On a gasoline engine any time hydrolock is suspected the first thing to do is pull all the spark plugs and see if you can turn the motor either by hand or with the starter. I would think the same would be true with a diesel. I would pull the glow plugs but others may have a better option. You dont want to force the motor to turn over without giving any fluid a placxe to go ie out the glow plug holes. Pull all of them and see which ones if any puke liquid when you turn it over and then you will know. Thanks

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83 300D 227,xxx miles, sold
86 300 SDL 130,000 miles, sold