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Why was my engine diagnosed as a write-off after driving through deep water?

Three days ago I stupidly drove my Citroen diesel ZX auto (190,000 on the clock) through a flooded road. The engine stalled and would not start. When the breakdown man arrived his first words were "if you have got water in the filter the car is a write off". He took off a filter cover and said, "yes, the pistons and valves would be smashed.” I was horrified, having had the car for 10 years and hoping to do another 100,000 miles in it.

When towed into my garage an hour later, the car started and spluttered. The immediate prognosis was that the timing was perfect but that I may have chipped a piston. I left it with the garage who next day said the car was a runner but I may have chipped a valve. I now have the car at home and it appears to be running normally. I am interested to know what the breakdown man and my garage were talking about. Can you please explain?

Asked on 30 July 2011 by LH, Preston

Answered by Honest John
A diesel engine is a compression ignition engine and most of them have extremely high compression ratios. The mixture of air and diesel injected into the engine explodes when compressed and that's how the engine works. But water cannot explode and cannot compress, so can cause the conrods to bend. Chipped valves and chipped pistons makes no sense to me. If only a tiny amount of water got in you have probably been lucky and no harm done.
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