My understanding is that it is related to oil pressure. I can't comment on engines without hyd valve lifters, except to say they are most likely flooded, and I haven't really experienced it at the same time as the impression of loss of compression. What I am led to believe is happening is that the oil pressure doesn't reach the lifters before the engine is switched off. In the few seconds of engine running the oil within the lifters is expelled so that when it is restarted the oil has to travel all the way to the lifters (and refill them) before the valves will open. In the meantime the injectors are putting plenty of fuel on to the back of the valves so that when they eventually do open the engine is flooded. Old Volvo 850's suffered quite badly with this so I recommended my customers used 5W30, instead of the 10W40 stated in the handbook for moderate climates (less viscous when cold and able to reach the lifters quicker). Conversely I ran my partners old Mondeo for 150K (250K total miles) on 10W40 instead of the recommended fully synthetic and never had the problem because we both made sure the engine was allways run for at least 2 mins.
For anyone not sure why you lose compression when the valves stay shut it is because if no air is being drawn in to the cylinder there is nothing to compress ie: it's a vacuum on the down stroke.
Regards to all,
Bryan
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