Fighting technology with science

Frank goes all-out looking for the cause of a vibration on an otherwise very well maintained car

Published:  13 February, 2018

I am sure all diagnostic technicians out there will agree vehicles are becoming ever more difficult to diagnose. Two obvious reasons include the increase in networked systems, and difficult accessibility.

The first step is to conduct a non-intrusive serial evaluation. This method often provides insufficient information to progress directly to a repair solution. What if the problem is a non-monitored component, or possibly a non-monitored component causing a negative reaction in a monitored component? Sounds confusing, then you will appreciate the following diagnosis and repair review.
Here is a conundrum: What has a vibration at around 100hz got to do with a EGR fault?  

The vehicle in question is a 1.4 16v mk4 Golf 1J chassis. The vehicle history is very well known to us as it was owned by our staff member, Annette. She had it well maintained for many years despite its 125,000 miles.

It had a minor serial error relating to EGR flow. A new OE valve was fitted many years ago without success. The vehicle performed extremely well so we ignored it. The vehicle passed into my ownership several weeks ago. My intention was to prepare it for my partner’s two sons as their first car. Totally new OE brakes front and rear, four new Goodyear 185/65/14 tyres… anyone spotted an anomaly yet?

Rear wheel bearings re-packed with grease, all fluids replaced. New OE exhaust system. The car drives superbly. Brake balance differential 1%! Perfect emissions. I decided to use the car for the Pico NVH-WPS course held during a weekend in November. On the Saturday we conducted several tests to confirm the mechanical efficiency of the engine.

The primary test, following a battery status and health check, was a relative compression test conducted in the Pico diagnostics platform. It’s very quick with only the battery connected to channel 1.

The result was excellent, all cylinders returning a differential of 100%. Let’s digest this for a moment, this does not confirm good compression or correct valve timing. It’s simply a balance of voltage drop whilst cranking the engine. You know what, a bad result here always indicates a serious internal engine problem.

Testing
We then discussed the issue of pumping losses and how this can be addressed with throttle control, variable valve timing and lift, and not forgetting cylinder cancellation! This progressed to dynamic compression tests on the engine using WPS. The results were excellent showing good pressure differential (note I don’t call it vacuum as there is no such thing) suggesting efficient cylinder and
valve seal.

The day ended with a prep talk on the advantages of noise and vibration monitoring. Sunday began discussing the information required for manual data entry into NVH platform. This includes PIDs, notably engine speed via a Mongoose serial interface. All the gearbox and differential ratios were entered together with the tyre sizes. Did you spot the anomaly yet?

Basically, the software can now calculate frequency and speed against noise and vibration signatures across all engine, gear selection, and wheel speeds. Remember frequency HZ x 60 = RPM.

RPM div 60 = HZ. Down the road we went several times sticking weights everywhere to demonstrate different vibration signatures. Due to the quality tyres and general smoothness of the car there was very little vibration to look at.

However, on closer inspection there was a vibration concern around 100 HZ. Apply the maths and you get 6,000 RPM. The engine E1 was around 50HZ! 3,000RPM and there was a E2 vibration, so whatever it was had to be  engine  ancillary related. Further inspection using a roaming microphone to pin point the noise confirmed a very noisy serpentine belt idle pulley bearing. This is where the shock on my part and the realisation of the incredible value of applying science and physics to an everyday problem pays off. I decided to conduct the repair myself the next day, stripping the front end exposed a fractured timing belt guide and badly impregnated timing belt tension pulley. The broken half of the guide was hovering inside the timing cover I guess just waiting to do its worst!

Pic pulleys
Several pulleys were singing like canaries despite no previous and obvious audible noises. So, three hours later and a total front end rebuild with OE parts, including water pump, we have an even sweeter engine. So, what else did I find? My original training was as a precision engineer specifically in engine remanufacture so instinctively I don’t strip out timing assemblies until I have checked the original position. It was one tooth out on the crankshaft!

Humming, I think timing out, manifold pressure will change, it’s a MAP sensed load system, so EGR is calculated from an algorithm based on throttle, map value and EGR control ratio, with feedback.

Eventually we discover the historical problem of a seemingly innocuous EGR DTC. In conclusion by recording vibration from the driver’s seat frame, yes, I do mean from inside the car,
we pin point a potentially engine critical fault.

A mechanical non-monitored component affecting a monitored sensor value! One last thought – the anomaly! The standard tyre specification for a Golf 1.4 1J IS 185/80/14. I deliberately wanted more responsive high-end tyres. The speedo is almost 10mph out, not a bad idea for two 24/25-year olds.

Want to know more?
If you want to get on the NVH bandwagon, email Annette @ads-global.co.uk or call 01772 201597.

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    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!

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