Putting pressure on a Polo

Frank explains how there can be more to a misfire than you think

By Frank Massey | Published:  07 December, 2017

I have always tried to express the importance of logic and process in any diagnostic challenge. Added to this foundation training principle should be common sense and simplicity.

With this in mind, I walked into the workshop last week noting a Volkswagen Polo with a misfire. It was familiar, and I recalled that this particular vehicle had recently been serviced by us, with the customer explaining of an intermittent misfire.
As I stood there, our technician was explaining his intention to replace the coils. I gave into a certain incurable habit I have, to the annoyance of all; I asked WHY? The response was vague, which confirmed that not enough testing had taken place.
By now my passive visit had already turned into an intrusion and was fast becoming an interrogation. I started with the recent known history. New sparking plugs had been fitted, and one of the three coils had been replaced when the service was carried out.


Prime causes
The error code, originally and in this case, related to an intermittent misfire on cly 2. I recall explaining recently the three prime causes of incomplete combustion, often worded misleadingly as a misfire.

  •  Ignition errors
  •  Fuelling errors
  •  Mechanical errors


With easy access to the coils I suggested a simple but effective coil test using a dummy plug adaptor. The two basic functions of the ignition system are to create sufficient electrical energy to completely burn the air fuel charge, and to deliver it without loss or leakage. The coil energy test qualifies both the above. This simple non-technical test confirmed that all coils were fully serviceable.

The next suggestion was to conduct a simple but very effective mechanical cly balance test using Pico diagnostics. This test compares the voltage drop against rotation frequency whilst cranking the engine. The test immediately identified a cylinder with 66% relevant compression. This is significant as it points to internal mechanical faults. At this point we don’t know which cly or more to the point, why.


Real time pressure
Based on this evidence I needed to conduct a much more focused and accurate test of mechanical internal function. The chosen and well-established method is to run the engine and observe real time pressure differential over the 4-stroke cycle. To do this the Pico WPS transducer is fitted to each cly in turn with the engine running at idle. The main advantages of this method over a traditional compression gauge is that all pressures above and below atmosphere are shown in real time. With a closed throttle, engine pumping losses are relatively high; this helps to assess any cylinder leakages over the complete cycle.
A common problem with this 3cly engine variant is worn valve guides. This has little effect at higher engine speeds, some of which is due to quicker cylinder cycle times and lower pumping losses. The powertrain PCM monitors the combustion anomalies, and eventually decouples the fuel injector on the faulty cylinder. This may have explained the intermittent nature of the reported fault.


Common sense
It is very interesting to note the response of most technicians when they view very low compression values. So why is this? With a closed throttle, the volumetric efficiency is very poor. Put simply, the piston cannot compress what it has not drawn in. We suffer the same problems, as oxygenating our blood for maximum effort requires an open, not closed mouth.
In our case, no2 cylinder was 3.25bar no1 4.35 no3 4.15, seemingly confirming the initial relative compression test. So, we had a problem that’s mechanically related. you may have also noted no tests or assessments have been carried out to the injectors.
This is the common-sense part; Given the age and value of the vehicle  I predicted that repairs would not be authorised. If they were then testing could be done at the manifold removal stage.


Pressure
Another good tip to increase the pumping losses to further challenge the cly seal is to accelerate the engine at high speed and snap the throttle shut. Then capture the data and note the increase in pressures below atmosphere. Note virtually no compression and little air drawn in.

The forces acting on the piston during the intake stroke increase, and the result is in effect an air spring increasing the pumping losses.  This principle is employed with cylinder select actuation, where all valves remain closed. You may be wondering what happens to the lost energy during intake? The answer is that most of it is returned on the compression stroke.
The basis of cylinder select is this; Accept the engine is inefficient at low torque; Close throttle; Turn off some cylinders. The remaining cylinders now operate on a wider throttle opening, therefore reducing pumping loss. Don’t just take my word for it; American heavy bomber crews in the Second World War used the same principle when the discovered that turning off one engine increased their operating range.


Want to know more?
Contact Annette at ads for details 01772 201 597 or visit autoinform.co.uk


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    Being part of Top Technician for the last few years, I have seen many technicians succeed and develop new skills. Typically all are good rounded technicians and have great knowledge, but what makes the difference and makes the good into the great?
        
    It’s not just that they are lucky. The difference is that a great diagnostic technician will have a well-defined diagnostic process (or procedure) that they stick to every time.

    Process
    Some technicians start their diagnostic procedure with a well laid-out and defined process that they have normally learnt, often from training courses. As with any new process, it starts slowly as theory is put into practice until it becomes natural.
        
    Many technicians typically revert ‘back to type’ during the early stages, as their older method seems to make the diagnostic process shorter. As a result they believe it could make them more money. Yes, in the short term they may be right. However, normally in the longer term a well-defined diagnostic process proves to be infallible especially when the fault is difficult to diagnose or a vehicle that has been to several garages and the fault is still apparent.
        
    Many technicians also try to shortcut the process, taking out some of the steps that don’t seem to help in finding the answer. Sometimes a simple fault is made more complex by the technician overlooking the obvious in the second or third step, jumping from step one to step four because that’s where they feel comfortable. In this series of articles I’ll be covering the 10 steps that make up a well-planned, well organised, tried and tested diagnostic process. Use the process and refine it within your business, it works.
        
    Many businesses use a similar structured process and base their estimating/costing model on it
    as well.

    Meaning
    Let’s start at the beginning, with the meaning of diagnosis. Most technicians will look at the word and think it only relates to a computer controlled system and they have to use a fault code/scan tool to be able to diagnose a fault. This is not the case. Diagnosis can relate to any fault, whether that is electrical or mechanical. Therefore, the diagnosis can relate to an electronic fault by the malfunction indicator lamp (MIL) indicating a fault exists or a mechanical fault that exists within a clutch operating system.
        
    The meaning of diagnosis is: ‘The identification of a fault by the examination of symptoms and signs and by other investigations to enable a conclusion to be reached.’
        
    Or alternatively: ‘Through the analysis of facts of the fault, to gain an understanding which leads to
    a conclusion.’
        
    Both can relate to various professions.
        
    With this in mind, what have celebrity chef Paul Hollywood, your doctor, the green keeper at the local golf course and a automotive technician all  got in common?
        
    They all use a diagnostic process within their profession. Paul Hollywood is often seen as a judge within baking competitions. He uses his experience and process to perform a diagnosis on why a bread is not cooked correctly.
        
    Meanwhile, a doctor uses a diagnostic process to find an illness. A green keeper uses a diagnostic process to determine why the grass does not grow as green as it should, while a automotive technician performs a diagnostic process to find the fault on a vehicle.

    Let’s begin to go through the steps of the diagnostic process.

    Step 1: Customer questioning

    Being able to question the driver of the vehicle of the fault is always a very important part of the diagnostic process. Using a well-structured and documented series of questions can determine how the fault should be approached. Many experienced technicians do this part very well, but when a business becomes bigger, the customer’s information on a fault can get lost  when passed between the receptionist and the workshop.
        
    A document can be developed to perform this task, and is often the solution here.
        
    A customer has often seen a ‘warning lamp’ on the dash. They can only remember that it was an amber colour and it looked like a steering wheel. The document shown has a variety of warning light symbols that they can simply highlight to let the technician know of the MIL symbol and in the circumstances that the fault occurs (driving uphill around a right-hand bend etc).
        
    Much of the diagnostic process is about building a picture before the vehicle is worked on. Trying to fix the fault by jumping to step 4 or step 5 can often neglect what the customer has to say. One of the last steps in the diagnostic process is to confirm that the fault has been correctly repaired and will not occur again (‘first time fix’). How can the fix be successfully tested if the circumstances where  the fault occurred are not replicated during the final stages of the process?
        
    The MIL illuminating again (recurring fault) when the vehicle is driven by the customer is not always as easy to fix a second time, as you need to fix the vehicle fault as well as fix the customer, who has been forced to return.

    Step 2: Confirm the fault
    Some technicians just seem to take the fault highlighted as by the job card (or similar document) and diagnose the fault without first confirming, which can take some time to complete. This step might involve a road test to confirm that the fault exists. The apparent fault may be just a characteristic of the vehicle or the receptionist/customer may have explained the fault to be on the other side of the vehicle.
        
    Therefore, it is imperative that the technician confirms that the fault exists and the situation that the
    fault exists within, all providing additional information on building
    the picture before actually working
    on the vehicle.

    Step 3: Know the system and its function
    In order to fix a vehicle fault(s) a technician will first need to understand how the system works. If a technician doesn’t know how the system works how can they fix it?
        
    Don’t be shy or foolish and indicate that a technician knows everything (even on a specific manufacturer brand). Every technician sometimes needs to either carry out new system training or just have a reminder on how a system works.  
        
    With all the systems now fitted to a vehicle, it’s not surprising that a technician cannot remember every system and its function especially to a specific vehicle manufacturer or the model within the range. A technician may just need to remind themselves on the system operation or fully research the vehicle system.
        
    Most vehicle manufacturers will provide information on how a particular system works and how that system integrates (if applicable) with other systems of the vehicle. Spending some time researching the system can pay dividends in terms of time spent diagnosing the system and it is also educational. System functionality can often be learnt from attending training courses but if these are not available the information can be sourced from various other sources such as websites.
        
    External training courses can provide additional benefits especially discovering how a system operates and understanding its functionality and how the various components work. They will also allow the technician to focus on the specific system without the distraction of customers or colleagues.
        
    Once the system is thoroughly understood, the technician may be able to make some judgements as which components are ok and those which may be faulty and affect the system operation.

    Refine
    Just to recap on the three diagnosis steps covered in this article, these were:
    Step 1: Customer questioning
    Step 2: Confirm the fault
    Step 3: Know the system and its function

    Remember to follow the process and don’t try to short circuit it. Some steps my take longer to accomplish than others and some may be outside of your control (it may be necessary to educate others). Practice, practice, practice. Refine the process to fit in with your business and its practices, align your estimating/cost model to the process to be able to charge effectively.

    Next steps
    In the next article I will be looking at the next four steps which are seen to be:
    Step 4: Gather evidence    
    Step 5: Analyse the evidence
    Step 6: Plan the test routine
    Step 7: System testing

    The last article in this series will indicate the final three steps and how to fit them all together in order to become a great technician and perhaps win Top Technician or Top Garage in 2018. Go to www.toptechnicianonline.co.uk to enter this year’s competition. The first round is open until the end of February 2018.
        
    Every entry is anonymous so have a go!


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