I opened my oil cap while engine was running and oil spluttered all over the place and smoke came out the cap actually spun off like the lid of a pressure cooker. Temp was a bit high at 100c. I'm going to flush system. But is the oil shooting out a sign my motor is going bad? Thanks!

The steam coming out, or air, is blow-by ... compression gasses escaping past worn piston rings into the crankcase. The oil sputtering out -- and someone can correct me on this -- is being flung off the timing assembly and isn't necessarily due to blow-by ... just make sure you have a good gasket under the cap and the oil sputters won't matter.

What does blow-by mean for your engine? A huge range of things. Yeah, you're motor could be dying. Or it could be perfectly fine for a long time to come. Better questions are: Does it start and run OK? Does it start cold? Does it burn oil? If it's yes, yes, yes, no ... the blow-by doesn't matter. Keep driving and enjoy it. My 240D has fairly significant blow-by ... but it starts great, runs great and, as far as I can tell in the short time I've had it, does not burn oil noticeably. No use worrying about the blow-by until I have a compression problem or oil use problem.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

The oil splattering is simply from the timing chain and/or camshaft slinging oil at the right angle to get it out of the Valve Cover. Keep in mind at roughly 80 degrees celsius, oil is much like water in terms of viscosity.

~Ian

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Regards,
Ian White

1995 E300 Diesel w124 OM606
2014 E550 w212 M278 biturbo

2001 BMW 740i E38 M62 (past)
1981 300SD w126 OM617 (past)

especially at 100c. You'll also have more blow by at 100c

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1981 300SD 512k OM603