Under pressure
By James Dillon |
Published: 18 May, 2017
Even apparently simple problems require thorough investigation if you want to diagnose faults right the first time
We accept a wide range of diagnostic and auto electrical work, primarily from trade customers. This means that some of the jobs we become involved with either come with no history or a nightmare history.
It is very rare to for us to see a straightforward diagnostic job. Vehicles appear with an array of replacement parts, some of these pattern parts are of such poor quality, and having been applied to the vehicle with the consideration of a cluster bomb, are often causing their own microclimate of vehicle running woes.
Healthily sceptical
This working environment means that we are healthily sceptical about everything concerning the vehicle in question. Our approach is one of initial evaluation, whilst tipping a nod to any history which may be available and focussing on the prevalent symptoms. We will then begin gathering data which may be relevant to the symptom. The data will come from a range of activities and tools such as visual inspection, road testing, the five human senses, leak detection, fuel hydrometer, pressure testing, diagnostic trouble codes, serial data stream, gas measurement, voltage and current with the meter and the scope, substitute signal and closed loop computer testing, etc.
Analysing the gathered data occurs upon the execution of each of the tests. Ideally, tests should be run only under the prevailing symptom. A list of good and bad factors is generated as the tests progress. More simple faults may give up their root cause early in the process. Some faults, particularly where poor quality parts have been used, have to be rectified in stages; we may have to undo the human causes before we can see the fault's original cause.
In curing a vehicle symptom, the data will provide the evidence as to the root cause. Sometimes the data will confirm a part or subsystem is definitely not at fault (eliminated), sometimes the data can infer that a part or subsystem may be at fault (suspected) and sometimes the data will confirm definitively that a specific part is at fault (confirmed). As the data gathering proceeds, the potential root causes are classified and the list of suspects shrinks from everything to just one thing. The skill in the job is defining the best diagnostic path to eliminate the suspects in the most efficient manner.
Lack of symptoms
A recent case which came to us with a lack of performance was a 2013 Renault Master fitted with a 2.3 dCi engine which was controlled by Bosch EDC 17 engine management system. Out of the blue the vehicle would just not rev past 3200rpm. The road-test gave an indication of fuel starvation or a lack of boost and the vehicle did not pull at all well during road test. Other clues (shown by a lack of symptoms) included that the vehicle started on the button and ran smoothly. There were no signs of exhaust smoke at idle. When the vehicle got into its upper rev range, it did produce a little blue/grey smoke though. The lack of symptoms data is as useful in diagnostics as the presence of symptoms data. For instance, if the EGR was in trouble, we would expect there to be rough idle and smoke; as these symptoms were absent, we could infer that the EGR was probably not in trouble.
An examination of the vehicle's DTCs showed the following codes, P2263 Boost Pressure Too Low (perm) and three glow plug open circuit faults (int). This vehicle had under bonnet stickers that indicated that it was fitted with a DPF and a quick visual inspection showed that there was indeed a DPF can in the exhaust and the two obligatory pressure pipes rising forth. My initial thoughts when seeing a DPF vehicle with glow plug faults and a lack of boost were to consider a blocked DPF as a potential suspect, however, a distinct lack of DPF blocked codes made me cautious about my suspicion. The vehicle did arrive from another repairer, so any codes relating to DPF issues may have been deleted previously and the van had not been driven far enough to complete a drive cycle. Sometimes knowing what we do not know can provide enlightenment and prevent distraction in the diagnostic process.
Substitute values
My next step was to take a look at the live data stream, as the symptom was ever present, the data stream would likely garner some clues as to the root cause of the issue. Choosing the parameter values carefully was probably the quickest way to build a picture of what was actually happening with the vehicle. I attempted to match the symptoms with relevant data parameters. I chose Accelerator Pedal Position, Rail Pressure, Boost Pressure, Boost Control Solenoid Command, Air Flow and DPF Pressure. A word to the wise regarding live data: There is a possibility of the engine computer using default or substitute vales in case of a problem with a component. This means that the values which appear on the scan tool may have been substituted by the engine computer.
When the engine computer calculates the fuel demand (which is then used to set the fuel quantity), it relies on several data points to look up the correct value for the operating conditions. For example, some of the values used may be engine temperature, mass air flow, boost pressure, throttle position. If one of these data values is incorrect, perhaps because the sensor has failed, the lookup value may be quite a way from what is actually required to make a good calculation, and the vehicle may run very badly or not at all. To reduce the impact of failed inputs, the engine computer uses a back-up or substitute value of data points for inputs that are out of range. This substitute behaviour is so that the engine computer can continue to run with limited inputs; the substitute values mean that although the calculation is off, it is still close enough to allow the vehicle to continue to run.
Verify
This substitution situation may mislead the unwary technician. In order that the technician gets a proper view of what is going on, the best policy is to verify scan tool data by measuring the physical output voltage from any suspect sensors. Obviously, this is more time consuming than simply observing the scan tool data, however, the payback is that the measured voltage is a true, raw, unadulterated indication of what is actually happening at the sensor. The Diagnostic Assistance software from Auto-Solve is a great tool in helping technicians understand raw sensor values to enable the performance of such tests.
Looking at the array of data, what is the weirdest parameter value? For me it was the parameter which supported the DTC: The Boost Pressure had dropped below atmospheric pressure. This indicated that either the sensor/wiring/ECU was bad, or that there was a vacuum in the intake manifold. This vehicle had a throttle valve. The symptom fitted with the data; the engine did feel as if it was being choked. But what was the cause? Could the DPF do this? No, it could cause a lack of boost, but it could not cause a vacuum in the manifold. The DPF data indicated that the DPF filter was not blocked.
Next step
So, the next step was to check out the boost pressure sensor voltage to prove its function. The sensor was easy to access and passed the voltage check, KOEO. I then used a sensor simulator to check the feedback logic of the engine computer and the sensor wiring. I sent a varying analogue voltage onto the sensor pin and assessed the live data on the scan tool and the multimeter in synch, which were shown as expected. Next up was a dynamic test. I ran the vehicle and took the revs up to the bad zone; the boost sensor voltage dipped under the KOEO value.
Root cause
Now to consider reasons for a restriction in the intake: A faulty throttle valve, blocked air intake, blocked air filter, blocked intercooler, broken/stuck compressor wheel or a collapsing intake pipe. I pulled the air filter and checked the panel filter and the air intake which were all sound and without blockages. I removed the pipe from the inlet manifold and checked the position of the throttle valve; it opened and closed as designed. I also measured the throttle valve feedback signal so that I could see the valve's position when the pipes were refitted. The turbo spun freely and was without significant play. I reassembled the pipework and then ran the vehicle at idle and measured the throttle feedback, it was open. I used a heavy foot to operate the throttle remotely so that the engine was in the "zone" and I returned to the engine bay. I looked and felt the intake pipework and I could see one of the lower intake pipes to the intercooler was collapsing (figure 1). The throttle was definitely open and the intake system was clear between the filter and the inlet manifold, a weak walled pipe had to be the cause of the problem. A relatively simple problem, the diagnosis of which was made simpler by a robust process.
Combination
Although the data provided a good clue to the root cause, the additional checks and tests provide surety in the quality of the diagnosis. It was possible to have diagnosed and replaced the pipe without any of the additional checks, but doing this would not have guaranteed a fix, as there could have been a secondary issue causing the vacuum. Using a combination of knowledge, data and a critical analytical process led to the root cause being determined, right-first-time. I was being paid to do a professional job and I could happily guarantee a fix and not a parts darts approach.
- Reasoning and diagnostics Part II
We began this journey last issue, so to recap: We need solid reasoning skills to carry out effective diagnostics; persistently good decision making doesn't happen by chance. Possibly out of convenience these skills are often underestimated and undervalued by people, both in and out of the trade. We must raise awareness of the discipline and precision of thought necessary for logical and critical thinking: so we can be better rewarded for our efforts; and to make sure they are consistently and properly applied.
Reasoning, arguments and hypotheses
We covered some fundamentals in my last article: we explain our reasoning using arguments, which contain statements supporting a conclusion; one type of argument, a deductive argument, should guarantee the truth of its conclusion (if it is sound); however, we need to use critical-thinking to check this, by making sure i) there are no other possible conclusions (which makes it a valid argument) and ii) the supporting statements are true.
- DENSO launches new sensors for Toyota and Lexus
DENSO has added 10 camshaft and crankshaft position sensors to its range. The five new crankshaft position sensors have 129 applications across the Toyota and Lexus range incorporating both past and present vehicle models. The eight new camshaft position sensors have 119 applications across the same vehicle pool.
- IAAF welcomes FIGIEFA ExVe ‘Proof of Concept’ testing
The IAAF has welcomed the news that FIGIEFA, together with other stakeholders, is conducting ‘Proof of Concept’ testing of a ‘Remote Diagnostic’ use case as part of the investigation of the Extended Vehicle (ExVe) concept.
- No codes, no clues?
Have you ever had a car in with a running fault or an issue, and you plugged the diagnostic tool into the OBD socket then read for trouble codes, only to be met with the message ‘no faults stored’?
For many reasons, this confuses technicians and stops them being able to progress with the job. They have no clues or starting point to work from. However, many other tests can be done to find the root cause of the issue. I have worked with many a technician who has been lost after finding a ‘no fault found’ message. I recently had a job where I was able to demonstrate to my colleague how knowing some numbers and how systems work and interlink can help identify what is wrong.
Call-out
The vehicle in question was a 2012 Land Rover Discovery 4. As we specialise in LR we have built up a good reputation in the area for being able to fix them, having also invested in dealer tooling and information. The customer’s first contact with us was via telephone and he explained he had parked the vehicle up outside his house and then having come to it the next day it would not start. The engine would turn over but it would not fire into life. He informed us his local garage had come out for a look and had been unsuccessful in finding the cause and recommended getting the vehicle recovered to us. He asked our call-out charge and asked for us to come and take a look before he organised recovery. This is not my favourite type of job as with limited tooling there is only so much you can do but we agreed to go and take and look and see what we could find.
No fault codes stored
Along with my colleague Jamie we went to the customer’s house that afternoon, taking a scan tool and the tool kit in our work van. Once we arrived we spoke to the customer to gather some information about the problem. He told us no recent work had been carried out on the vehicle and the other garage had done some basic tests on the battery and fuel system where it sat but could not find an issue. I sat in the vehicle and cranked the vehicle to verify the complaint, doing this also allowed a few checks to be done by listening to the sound of the engine cranking. A trained ear can pick up a compression issue, whether it is spinning fast enough or anything mechanical which doesn’t sound correct.
On this vehicle though all sounded ok. I then let Jamie do some checks to see what he could find. As a younger technician he mainly does MOT and general service work, so it was a good opportunity to possible teach him something along the way without the distraction of a busy workshop. After some basic checks he decided to plug in the scan took and see if any fault codes were stored. Upon carrying out a fault code report he was met with the message ‘no fault codes stored’. I then asked him what his thoughts were and where we go next. His reply was “I don’t know?” I am sure this has happened to some of you reading this article, we have all been there.
Live data
I explained to him that live data was a key element here and we should use it to our advantage. We need to look for data relevant to the complaint to rule out what it can’t be, and knowing what the numbers mean will do this quickly. Unfortunately, this takes years of looking at good data, taking notes and memorising it. Luckily for him, I was able to assist. My first checks were to be engine RPM, fuel pressure, immobiliser status, cam/crank synchronisation and a plausibility check of all temperature and pressure sensors to make sure they were in spec. Working through them all with ignition on, then cranking everything looked good so the engine should start but why wouldn’t it? This is where it pays to step back for a moment and evaluate what you know already and what you should do next.
Smoke/air pressure
An engine in its simplest form is an air pump. We know it needs compression, fuel and air to run. With what seemed to be good compression, and from what I had heard, also good data from the scan tool, with limited resources, I decided the next test would be to see if any smoke was being emitted from the tail pipes. This would show if there was any sign of fuel delivery to the engine. With good RPM and fuel pressure, if the ECU is happy, it should be firing the injectors. There was no smoke, however when I felt the tail pipes there was no air pressure whatsoever from either tail pipe. Was this a clue to where the issue may lie?
My first thought was we have a restriction and the engine cannot breathe, so we are missing the air section of the triangle for the engine to run. I then had a good visual inspection of the engine. Knowing the design well, I decided to open the inlet up to atmosphere by removing the map sensor to see if there was any change. If there was a blockage, this test would prove it and allow the engine to run. In this vehicle, the engine is a V6, so it uses a conventional V configuration. To allow air to flow into both intakes of each bank there is what Land Rover call an intake throttle manifold which also houses the MAP sensor, the EGR inlet pipework and a throttle butterfly flap with a rubber hose to direct air from the intercooler into the manifold (fig1). Removing the MAP sensor would allow air to be released if there was an issue from either EGR valve or upstream from the intake i.e. throttle butterfly, failed turbo just to name a few. On removing the sensor and cranking the engine it now fired into life and idled fairly well, this confirmed we had a blockage somewhere manifold side starving the engine of air.
Throttle butterfly flap
Checking the clock, we still had some time left allotted for the call out. I decided as it was easy to remove the intake hose to the intake throttle manifold just to see as a quick test if the issue was before or after. Upon removing the pipework and refitting the map, the engine no would not start, again proving the issue was on the engine side of the pipework. Removing the air intake plenum to the throttle manifold then revealed the issue. The throttle butterfly flap used to strangle the engine of air on shutdown had jammed shut and never reopened as the housing was heavily covered in carbon. This butterfly, when working correctly, should spring back open ready for the next engine start. Questioning the customer and his driving style revealed he mostly done slow speed and town driving and used supermarket fuel, all of which were a contributing factor to the issue as the valve sits closely to the flow of EGR gas from both valves. Forcing the valve open and refitting the components allowed the vehicle to be driven back to the workshop for a repair to be carried out.
Upon the removal of the entire assembly (fig2), it was found the unit would be better to be replaced as cleaning would not remove all of the carbon deposits and could cause the issue to re-occur. The EGR pipework was also removed and cleaned as a preventive measure along with an oil and filter change and the vehicle was returned to the customer.
Further learning
Why were there no fault codes stored you ask? Well on this engine the position off the butterfly flap is monitored and it should have stored a stuck closed fault but this may not be part of the software’s strategy so I am unable to answer why. However, this article shows that if you have an issue and no faults are stored, there are tests you can do to find the issue. So next time you have a scan tool connected, grab for example 10 good live data PIDs and store them then learn them off by heart. Once you have mastered that section move onto some more and soon you will build up a good mental library of what good data should be, which helps massively to fix cars!
- Lemförder switchable engine mounts
Under its Lemförder brand, ZF Aftermarket has introduced a range of switchable engine mounts for a number of Audi and Mercedes Benz models. Using an integrated air spring this can actuate two different characteristic curves depending on vehicle and rotational speed. The mounts are currently available for Audi and Mercedes-Benz vehicle models and includes the Audi A4, A5, A6, Q5, Q7 and Mercedes C and E Class, GLK, GLC. ZF Aftermarket recommends that all engine mounts always be replaced during repairs, not just the defective part. Otherwise, there is a risk that the new component will have to absorb higher forces and vibrations because the remaining mounts are already heavily worn which may accelerate subsequent damage.
www.zf.com
