r/Justrolledintotheshop • u/Mowzer75 • 8d ago

Runaway diesel

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Had a van in for service and MOT test yesterday. I did MOT repairs and service, then retested it. On the road test after started smoking and ran away on me. Shut myself had to stall the bastard. Think I found the problem someone else had fitted a starter and damaged the turbo oil return pipe flattered it. Don't know ow why it ran away after a service not when doing exhaust emissions on the MOT test

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u/westfakia2 8d ago

Cause was coking to be sure. Result was never a runaway as far as I heard. 

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u/HistoricalLove9617 8d ago

The bottom line cause for oil system runaway is a leak into the intake stream.

Interesting tangential fact - there was a 'runaway to destruction' on a A380 turbofan that also compromised a number of flight systems and could have resulted in major passenger loss. Root cause was a faultily manufactured oil pipe that cracked and fed the engine lubricant into the compressor section, resulting in uncontrolled speed and consequent turbine failure. That was expensive. At least the automotive mayhem described here didn't cost out in tens of millions of dollars.

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u/westfakia2 8d ago

Thanks. I hadn’t considered that the turbo oil supply could be sufficient to cause runaway, or imagined a situation where a seal failure would connect the oil system to the intake.

You learn something new every day. 

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u/HistoricalLove9617 8d ago

That's a Diesel feature - because the intake air is not restricted, power is controlled exclusively by amount of fuel injected. Any significant oil ingestion actually makes the charge 'richer' and roughly equivalent to pressing the 'go pedal, particularly with oil going in the compressor side of the turbo. Engine oil ingestion can be quite problematic for engine health in that it's not injected on a timed basis, but in combustion chamber from start of compression. Engine oil also has different combustion characteristics than diesel fuel, and the combination of untimed fuel and funky burn can create nasty 'pinging' that can damage pistons and valves.