Process not problem

Part two

Fig 1

By Frank Massey |

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

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