Aftermarket April 2022

APRIL 2022 AFTERMARKET 31 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. Fig 4.