PMM catches up with Joshua Jones, who recounts a couple of EGR system faults that he has recently encountered.
This month, I would like to draw on a couple of examples of EGR system faults, to try and highlight the potential difference in the diagnostic techniques required for petrol and diesel engines. The case studies will also serve to demonstrate how the fundamental differences between ‘spark ignition’ and ‘compression ignition’ power plants, if relatively well understood by the technician, can be a valuable diagnostic tool.
Scenario 1
I recently worked on a 2002 Vauxhall Astra with a petrol engine fitted. The complaint from the driver was an erratic idle speed, illuminated EML and generally poor performance. It was also stated that the customer had been using the vehicle for days before having the time to drop it off for investigation. During this period, the car had not performed properly but was drivable.
For the purposes of what I am trying to explain in this article, I will not put too much emphasis on the diagnostic procedure that was carried out at this point. The fault turned out to be simple enough – a jammed open EGR valve. This was caused by a small piece of debris that might have been a piece of grit (which may have sneaked past the air filter and had luckily not caused any other damage). It was lodged in between the valve body and valve itself, holding it open.
As the fouling experienced by EGR equipment for petrol engines is generally much less severe than on their diesel cousins, the debris was simply removed and the valve cleaned and refitted in order for the owner to drive away happy. Job done, but it did really make me think how the same fault in a diesel engine would show itself. How would it compare to the example above and how may this affect diagnosis if not properly understood? This type of problem is so much more commonplace under a diesel hood, so it’s important to understand the difference.
Scenario 2
I experienced a very similar fault with a 2.2 Peugeot diesel engine fitted in a 2005 Peugeot 407. However, the situation and the symptoms experienced here were very different to the example above. The vehicle had broken down completely, and was not drivable. If an attempt was made to start the engine, it would crank as normal, start normally and then almost immediately cough to a stop.
These issues were accompanied by a worrying amount of black smoke from the exhaust. There was no chance this vehicle could be driven for days before being booked in for repair. This cranking/starting process could be repeated over and over and the results would be the same. After yet another very simple diagnosis, which was that the valve was jammed open (in this case the valve was replaced due to mechanism damage, along with some serious soot removal being carried out) the vehicle ran totally normally. The same outcome as before, but very different symptoms…
Why were the symptoms so different?
Assuming that the rest of the engine systems are operating correctly, a conventional petrol engine adjusts speed at idle by controlling the density of the charge air that is available at the combustion chamber. The proportionate amount of fuel is then added and burned. This is achieved by restricting the size of the inlet of the plenum chamber (throttle).
The pressure in a conventional inlet manifold with the engine at idle is normally around 350mbar. If an additional source of gas is introduced (open EGR valve), two main things happen that affect the way the engine runs. The first is that the extra gas introduced contains much less oxygen than the air that has come directly past the throttle flap, meaning that a percentage of the charge that ends up in the combustion chamber is not burned (this is because it has already been through the combustion process).
The second thing that happens is that the density of the charge reaching the cylinders increases because of the extra gas, causing the control unit to add more fuel. This would be burned and cause an increase in engine speed if the charge was made up entirely of air, but in this case, it would be made up of mainly burned exhaust, which means the engine ‘chokes’ on the extra fuel and runs erratically as a result. An EGR valve on a functioning petrol engine is invariably shut at idle for these reasons.
A diesel engine controls its speed at idle by means of fuel delivery adjustment only. It does not adjust the pressure in the plenum and a MAP sensor would detect atmospheric pressure with all systems nominal. Due to this principle, an amount of EGR can be introduced at low engine speed (idle) with the charge pressure available to the cylinders remaining similar. However, in order to cool the combustion process enough to reduce emission levels to the required amounts, at higher engine speeds significant amounts of EGR are used in diesel engines (sometimes around 60% of the inlet charge). This means that if a valve is jammed fully open at low engine speed, then a lot of gas will be pushed through from the exhaust stream into the intake system.
At idle speed, this can be enough to stop the engine, as it makes up a significant amount of the inlet charge. A symptom commonly encountered when an EGR valve jams on a diesel engine is that the engine will start almost perfectly and then stop. This is because exhaust gas is not exhaust gas until it has been burned. During cranking, gas is being pushed via the jammed valve back into the plenum – but at this point it is still air. When the engine starts, this gas becomes exhaust gas, which, as mentioned, contains little oxygen and thus causes the engine to stop again.
These are the main reasons for symptoms experienced with EGR faults on petrol and diesel cars and I find it interesting and very useful applying this knowledge when diagnosing faults.
Fault codes, what fault codes?
As I have mentioned in previous articles, solely relying on fault codes does not always tease out the best outcome. In the case of the Peugeot and the Vauxhall above, both EGR valves have position sensors built in. This means the control unit has a direct way of measuring actual valve position, as opposed to the desired position. On some vehicles, the control unit relies on indirect measurement (e.g. measuring airflow in relation to desired valve position etc.). These calculations are open to an almost infinite number of variables, which can cause a misleading fault code to be triggered (e.g. air leak).
For this reason, I would always attempt to analyse the physical symptoms the vehicle is suffering before analysing the fault codes. This way, by the time you reach for your scan tool you can be sure that you have done everything you can to rule out the possibility of a very simple diagnosis. The beauty of a jammed valve is that it is easily visible on inspection, but removal to inspect can be costly and time consuming if you are not confident the valve is at fault!
I believe that evolving your approach to this type of situation will help you provide great service to your customers, and ultimately make your business more profitable.
