My 2001 C-320's engine has produced a knock-knock-knock-knock (presumably until oil pressure comes up) on 90% of it's engine starts since it was new. Hot or cold, makes no difference. This is the same sound one hears during the first start after an oil change until pressure comes up. My '95 C280 did this (it had an oil pressure gauge, unlike the C320) so I know that it quieted down as soon as pressure was seen on the gauge and was not heard again until the next oil change.

Looking at the microfiche, I see that the V-6 has a check valve at the oil pump outflow and I'm theorizing that the valve is sticking open so that the pump loses it's prime on most shutdowns.

The question is, is this a rod knock and does it in fact do any damage, anyway? BTW, oil used was the factory-fill Mobil 1 0W40 which is now Mobil1 15-50 since 0W40 is not available at retail yet. When it is (and I don't mean at dealer prices) I will be switching to it.

Roger Ellingson/Seattle