Cold start damage - why?

[MearCat]

I've read a few discussions in the forums on how long people idle their car for (if any) before they drive off, but there's no discussion on WHY it's better to idle for a few minutes or just drive straight off...

Now for the question : I was talking to a thermodynamics professor a while ago and he mentioned that while it is true that oil doesn't fully protect your engine until it is at operating temp, your car is better protected if you drive off straight away and get it up to temp as quick as possible (as long as you don't rev the engine too much); instead of idling for a few minutes to warm it up at the first start in the morning.

Whether you have a turbo or not is irrelevant for this question.

Comments?

[carlo]

I cant give you the technical reasons, but I have been told during discussions with a engine builder that once the engine has full oil pressure you should start driving it. Gently of course (off boost).

I would guess this allows for the oil to start flowing properly, heat up more evenly etc.

[MearCat]

At what point does the engine reach full oil pressure? Within a few seconds of startup? Or once the oil has warmed up?

[Demon Dave]

Basically, before you first start your car from cold (say in the morning) all the oil has drained down into the sump. It's at ambient air temperature, so it's going to be pretty viscous and not likely to flow freely...

You turn the key and start the engine. Since the oil pump only works when the engine is running, and it takes time for the pump to pump cold oil from the sump up into the valve/cam area (and throughout the engine block)

So, you can see that for a certain amount of time the engine is running without any oil!

The exact time it takes for the oil to start flowing freely through the engine depends on many factors - pump efficiency, oil grade, oil temp, climate and the size of the engine. I remember reading that on some older american muslce cars it could take around 2 minutes for the oil to start circulating around the block!!

Since an engine running without oil is BAD, the best thing to do is minimize the wear by not driving off imediately after you've started the engine...Basically you're giving the engine time to become fully lubricated before you put any pressure on it.

Ohh, and turbo motors are even more susceptible to this since the turbo is sitting right on top of the engine - far from the oil pump...

That's how I understand it anyways...

[SmoothLine]

Usually reaches ful oil pressure "about 4 bar on a skyline' after 5-10 seconds, after that u can drive, but as carlo said, keep if off boost, ie under 3k rpm

[Demon Dave]

Additionally,

you may note that very soon after turning the key the oil pressure is going to rise to quite a high level (say above 4kgcm) but that is mainly due to the fact that the pressure switch is located lower down on the block...

[Zahos]

After about 15 mins of driving, my oil preasure drops to about 2 Bar, sometimes a little less (on idle that is), while driving it is typically around 4 Bar. I assume this type of behaviour is correct?

I always let mine warm up at least 1 minute, revving it a little (2000rpm - and not continously), so oil can circulate.

[Grim]

yeah i notice that within 10 seconds of turning the key and starting the engine my oil pressure reaches about 4-6bar (depending on how cold it is). After those 10 seconds I'm happy to drive off and after a couple of minutes driving the pressure drops to about 2 bar as the oil heats up. I think most of the damage is done in the first few seconds after startup. Once oil pressure is up then the engine is fully protected.

[Guest hooey]

I used to work in a generator factory and in my experience, running an engine unloaded is a slow way to get any heat into it. Therefore the quicker you warm it up, the quicker the oil heats up. Driving it quietly until the temp needle moves up to running temp is my guess.

[benm]

I let my car idle for whatever time it takes to start it up, reverse out of the garage, shut the side garage door, get a popper out of the fridge in the garage, put the roller door down, get in, put my CD Face in, window down, seatbelt on and get comfortable......

about 2 mins

[Ronin 09]

There are many schools of thought on the warm up issue. Some people like to warm their engine up till the temp gauge rises, and then drive off, ensuing their engine is protected.

HOWEVER, you also must remember that the diff and gearbox oil are NOT at optimum operating temp from cold, and even though your engine is warm, doing burnouts etc with cold gearbox / diff is a recipe for a large bang - trust me, i've blown up a number of diffs in my time, a couple backing out and squirting it too hard (not a skyline though).

So, while i think it is important to warm up a bit before driving off (say 2 mins), you should really take it easy for the first 5-10 mins of driving time (under 3000rpm) to make sure diff and gearbox are happy too.

my 2c

[Demon Dave]

Agree with you on that.

In the past I thought I had a problem with my gearbox - difficulty engaging second gear. Turned out that I just wasn't given the gearbox oil enough time to warm up (especially in winter). Not moving, or moving a slow speeds isn't going to warm up the other oils in the car....

[WHITE R32]

okay well from my experience with my old torana race car.... i was young and silly and i had a mate i had to impress so i started it and revved it pretty hard after a little while of idling... about 8k.... went for a spin and wemt to like 8.5k several times and when i put it back in the shed it was fine ... when we went to take it to the drags the next wekend it developed a very bad miss...we changed coild ,plugs,leads,everything......ended up been a broken valve spring......all brand new done like 5 hours on the dyno......was broken from it not been warm enough and the wire in the spring not been ductile(flexible) enough which cause it to break...personally i would rather letting it get to operating temp before driving....reason been that the oil may not be at full protection till operating temp but it has no load on it unlike driving the car......hence idling it will not stress anything till it warms up and you take off..... i alwats warm mine fully before driving.......