Skoda car handbook… Check oil when engine is HOT?!

pinkteapot

I’ve checked the oil when my engine is cold for 20+ years, because my dad taught me to. Happened to notice in our 2016 Skoda Superb (diesel) handbook that it says to check it when the engine is at full temperature! (But to wait “a few minutes” for the oil to drain back down.)

Is that right? On Googling the entire internet still seems to say you check oil when it’s cold. Is that just an old wives tale with modern engines though?!

Did everyone know this apart from me?! 

flashg67

Never heard this, but in theory, as long as you've allowed time for the oil to drain back down then there shouldn't be any/much difference, but I'd allow 10 mins plus after switching off. Might be a VAG 'thing' - Google suggests the same thing in the Seatforums!

pinkteapot

Ah ok, glad it wouldn’t make much difference - wondered if we’ve overfilled it.

Other car is a 2012 Audi so also VAG - I’ll have to see what that says. I’ve never in my life looked up how to dip a car before.

chrisw

Things do seem to have changed, particularly with electronic dipsticks. A lot of BMWs need the engine running to measure the oil level.

Belenus

chrisw said:

Things do seem to have changed, particularly with electronic dipsticks. A lot of BMWs need the engine running to measure the oil level.

Our 2015 BMW 116i Series1 F20 petrol car measures the oil electronically with the engine running but it also has a dipstick.

oldagetraveller1

"Checking the engine oil
Park the vehicle on level ground. After warming up the engine
and turning it off, wait more than 5 minutes for the oil to drain

back into the bottom of the engine."

The above is for a Toyota. A warmed up engine (warmed to what degree, they don't specify) but allow only sufficient time for the oil to drain, at least 5 minutes.

I ignore that and check cold! Always have done with any vehicle.

Ebe_Scrooge

Assuming we're talking about a good old-fashioned dipstick with no fancy electronic wizardry, I can't fathom any reason for the engine needing to be warm.  Yes, make sure you're parked on level ground, yes give it a few minutes for the oil to drain back into the sump if you've just been running the engine.  But aside from that, you're just checking the level of the oil - makes no odds whether it's warm or cold.

It's different if you're wanting to drain the oil, of course - then you do want it warm so that it drains more easily and completely.

pinkteapot

Yep, it’s a proper old-fashioned dipstick.

Next time I’ll dip it cold, then when warm immediately after the next journey, and see if there’s any difference at all in the level.

Goudy

Don't forget oil expands when hot.
It's around 10% at 100c.

So if the level is meant to be read "hot" the mark on the dipstick would account for that.
Reading it cold would show it low, when it actually wasn't.

If you think the difference between high and low mark on a dipstick is usually around 500ml and a car that takes 6 litres can expand by 10%, taking a reading cold when it should be read hot could cause you to over fill by quite a way.

Jenni_D

https://www.engineeringtoolbox.com/cubical-expansion-coefficients-d_1262.html

According to that website oil volume expands by 0.0007 per °C. Therefore a 100°C raise in temperature would increase the volume of 1 litre of oil by 0.07 litre, or 70 ml. That's 7% so your 10% estimate isn't too far adrift.

Goudy

You have to consider what what else is in the oil.

OK, you pour clean engine oil in and the guide gives a indication of how that unused engine oil reacts to heat, but after running your car it's no longer just engine oil.

Byproducts of combustion like barium and zinc, moisture, various heavy metal particles and unburnt fuel mix with it and they all expand and contract at different rates.

As this is a diesel there will always be fairly health proportion of diesel fuel in the engine oil either through combustion (passing the rings) or via post injection from DPF regeneration which expands slightly more more than straight oil (0.00083 per °C)

There will also be a large amounts of soot in the oil as well. (which is why clean engine oil goes black the instant a diesel engine is started) 

Not only will all these have an effect on the volume of the oil you poured in, these "extras" all expands and contracts at different rates so have an effect on the level between hot and cold above that of clean unused oil.