Mercedes W202 - Why would low oil level make an engine louder?

Honest John Back Room » Motoring discussion
View Threaded
Sat 1 May 2010 14:43
Mercedes W202 - Why would low oil level make an engine louder? - Pondlife

I recently PX'ed my 1999 W202, but one thing I never understood was that, when the oil level was low, it sounded like a bag of spanners when started from cold. But it was much quieter when I topped the oil up.

By "low", I mean below half-way on the dipstick, not below the minimum mark. So there should not have been low pressure, and anyway I never saw the low pressure light come on even at slow idle.

It was an 2.4 petrol engine with a chain cam which had done 120,000 miles, so I can understand the cold-start rattle. What I don't understand is why adding oil should help. Surely as long as the oil level is below the minmum, the oil pump will generate the same pressure won't it?

This is just idle musing really as I've sold it now, but does anyone know why an engine would "prefer" more oil?

Reply | Report as offensive | Link
Sat 1 May 2010 16:15
Mercedes W202 - Why would low oil level make an engine louder? - SteveLee

Perhaps the reduced quantity of oil ran hotter, thus thinning out and increasing noise?

Reply | Report as offensive | Link
Sun 2 May 2010 20:44
Mercedes W202 - Why would low oil level make an engine louder? - Cliff Pope

so I can understand the cold-start rattle. What I don't understand is why adding oil should help.

So it did it with cold oil, nothing to do with thinning when hot.

I can think of several possible reasons:

1) Oil pump worn, so it struggled to build up initial pressure, but immersion in a greater depth of oil would help it to prime.

2) Incorrect oil filter lacking a non-return valve, so took longer to re-prime.

3) Is the timing chain pressure lubricated or does the crankshaft sprocket sit in oil gravity-fed from the sump?

Reply | Report as offensive | Link
Mon 3 May 2010 23:31
Mercedes W202 - Why would low oil level make an engine louder? - Pondlife

3) Is the timing chain pressure lubricated or does the crankshaft sprocket sit in oil gravity-fed from the sump?

That's possible. I never thought the chain might just sit in the oil and not be fed from the oil pump. I'd only been thinking about the oil pressure, but I can imagine how a greater depth of oil in the sump could get the chain lubricated quicker in these circumstances.

Reply | Report as offensive | Link
Sun 2 May 2010 22:13
Mercedes W202 - Why would low oil level make an engine louder? - galileo

Oil that has been circulating in the engine has been picking up comtaminants and long-chain molecules in oil tend to get broken up in use - new oil will have intact long-chain thickeners and this may quieten things down for a while.

Reply | Report as offensive | Link
Mon 3 May 2010 16:46
Mercedes W202 - Why would low oil level make an engine louder? - Cliff Pope

But this isn't new oil. it's the old oil topped up.

Reply | Report as offensive | Link
Mon 3 May 2010 23:33
Mercedes W202 - Why would low oil level make an engine louder? - Pondlife

Topped up with new oil though. I generally kept it between half and full, so added about a pint or so of oil each time. I guess that's about 10% of the oil capacity, which would make some difference I guess.

Reply | Report as offensive | Link
Tue 4 May 2010 09:15
Mercedes W202 - Why would low oil level make an engine louder? - Cliff Pope

OK, I give up. I can't see any reason why a mere 10% should make any difference.

Final shot:- If you pour a pint of oil in and then immediately start up it might be quieter because you have just doused the valve gear in oil and it hasn't yet had a chance to run down into the sump.

But obviously that would only apply the first time after a top-up.

It must be psychological.

Reply | Report as offensive | Link
Tue 4 May 2010 09:29
Mercedes W202 - Why would low oil level make an engine louder? - madf

I have had this experience in lots of cars.

I put it down to the effect of liquids on noise. Large quantities absorb sound. Lesser ones absorb less.

Reply | Report as offensive | Link

Looking for a garage you can trust? Honest John's Good Garage Guide has thousands of reader recommended garages, plus how-to guides and tips on servicing.

The Good Garage Guide