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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    You may have read about some of the challenges that the aftermarket has faced over the last year or two as part of the vehicle Type Approval revisions – with their inherent ‘rights of access to repair and maintenance information’ and the associated fight to maintain access to the vehicle data via the ever-so-not-so-humble 16 pin OBD connector.

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    Legal reference
    Firstly, there is the additional problem of existing Block Exemption and Euro 5 Regulations which do not provide the critical legal reference to enable access to in-vehicle data beyond just emissions. The standardisation requirements are included, but not the data and information for the wider diagnostic, repair and maintenance data. This means that vehicle manufacturers can claim that access to the vehicle and the corresponding ‘wider data’ does not have to be provided. This is currently being challenged by the Aftermarket Associations in Brussels, but no solution has yet been agreed for those contentious claims and there will be many vehicles on the roads with restricted access before a workable solution can be agreed and implemented.

    As vehicle manufacturers are likely to be in contradiction with these existing Type Approval requirements, it is also likely that they will have to provide access, but this may well be through the use of electronic certificates. As each vehicle manufacturer has their own certificate strategy (process, access criteria, data available etc.), this is still a significant problem and in some cases could mean multiple certificates are needed to work on the different vehicle systems on specific models. It is also important that certificates can be used without the necessity of having to use the vehicle manufacturer’s dedicated diagnostic tool and an online connection to their server to generate the required certificate when using the 16 pin connector.

    However, the new vehicle Type Approval legislation should now provide the legal reference for the physical connector and critically, also contain a reference to the data needed for diagnostics, OBD, repair and maintenance, but beyond these important requirements there are still other elements which have yet to be discussed or agreed.

    Logical cascade     
    These other issues revolve around the secure access for independent operators, together with the exact data that will be made available once access has been granted. This may sound strange, but the 16 pin OBD port is increasingly seen as a high security risk access point into the in-vehicle networks. Consequently, some form of controlled access is highly likely to be implemented, even for such seemingly mundane tasks as checking safety system trouble codes when conducting an MOT test. This is also likely to be a ‘certificate based’ system and this introduces a whole range of new challenges!

    To understand these various issues more clearly, there is a logical cascade which starts with the legal requirement for a connector to be fitted to a vehicle. This is covered as part of vehicle Type Approval legislation, and this legislation also includes the need for the connector to be standardised from both the aspect of the physical shape and connector pin layout, but also what data or information is needed for emission systems, as well as the communication protocols that must be used. All these legislative elements have been in place for more than two decades, but the wider use of the 16 pin connector for diagnostic, repair and maintenance requirements had until the current revision of the vehicle Type Approval legislation, not been legally referenced. Now that this has (hopefully) been addressed, the next key discussions will be about who can access the vehicle via this connector, how this can be authenticated and once access is provided, what data, information and functions will be supported.

    As mentioned earlier, this is likely to require electronic certificates, but to avoid the ‘wild west’ of different processes, access conditions and data availability, a standardised process should be considered by the legislator which also uses a single and independent point of access for certificates from all vehicle manufacturers. It should also be possible to access in-vehicle data without a certificate when the vehicle is in the workshop, although software updates may require certificates. When the vehicle is being driven, ‘read-only’ data should still be available and a certificate should only be needed if some form of ‘functional’ testing is required, but this should be considered as the exception. As there is an increasing use of ‘plug-in’ devices being used to allow remote communication with the vehicle when it is being driven for services such as insurance, or remote monitoring for prognostics and predictive maintenance, arguably, the importance of the OBD connector is increasing for these telematics services – even if the data it can provide is restricted in relation to what is available via the vehicle manufacturers’ embedded
    telematics systems.

    Further requirements
    Once data is accessed, the new General Data Protection Regulation (GDPR), which comes into force in May this year, will impose further requirements for the use and handling of personal data.  A fundamental issue will be that much of the data contained in the vehicle can also be considered personal data and is subject to data protection legislation. Critically, the customer must give their consent to the use of this data by a positive action or statement – it cannot be assumed.    

    As you can see, it may be ‘so far, so good’, but the simple task of continuing to plug into the 16 pin connector and diagnosing or repairing the vehicle is going to be far from simple, with many hurdles and challenges yet to be addressed, but the aftermarket associations, both in the UK and with their pan-European partners, are continuing to fight for the ability to do so.


    xenconsultancy.com


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