Highs and Lows

Karl shares his insights on the conundrum of EGR. A riddle, wrapped inside an enigma, recirculating exhaust gas through the system

Published:  30 July, 2018

When faced with diagnosing a fault, in order for us to be able to test the system it is crucial we understand the system’s layout, components and function. We recently faced a fault in a system we had little experience on, so it was an ideal opportunity for a bit of studying.

Technical information is readily available from many sources, be it manufacturer or generic information, and does not take too long to find. While Google isn’t really a substitute for diagnostics, in situations like this it can be very useful for generic information. The fault on this vehicle turned out to be something so trivial I won’t bore you with it. What I would like to share is the valuable information I picked up along the way.

Main purpose
Exhaust gas recirculation (EGR) is nothing new, it’s been used on petrol and diesel engines for many years and while layout and control has varied in design the principle has remained the same. It is important to understand that manufacturers use different methods and configureuration, and for this article I’ve studied several and have tried to demonstrate a generic system.

The main purpose of EGR is to reduce the level of harmful Nitrogen Oxide (NOx) gases emitted from the vehicle’s exhaust. NOx is present in exhaust emissions due to high combustion temperatures and pressures. Under light load/cruising conditions the EGR system directs a proportion of the exhaust gas back into the engine’s air intake. This reduces the oxygen levels which in turn reduces combustion temperature resulting in a lower NOx emission. When power is required from the engine the EGR system closes to insure a more efficient combustion (see figure 1).

EGR on/off
This is the conventional system in its closed (off) position.  During operation exhaust gases are taken from the exhaust manifold (pre-turbo), passed through a cooler (10) up to the EGR Valve (6). The cooler is a heat exchanger that not only uses the engine coolant to cool the gases to increase the mass but utilises the heat to warm up the coolant faster which helps the interior heater warm-up faster. The EGR Valve (6) can be either electrical of vacuum operated. The  powertrain control module (PCM) commands the EGR valve to open by a specified amount dependent on engine conditions (see figure 2).

Some EGR valves have a position sensor that provides feedback to the PCM to ensure the correct position has been achieved. In a system where the EGR valve is not equipped with a position sensor, the PCM monitors the Mass Airflow signal in order to regulate EGR flow. This is achievable due to the fact that as the EGR valve is commanded open and gases start to flow, the air flowing in to the Mass Airflow Sensor will decrease. The calculation is made using tables of data (mapping) within the PCM’s software. Understanding this is crucial when diagnosing running faults as a fault in the Mass Airflow can easily affect the EGR system and vice versa.

Understanding and diagnosing airflow and EGR faults I find can be easier if you look at it pressure differential. If air is flowing through a tube with a restriction in it, the air pressure after the restriction will always be lower than the pressure before the restriction. The difference in pressure will vary depending on the mass or pressure of the air and the size of the restriction.

Air intake/throttle flap
The air intake/throttle flap (see figure 3) generally defaults to the fully open position while the EGR valve defaults to the closed position. The purpose of the flap is to reduce the pressure on the engine side. As the intake flap starts to restrict the airflow, the pressure decreases to a pressure lower than that of the EGR pressure and the EGR gases start to flow into the engine’s air intake. If the exhaust gas pressure was slightly lower than the air pressure entering the engine then the gases would flow in the wrong direction.
    When in good working order this system serves its purpose. However, due to the fact that there is particulate matter in the exhaust gases, the system and components will slowly become blocked, causing reduced flow and valves starting to jam or not seal correctly. The air intake system often contains oil residue from the engines breathing system and slight oil loss from the turbo itself. When this oil is mixed with the particulates in the EGR gases it makes a very sticky gunk that starts to block the inlet manifold and intake ports.

When the engine is under load and turbo boost pressure is required, the EGR valve needs to close and seal. If an EGR valve isn’t sealing correctly when closed then boost pressure will be lost into the exhaust system. The lower boost pressure and reduced oxygen level affects the combustion which in turn causes more particulate matter which only adds to the issue. If the EGR valve is stuck wide open then in most cases the engine will barely run.

High pressure system    
Euro 6 was introduced in September 2014 which demanded much tighter emissions than previous which required an advance in emission control technology. While the precise control of the fuel side of the engine management system has gained precision with higher fuel pressure and multiple injections within the cycle, the air intake, exhaust and emission control systems have too. Most manufactures use a high and a low pressure EGR system.  Prior to this most EGR systems were relatively simple and fell under the ‘High Pressure EGR’ title (see figure 4 and figure 5).

The high pressure system is similar in layout to previous systems but serves a slightly different purpose. The system is only used during the warm-up phase of the engine from cold start. There is a pre-turbo passage from the manifold directly to the high pressure EGR valve (6). As the system is only used in the warm-up phase there is no need for a cooler. In this particular system there is a distribution channel that directs the gases equally into each inlet port. The purpose of this system is to raise the intake air temperature in order to improve combustion and reduce the warm-up time for the catalytic convertor/NOx storage catalyst (7) allowing them to function sooner. Once at operating temperature the system is pretty much redundant.

Low pressure system
The low pressure system (is active under most engine operating conditions and its purpose replaces that of the older systems- to reduce NOx gases (see figure 6). A proportion of the exhaust gas is collected after the Diesel Particulate Filter (8) and passes through a Wire Mesh Filter (9), through the EGR Cooler (10), up to the Low Pressure EGR Valve (11). The EGR valve then controls the flow through a channel up to the intake side of the turbocharger. The wire mesh filter ensures there is no particulate matter entering the system and also in the event of the particulate filter substrate breaking up, it also protects the rest of the system including the turbocharger, air intake and engine internals from damage. The cooler reduces the gas temperature which in turn increases the mass allowing a higher volume of exhaust gas to be recirculated. Due to the exhaust pressure after the particulate filter being quite low and also the air intake pressure before the turbo charger also being low there is and Exhaust Flap (12) fitted. By closing this slightly the exhaust pressure increases which causes the gases to flow back towards the turbocharger.

Key benefits
These systems usually have between three and four  exhaust gas temperature sensors each placed at key points of the exhaust system and two pressure differential sensors. The first is measuring pressure before and after the particulate filter (to calculate soot loading) and second between the DPF outlet and the point after the EGR valve, before the turbo. Coupling these six signals with the Mass Airflow sensor, the positions of both EGR valves and the intake flap, the turbo variable-vane position and the intake pressure (MAP), using the mapping within the PCM’s software means it can also make all calculations necessary. This provides an extremely high intake pressure and exhaust after treatment control.

The key benefits of this system are that the exhaust gases are free of any particulate matter which keeps the entire system much cleaner and therefore reliable. The gases are also cooler meaning a greater mass can be used in a more effective way. Finally the gases re-enter the system before the turbocharger, allowing for the increase in boost pressures at lower engine load and RPM.

Does this make diagnosis harder than before? Not if you take the time to study the purpose of each component and how it works. I’ll openly admit it wasn’t that long ago that I would have taken one look at this system and sent it on its way! Nobody likes being beaten by a job but neither should we have to waste too many hours trying to guess what’s wrong with it, worse still start throwing parts at it. It took me half an hour to locate this info, an hour studying it and a further hour planning what tests I was going to conduct and what results I was expecting to see. What was wrong with it in the end? A faulty sensor confirmed with no more than a voltmeter! After replacing the sensor I wanted to confirm the repair and monitor the function of the components using serial data. Something I highly recommend doing is picking five lines of serial data on every car you work on that requires an extended road test and monitoring them to see how they behave and what effect driving style (engine load) has on them. I guarantee after 10 cars you’ll know what to expect and be far more confident in diagnosing related faults. It works for me!

Related Articles

  • Dirty work: Keeping diesel exhausts clean 

    The exhaust is a lot more than just an exit route for waste gases for some time now. Tim Howes, deputy general manager – supply chain and technical service, NGK Spark Plugs (UK) Ltd, provides some context: “In 2009, The Euro V emissions standard for passenger cars demanded a significant reduction in NOx, HC and particulate matter and in 2014 the Euro VI standard brought a further tightening of these emissions, primarily for diesel engines.”


    Complexity
    For diesel powered vehicles this has meant a significant increase in the complexity of exhaust gas recirculation (EGR) and after treatment resulting in the fitment of various combinations of diesel oxidising catalyst (DOC), selective catalytic reduction (SCR), lean NOx trap (LNT),  diesel particulate filter (DPF) and other associated devices and control systems.
    All these additional components have led to an increased need for sensors in the system.

  • In the heat of the fault  

    At the workshop we cover all kinds of vehicles, old, new, big and small but with all these vehicles we need up to date diagnostic equipment to be able locate faults within the electrical system.
        
    In the workshop this summer was a 2009 Volkswagen Golf that had an intermittent issue which meant the car would go into limp mode, the cruise control was disabled and the climate control wouldn’t work. Understandably in the weather we were having the lack of air conditioning was a major concern to the customer. No one wants to be without air conditioning in 30Cº.
        
    I plugged in the trusty diagnostics reader and came up with four faults. These included turbo boost sensor, manifold pressure, throttle pedal position sensor and ‘fuel system
    too rich’.
        
    In my experience cars can throw up all kinds of trouble codes even when there is no issue with that part. I wouldn’t say some manufacturers are more troublesome than others but if a light does appear on the dash it’s best to get it checked out as soon as possible.

    Issues
    I cleared the fault codes and told the customer to see how it drove and if the issues resolved themselves. The customer had the car for just an hour before they called and said that the problem had reoccurred, as much as this is a pain for the customer I always clear the faults and see if it happens again rather than changing unnecessary sensors. I got the Golf back into the workshop and once again plugged the computer in, which brought up one code. This was the throttle position sensor. A quick call to VW and a discussion with their parts people showed that this particular issue can lead to the cruise and climate control not working.
        
    Next day delivery on the part means the car came back in the following day. One bolt, two plastic clips and an electrical connection later and the pedal was off. Gone are the days of the throttle cable. The throttle response is now done by a sensor on the pedal which works out how far the pedal is being pushed and tells the engine how to respond. It is clever stuff,  when it works.
        
    A pedal replacement on the Golf only takes five minutes and another clear of the fault code before taking the car for a road test. On the test drive cruise and climate control were checked as well as making sure no dash lights had appeared.
        
    Modern mechanics have become very computerised. Dash lights appear whether it is indicating an issue with the airbag systems, ABS or engine and diagnostic computers are so important to narrow down what the issue could be. I dislike the reliance that some workshops put on just trusting what appears on the screen of the diagnostics. It is still imperative that mechanics test sensors and look into live data to make sure that unnecessary components are not replaced and the costs put onto the customer, who will have to pay.


  • Part two The good and THE GREAT  

    In part one, we looked at the start of the ‘diagnostic process.’ The first steps were customer questioning, confirming the fault and knowing the system and its function. These help the technician to build the ‘big picture’ necessary to repair the vehicle correctly.
    In this article we will look at the next four steps.

    Step 4: Gather evidence
    It is easy to overlook this step as many technicians think of it as the overall ‘diagnosis.’ However, once the technician understands the system, gathering evidence will provide key information. This step is normally best carried out with the use of test equipment that does not mean the dismantling of systems and components.

    Many technicians have their own favourite tools and equipment but this list can include (but not limited to)
    the following:
    Scan tool – It is always best practice to record the fault codes present, erase the codes, and then recheck. This means codes which reappear are still current. Remember that a fault code will only indicate a fault with a circuit or its function. It is not always the component listed in the fault code that is at fault

    Oscilloscope – An oscilloscope can be used for a multitude of testing/initial measuring without being intrusive. Some oscilloscope equipment suppliers are looking at systems within high voltages hybrid/electric vehicle technology. The waveforms produced by the test equipment can be used when analysing the evidence and may indicate that a fault exists within a system. An understanding of the system being tested will be necessary to understand the information. This may even include performing sums so all those missed maths lessons at school may come back to haunt you. It may take time to become confident analysing the waveforms, so be patient

    Temperature measuring equipment – This can include the use of thermal imaging cameras. Most systems that produce energy/work will also produce some heat. The temperatures produced vary from system to system. Examples include everything from engine misfires to electrical components, as well as air conditioning system components and mechanical components such as brake and hub assemblies. The possibilities are endless and results can be thought provoking.

    Emission equipment – By measuring the end result, an exhaust gas analyser can show you if the engine is functioning correctly. The incorrect emissions emitted from the exhaust help indicate a system fault or a mechanical fault with the engine

    Technical service bulletins – Many vehicle manufacturers produce technical service bulletins (TSBs) that are generated by a central point (usually a technical department) from the information that is gathered from their network of dealers. Some of these may be available to the independent sector either through the VM or through a third party – It’s always worth checking if these exist. They may indicate a common fault that has been reported similar to that the technician is facing. Some test equipment suppliers may provide TSBs as part of a diagnostic tool package

    Software updates – Many vehicle systems are controlled by a ECU. Most vehicle manufacturers are constantly updating system software to overcome various faults/  customer concerns. Simply by updating the software can fix the vehicles problem without any other intervention of repairing a possible fault. This is where having a link to a vehicle manufacturer is vital in repairing the vehicle

    Hints & tips – Most technicians will have a link or access to a vehicle repair forum where they can ask various questions on vehicle faults and may get some indication of which system components are likely to cause a vehicle fault

    Functional checks – Vehicle systems are interlinked and typically share information using a vehicle network. The fault may cause another system to function incorrectly, so it is vitally important that the technician carries out a functional check to see if the reported fault has an effect on another system. By carrying out this check the technician again is building the big picture

    Actuator checks – Most systems today are capable of performing actuator tests. The technician can perform various checks to components to check its operation and if the system ECU can control the component, often reducing the time to the diagnosis, by performing this task the technician can identify whether it is the control signal, wiring or component or it is sensor wiring. This function can be used in conjunction with serial data to see how the system reacts as the component functions

    Serial (live) data – The technician can typically review a vehicle system serial data through a scan tool. Having live data readings to refer to can help you review the data captured. Using actuator checks and viewing the serial data can also help the technician to identify a system fault

    Remember to record all the evidence gathered so it can be analysed during the next step in the diagnosis. We can’t remember everything. If the technician needs to contact a technical helpline they will ask for the actual readings obtained recoding the data gathered will help.

    Step 5: Analyse the evidence
    Analysing evidence gathered during the previous steps can take time. The technician needs to build the big picture from all the evidence gathered during the first few steps. You need to analyse the information gathered, and decide on what information is right and wrong.

    This step may rely on experience as well as knowledge on the product. You should take your time – don’t be hurried. Time spent in the thinking stages of the diagnosis can save time later. Putting pressure on the technician can lead to errors being made. It may be necessary to ask the opinion of other technicians. If the evidence is documented it may be easier to analyse or share between others.

    Step 6: Plan the test routine
    After analysing the evidence gathered it’s now time to start to ‘plan’ the best way to approach to the task or tasks in hand.

    The technician should plan their test routine, decide on what test equipment should they use, what results are they expecting, if the result is good or bad  and which component should they test next.

    Document the plan – this enables you to review decisions made at this stage in the next step. The technician may not always get it right as there may be various routes to test systems/components. The test routine may have to be revisited depending on the results gathered during testing. Documenting the test routine will provide a map.  Also, don’t forget to list the stages, as this is something that could be incorporated into an invoicing structure later.

    The technician should indicate on the routine what readings they expect when they carry out the system testing. This can be generated by their own knowledge/skill or the expected readings may come from vehicle information which they have already sourced. If the information is not known at the time the test routine is planned, then the test routine may highlight what information is required and what test equipment is needed. You shouldn’t be afraid to revisit the plan at any time and ask further questions on which direction the tests should take. If the plan is well documented and the technician becomes stuck at any point, they can pause the process and revisit later. Also the information can then be shared with various helplines that support workshop networks.

    Step 7: System testing
    The technician then follows their pre-determined plan, if it is documented they can record the results of the test(s) as they follow the routine.

    Many technicians tend to go a little off-piste when they get frustrated. Having the routine documented can keep the technician on track and focused on the result. If the routine is followed and the fault cannot be found the technician may have to go back to the analysing the evidence or planning the test routine. The technician shouldn’t be scared of going back a few steps, as I said previously analysing the evidence takes practice and can be time consuming, not to be rushed.
        
    Summing up
    Remember to follow the process. It is easy to be led off track by various distractions but don’t try to short circuit the process. Some steps may take longer than first thought to accomplish than others. Some distractions may be outside of your control, and it may be necessary to educate others. Practice, practice, practice. Refine the process to fit in with your business and its practices, the business could align its estimating/cost modelling to the process, being able to charge effectively and keeping the customer informed at each stage of the process.

    Coming up...
    In the next article I will be looking at the next four steps which are; Step 8: Conclusion (the root cause), Step 9: Rectify the fault and Step 10: Recheck the system(s). 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 succeed in Top Technician or Top Garage in 2018.



  • Four wheel aligner compatibility  

    The Bluetooth Pro Wheel Aligner from Absolute Alignment is fully wireless to ensure fast and easy use, and can be used with four and two-post lifts and in-ground wheel-free scissor lifts. The direct contact charging and calibration system means the whole wheel alignment package is 100% cable free for a safe workshop.  Absolute Alignment supplies an array of wheel alignment equipment to suit workshops of all sizes, and is the only UK provider with a full range of Bluetooth wheel aligners suitable for cars and commercial vehicles – including those equipped with the latest generation of ADAS.

  • Exploiting Aircon 

    Although it may be hard to believe given the weather so far this year, but a lot of customers will soon be starting to use their aircon systems only to quickly realise that their system is not working as expected, leaving them hot under the collar! So an ‘exploitable opportunity’ exists as the people in suits might say, but will you be in a position to exploit it most profitably?

    Modern systems
    With the majority of new cars now having some form of HVAC (heating, ventilation and airconditioning system) fitted as standard, it is no longer considered a luxury, just another part of the vehicle’s array of functions that should work when needed – summer or winter.
        
    Many modern systems are designed to be highly efficient and rely on much less refrigerant than previously. Unfortunately, most customers do not understand that the system will naturally leak the refrigerant at a rate of between 10% and 20% per year (depending how often the system is used to circulate oil around the various pipes and seals) and it therefore requires regular servicing and maintenance to ensure continued efficiency. Ultimately, if the refrigerant level gets too low, the system will not operate at all.

    Added value
    The easy way to deal with this is to offer an ‘added value’ service whenever the vehicle is in your workshop – namely a free air-con system efficiency check. If the system does not perform to expectations, or emits a bad odour, then the opportunity exists to sell the service to your customer. So, make sure that you optimise the opportunity that aircon maintenance and servicing presents ensuring that your customers can keep their cool when summer finally arrives.
        
    The fully automatic units available from the leading suppliers will allow full functionality with a minimum of technician’s time – who can still be servicing other aspects of the vehicle while the unit does the work. A printout at the end details what was done and if any problems exist – useful for both the customer and as an activity record for the F-Gas regulations.
        
    If a problem exists with the vehicle air-con system then there is a requirement to diagnose and repair. A good understanding of the principle of operation and system design is necessary to both identify and repair profitably, in terms of time and for fitting the correct parts. The typical mathematics for the return on investment (ROI) are something like (prices as of May 2013 for illustration purposes only):

    Cost of equipment                     £2,795
    Cost of training                         £350
    Marketing materials                   £250

            Total costs:                       £3,345

    Air-con service                          £65 (net workshop revenue)
    2 x air-con services per week     £130 net income

    ROI    3345 ÷ 130 = 26 weeks.


    This excludes any additional repair/parts revenue and is based on only two vehicles per week. With this in mind I really think the decision to invest in the training and technology is a no-brainer.

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