to do this whilst driving. There is an option of using the WPS to monitor the vacuum when driving instead of the gauge. This gives the added bonus of being able to replay the data observing command, control and position – all at the same time. These three tests took a total of 45 minutes. It would have taken at least that to remove, clean and refit the EGR valve which in this case wouldn’t have made any difference.

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