Checking your engine oil, warm or cold engine?

My dad always told me to check when the car was cold as this gave a true indictation of how much oil was in the sump, but then it seems that a warm engine is the 'norm'.

What do you do, and if warm is it ok to check just after switching off, ie normal running temp, or leave it to cool abit?

PS I haven't ever run out of oil, so can't have been doing much wrong for the last 17 years…

Always cold and on a level suface.

Where is a warm engine the norm?

Yes, cold and level I believe.

Cold. Warm is never the norm, your oil is off round your engine in an unknown quantity. The only items that are normally tested when running/warm are some cars auto box fluids.

I've always checked when the engine's cold.

from asking what other people do…one explanation was that it then shows how much is left in the sump after the engine has coated itself while running…which seemed a reasonable point of view.

I've always done it when warm as the oil thins. Maybe it's different with classic cars.

Hot or cold it makes no difference to the volume of oil in the sump. The problem arises when you check the oil too soon after running the engine as the oil will have been pumped into all the oil gallerys and the cylinder head, and it takes a little time to work its way back down to the sump. 5 minutes or more after stoping the engine should be sufficient time to wait, and obviously on level ground.

Its irrelevant whether its shows how much is in the sump when the engine is "coated" or whether the oil is thin. They are designed to be checked when level and cold, that is to say the manufacturer expected you to check it cold and so set the dipstick levels at the right height for cold oil in the morning. The correct amount is the max mark when level and cold, any higher and you risk problems, much lower and you risk bigger problems.

For those working on the warm theory ask yourself this – last time you turned it off was it not warm? And therefore all that nice warm oil slithered down to the sump where it now resides.

Grimy – at 90C your oil is nearly 6% greater volume than when at 0C.

changing the oil when it's warm (though only just warm!) – yes. checking the level – cold on a flat surface.

if you check the oil just after the engine has run, a lot of it is still round the engine and you'll end up over filling it, which can actually cause quite a bit of damage in some engines

Likewise changing the oil when warm just allows the sump to drain a little faster, it's a time saver. It doesnt "get more" from around the engine as some people believe (not suggesting you do CP).

cool, seems like i was right for once (well my dad anyway!)…nice to get a consensus on here 😉

You should check engine oil level when warm-just after switching off.
Particularly important if you have an apollo tank(air oil separator) or a dry sumped fitted

I always used to check when cold.

However, there's a sticker under the bonnet on my 9-3 which says get the engine to "normal" operating temp, let it cool for 10 minutes and then check (ie not stone cold).

then someone goes and spoils it all…:-)

good night!

MikeWW – thats simply not true and not useful for the vast majority of readers. 99.9% of standard vehicles are designed to be tested when cold and level, checking any other time will give a false reading. For example, on a subaru with the boxer engine even the manual suggests checking when warm will give a disparity of over 1" on the dipstick. Dry sumps are a different issue entirely and not related to normal car engines.

My merc vito's have muti function dash displays which include an automatic oil level check. It also comes up with a big message saying unable to check oil level if the engine has been running. Has to be cold.

Coffeeking
cxi and Saab not true either then?
I guess it depends in reality

edited to add:

Some of the engines I've had its been particularly critical to check when warm as with a shallow sump and oil surge it can get expensive if you get it wrong. On the one hand you have oil expansion but on the other its the amount of oil that drains back to the sump especially if the engine has been stood for a while.

Hot or cold, as long as youve given the oil suitable time to drain down to the sump your going to get a neglidgable difference in readings, especially on a dip stick which is hardly a suitably acurate means of mesurment if your worried about 6% expansion to freezing temperatures.

Oh, and given the tractor lump in my 9-3 is a GM / Fiat engine share, I would suspect the same oil-check advice applies to a whole pile of Vectras, Alfas and Fiats.

I'm sure I've checked the dip stick when totally cold and the oil doesn't even show up on it. Which would be worrying if the advice wasn't to check from warm.

Mike – That depends on the manufacturers suggestion obviously, these are not the usual suggestions I've seen on countless car handbooks over the years. Regardless, 10 minutes is sufficient for your oil temp to have halved from operating temp based on measurements on my own car.

Grimy – not sure what other means of measuring you expect to be more accurate? 6% is a fairly large amount as your dipstick only shows a fraction of the volume, min isnt zero and max 100%. On a gallon of oil you're talking ~250ml of oil heigh difference. On my two cars that's a good half a max-min distance and around half way on the dipstick makes a notable difference in engine note on at least two of the cars i've owned.

Reading the above i've got to accept that my previous statment would perhaps only apply to conventional engines. I cant explain the phenomenom cxi explains, where no oil shows up until warm, thats weird, If i saw no oil on the stick from cold, I wouldnt even turn the key. Boxer engines although as perhaps as rare as dry sumps, may also be different due to there unique layout, I can see why the oil would take considerably longer to return to the sump. I guess youve just got to go of what your perticular manufacturer recomends.

Grimy – not sure what other means of measuring you expect to be more accurate? 6% is a fairly large amount as your dipstick only shows a fraction of the volume, min isnt zero and max 100%. On a gallon of oil you're talking ~250ml of oil heigh difference. On my two cars that's a good half a max-min distance and around half way on the dipstick makes a notable difference in engine note on at least two of the cars i've owned.

Bugger, I got that one wrong in my head and you even saw my mistake, I assumed 6% over the length of the gauge. I stand corrected! lol.
I know what you mean about the oil levels influance on engine note, Its very noticable on my motor too.

Porsche 911 (the proper air cooled ones) – check the oil when the engine is fully warmed up and running 😀

Ok checked with a friend who works in powertrain in Italy and it should be when the engine is warm (water temp of approx 90 deg c) but the vehicle has sat for at least 10 mins to allow the oil to drain back into the sump

On some cars you can get a false reading if the car isnt totally level (daft). My Forester is one of them.

On some cars you can get a false reading if the car isnt totally level (daft). My Forester is one of them.

I think that's called the law of physics.

I've spent over 20 25 years checking oil 5-10 minutes after turning off a warm engine…. That's what I was always told!!

Rationale – you never check the oil level on the first pull of the dipstick. If the engine is cold and the oil quite old and gungy, it might have congealed / be quite viscous in the dip tube, so wiping the dipstick and sticking it back in the dip tube might give a false reading if the oil is not warm and fluid in the sump.

This may not be as applicable in today's world of modern synthetic oils etc…

Drac that would bethe Law of Gravity. Prof Hawkins still hasn't finished his unified theory yet! 8)

Well it seems the question is still open then. I'm going to have to think on my position and gather more evidence to review – I could well have been going on mis-information for many years! I'm off to RTFM and see what I find. I know that if my pug is < full (when cold) it rattles like a can of bearings, if I check that when hot the subsequent cold reading is almost off the bottom of the min mark.

On Fifth Gear this week the bloke said always check it when warm. I always thought cold, though.

Just checked my Toyota book

With the engine at operating temperature

I've always done it cold. However there was a news article on the other day about how many cars go kaput a day due to lack of oil and on that article they suggested measuring when warm.

ITs impossible to test it with the engine at operating temp – by the time you've waited for the oil to fall it'll have cooled, and by some unknown quantity depending on ambient temp and length of time left. Seems like a very silly way of measuring to me.

how many cars go kaput a day due to lack of oil and on that article they suggested measuring when warm.

Measuring when warm is MORE likely to give you a lack of oil, not less. It'll give you a "false high" due to expansion (assuming you've allowed it time to settle back to sump correctly).

I understood it as if it's cold you can check it but if the engine has been running you have to wait a few minutes for it to settle down. At work the engines are never cold and are checked every shift not been a problem.

Ultimately providing you keep it up near max, cold or hot, you'll have no problems unless you operate on the raggedy edge of thrashing and cornering.

TBH – I never check mine
It gets changed at service time & unless the light comes on, I don't bother with it

scary but true 😀

you'll have no problems unless you operate on the raggedy edge of thrashing and cornering.

Certainly are.

each manufacturer will always have a different way of checking your oil level. If you follow the manufacturers guidelines of your car you cant go wrong. Dont do it the way someone tells you check for yourself to make sure you do it right