Process not problem

Part two

Fig 1

Published: 30 May, 2022

Frank’s ongoing look at the recalcitrant VW Golf R serves as an example of why process will win every time

I left off last month with a road map and a suggested list of diagnostic options, with several serial errors, some of which were predictable given the mix. It is easy to be drawn into a quick fix based on experience, so I intend to explain in detail just how complex this drivetrain is and hopefully illustrate how we should be reacting to our waypoints.

Are fault codes clear enough? Can we trust them? More importantly, what caused them? We should always approach faults in a progressive logical order; However, this is often modified due to accessibility restraints and cost. For example, conduct the quick and easy tests first, while the engine runs. Once disassembly starts our test options are limited.

One misfire count per 1,000 RPM: Direct or coil on-plug ignition is easy to evaluate providing you use an oscilloscope. Each coil has three circuits, an independent 5-volt square trigger from the PCM and the thin cable. Power and ground circuits are shared across all the coils. Current flow is an internal function of the coil assembly, however dwell or saturation and ignition point is controlled via the 5-volt PCM trigger. Please refer to Fig.1. This shows a Pico screenshot illustrating normal function, spark burn time 2.5 m/s and peak current flow of 15 amps. The slope (burn line angle) suggests a clean plug with normal physical loads. The secondary image was achieved by an inductive probe. Current flow via power or ground is achieved by an inductive Hall probe.

Our vehicle displayed good patterns across all coils. However, the expression misfire can be accredited to ignition, fuelling or mechanical faults. We have yet to establish the validity of the cam crank position error codes. The map sensor value error cannot be addressed, as a valve timing error would directly cause manifold pressure deviation.

The next interesting feature of the 888 engine is the dual injection system. Cranking from cold, the high-pressure injectors operate with three delivery events. Once running and below 45°, high pressure fuel is injected over two delivery events at 60-80bar for approximately two minutes. The port injectors now take over fuel delivery at 4-6 bar pressure via the J538 pump control module. A word of advice; This is subject to adaptive control. Port injection then provides fuelling across all the low mid load ranges therefor it seemed reasonable to evaluate the port injectors next.

All port injectors were evaluated in our ASNU test bench. As the high-pressure injectors required considerable disassembly, our next test focus was the camshaft-crankshaft synchronisation.

It is helpful to explain how the valve timing is adjusted across the operating range. From cold, with the high lift exhaust cam operating, and 30° advance angle, the inlet timing is set to 0°.
Above 45°, the low lift exhaust cam is triggered, retarding the cam to 0-4° advance angle. The inlet now adopts 15° advance angle. So, you must rely on serial data to establish the actual request cam positions before continuing to in-cylinder pressure differential testing.

Now please refer to Fig.2 and Fig.3. Traditional cylinder assessment directs you to compression testing. The problem with this method is only pressure above atmosphere is recorded. To make things worse, it is stored by a one-way valve which may mask cylinder leakage, in contrast with an in-cylinder pressure transducer and with the option to run the engine; Less ignition, allowing real-time pressure measurement above and below atmosphere. Note I use the term pressure, as the is no such thing as vacuum in the Otto cycle.

Because of the very subtle changes in-cylinder pressure at and around atmosphere (+1 bar) we can accurately determine the effects of valve open, valve close position. Please refer to Fig.4.

Please excuse my over-simplified explanation of this incredible diagnostic opportunity as a full explanation would fill a topic itself. The use of crankshaft rotation angle cursors is required. I also suggest a high sample rate with no bandwidth filtering. This will provide the best image quality.

Let’s get the physics out of the way so you may best understand the changes in pressure that occur during the 4-stroke cycle. Boyle’s Law sets out that pressure and temperature within a sealed cylinder will reduce as volume increases, with an increase in pressure and temperature as volume reduces.

All that’s required is to understand which direction the piston is travelling, hence the angular cursors, to determine when the valves are open and closed. So, during the intake stroke, the piston is descending, volume increasing. The pressure drops below 1 bar (atmosphere), due to intake restriction (pumping losses) so not a vacuum. The intake valve will close just after BDC, allowing the best possible cylinder charge. At this point the pressure will rise, and continue until TDC. The stored energy will act on the piston crown until just before BDC. As the exhaust valve opens, you will note a small rise in cylinder pressure despite the piston still descending due to the higher pressure in the exhaust system. As the exhaust stroke continues, pressure should remain equal or below atmosphere, with no restriction, until the exhaust closes, with a small sudden rise in pressure as the piston is still rising.
   As a note of interest, the piston speed is greater in the first and last 90° of rotation due to crank/con rod angles. The entire cycle relies on a transfer of pressure from high to low, with no suction. We can accurately measure these events from an intact rotating engine. Our vehicle confirmed a physical shift in valve timing against a known waveform sample.

Confidence
With the confidence to strip the timing cover and adjust the timing error, we can proceed to the question of why it happened. There is a modified tension kit, and we did note a slight rattle from the timing chest on start up. This quickly disappeared once it was running.
The next interesting feature of the 888 engine is the mapped variable displacement oil pump. At lower engine speed load, it delivers oil at 1.8 bar rising to 3.8 bar. You will note two oil pressure switches on the filter housing, brown reduced pressure 0.5-0.8 bar, high pressure 2.5-3 bar.

The oil was drained, with its poor visual condition noted, followed by the filter element, with what we are confident is the cause of our timing slippage; The cartridge had collapsed due to blockage. When a filter is partially blocked, a pressure drop will occur with the higher pressure differential crushing the filter, causing a reduction in flow.

Given the low start up oil pressure at 1.8 bar, it is probable that normal chain tension was delayed. This gave rise to concerns over further journal damage, so we removed a cam bearing cover to check for bearing wipe. Having confirmed no further serious internal engine damage, we proceeded to complete the repair. We applied a preliminary flushing oil change, then a final filter and oil service.

So, poor servicing and incorrect oil specification caused the faults. Re-setting all default values, our vehicle performed without fault over an extended test drive. The ignition misfire may have been due to restricted piston crown oil cooling.

Fig 2

Fig 3

Non-intrusive diagnostic techniques
The last two topics in recent issues focused on combustion issues and the various tools, service and repair process available to us. Two reasons have directed me to develop this debate further, firstly an email from my much-respected friend Phil Ellison at ASNU, and a VW Golf edition 30 presented to our workshop with poor running at low and transient throttle position. I was also involved in a conversation with friends in Perth, Australia over valve timing issues.

I’m going to respond to Phil’s interesting input first and clarify something especially important to all diagnostic techs. All decisions we make must be evidence based and not opinion. This is an extremely broad statement, but simplifies the fact that if you do not have access to the required tools, software, or process skillsets your decisions will be opinion-based!

I can relate this to my time building military aircraft, where nothing ever happens as a result of opinion. You could quite literally switch off and simply follow the build schedule and submit your work to inspection. You were not paid to have an opinion. This is why I left!

I may have previously left an impression that it was not necessary to fully evaluate injectors in a test bench, if this was so, then I apologise as my thoughts are the exact opposite. My intention was to ensure that you fully explored all causes of incomplete combustion while the engine is running, as most engine work now carries a high labour content! Do not, however make the mistake of letting cost dictate your process. Phil did pick up on the common issues of injector removal damage where specialist tools are required. The use of fuel additives, which can be a common cause of internal injector damage especially to plastic filter baskets, where any debris is then deposited in the basket effecting fuel flow. Direct injection technology now demands the absolute best fuel quality, often reinforced by manufacturers placing fuelling advice inside the filler flap.

Phil also picked up on a common issue I did omit; Stop/Start. Hot engines with an increase in stop events, with fuel trapped in the injector often causes lacquering of the pintle. Heat in the combustion chamber dries any combustion residue and oil on the injector tip. I’m coming to the inlet valves very shortly…

Fuel trim or correction does not fix problems, it can exacerbate them, imbalance in injector delivery or as Phil pointed out deterioration of the spray pattern will cause bore wash, premature lubrication failure, and an increase in crankcase emissions, larger fuel droplets do not combust fully.

Interestingly, he pointed out that new injectors are produced with a +/- 5% tolerance.

Potentially misleading evidence
The Golf appeared in our workshop just a few days after I had finished my topic. I was not involved in most of the diagnostic process or repair but was in discussion over potentially misleading evidence.

The vehicle had covered 106,000 miles, and was suffering from poor idle and incomplete combustion, with a mil light indication.

Step 1/ serial interrogation
0568/P0238 boost sensor, signal high, frequency 1
0768/p0300 random/ multiple cyl misfire, frequency2, counter re-set 255
0772/p0304 cyl #4 misfire intermittent frequency2 counter re-set 255

The next step taken was a cranking current differential test, showing no apparent mechanical imbalance? Back to this later.
Coil and plug failure is a common problem and is an obvious job for the Pico scope, no problems with burn times or primary current saturation here.

David Gore, our diagnostic tech, opted for the first look sensor in the exhaust next. I’m not sure if he opted for WPS in cylinder or not. This would have been my preferred choice, but as the saying goes too many chefs…
If you refer to Fig.1, The image is triggered from ignition, sequentially 1342 from left to right. I’m going to let you debate this image, as I intend to cover this in detail next month. I bow to Brendon Stickler’s wisdom on exhaust pressure evaluation. My debate is focused on the properties of pneumatic pulse delay from the cylinder head to tail pipe. I have since proven this and will discuss this in the October issue.

The next and obvious decision was to remove the manifold and check the intake tract and valves for carbon.
So, as you can see in Fig.2, there is excessive intake valve carbon. This is due to several factors, the most common of which is no self-cleaning from the fresh fuel air intake cycle. Other factors include, lengthy oil service intervals, not replacing oil separation filters, poor fuel quality, driving environment, poor or incomplete combustion cycles, incorrect atomisation and air swirl during the intake and combustion preparation cycles. Remember, direct injection can separate the fuelling into several events on both the intake and compression strokes.

Value
Back to a comment I left open earlier, I hope you are still interested? The value of compression is determined by the mechanical engine efficiency and volumetric efficiency, Pumping losses! So why didn’t a problem show up during the cranking balance check? As this test is based on compressional resistance. Accepting that when the engine was at idle it ran badly and would eventually disengage the injector cycle in cly #4? the answer is rotation speed increase reduces the available time to draw in fresh air. If you compare nominal compression values say 10-12bar against the value at idle they will only be around 3.5 bar!

The detrimental effects of intake fouling only tends to occur at closed and partially open throttle, where the pumping losses are the greatest. The dtc relating to boost pressure sensor value high, can be caused by ignition misfire or unstable intake pulses.
Finally, the injectors were subject to the Spanish Inquisition in the ASNU bench. The results (see Fig.3) confirm substantial fuelling imbalance causally relating to my previous comments.

My grateful thanks to Phil, David (and myself), for the technical input in this topic. I’m off to the workshop to check the delay characteristics with WPS in cylinder and FIRST LOOK sensor in the exhaust.
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