Which is the correct oil level?

[uumode]

The Mercedes manual gives two methods to check oil level.

1. When engine is cold (my usual method because it's easier to remove the oil cap and top up if necessary). Or has been started temporarily wait 30 mins.

2. When engine is hot at normal operating temperature, stop engine and wait 5 mins to take reading off dip stick (didn't do this method as on my previous cars the oil cap expands with the heat and is difficult to get off when hot to top up)

I took a reading cold, and discovered the oil level at min and figured my new car was consuming oil as expected and indicated in the manual. So I topped up the oil to the max line on the assumption it would consume more oil as the engine was being run in.

Then the other day I wondered what the level was and the engine was hot, so waited 10 mins to take a reading, and was surprised it was about 5-6 mm above the max line.

I was concerned of overfill complications (which may or may not be real at 5-6 mill above) and syphoned off a little excess.

I took a fresh cold reading and the oil was just past halfway on the dipstick.
Then warmed up the engine to normal temperature, waited 15 mins and the reading was just below max which I feel more comfortable with.

But which is the true reading?

 

[DieselPete]

Most mercs if not all i think will come up with a message if the oil level is to high, oil expands when hot, the only oil i ever check hot is the transmission oil, engine is always cold and on level ground.

 

[markjay]

In the olden days we used to top up to the Full mark when cold. But this is no longer how it's done....

According to MB techs, ANYWHERE between the Min and Max if fine.

The oil level should not be lower than Min when cold, or higher than Max when hot.

But to my mind, the oil level should always be a smidgen under the full Mark when hot.

Incidentally, while overfilling is bad for both petrol and diesel engines, for the latter it could be terminal!

 

[John Jones Jr]

My rule of thumb is never to top up to the MAX mark (always just below), engine hot or cold. A slight underfill is always better than an overfill.

 

[Rory]

Don't have dipstick on mine, but the oil level in the cluster display reads highest when the engine is cold.

 

[190]

Oil expands significantly when hot. Where the level is fairly critical as in auto transmissions they make a big fuss over the correct method and temperature for checking the level.

The level is not so critical for engine oil as long as it's not lower than the pump pickup or so high that it causes frothing in the crankcase or self digestion in a diesel.

As a matter of principle in any of my vehicles I prefer to provide the maximum breathing space in the crankcase by running the oil level no higher than the half way mark when hot. The positives to that is the oil will warm up a little faster and the reduction in crankcase compression can potentially reduce oil consumption. The only potential negative is quicker depletion of the oils additives but that is not remotely a concern with my oil change intervals of 4000 miles.

If I was running an extended oil change interval of say 15000 miles or whatever the maximum allowed is I would set the oil level to the full mark when hot.

Thankfully my MB has a dipstick which is a small touch of sanity in an otherwise needlessly complex car.

 

[uumode]

In the light of this I will now also take readings after a journey when the engine is hot and oil expanded.

 

[zipdip]

The difference on my car from min to max on the oil dipstick is around a 1.5 litres so not that much,I normally check the engine oil cold,I would check the gearbox oil hot but I do not have a dipstick for it,being the 722.9 7 speed box,I don't think I have ever seen one for sale.

 

[whitenemesis]

Not sure the 722.9 'box has a dipstick tube, as it is filled/drained from the same plug on the sump.