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In another thread, Cyd said "Do you drive off immediately after start up? Try letting the engine idle for 5 to 10 seconds each time you start up before driving off. It does wonders for engine life." The "5 to 10 seconds" notwithstanding, that's an interesting comment in the face of advice from all the car manuals I have ever read, which was to get moving immediately ? cold idling being "incredibly damaging", if I recall correctly the words of a Ford engineer as reported by HJ some time back. I'd be very interested in further views on this, from Cyd and others. Perhaps 5?10 seconds doesn't matter after all.
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Driving off straight away is the standard advice. I give it a few seconds for the oil to circulate first.
But the manual for my new Yanmar marine diesel says it is very important to let the engine idle for 5 MINUTES before increasing the speed or engaging gear, to allow the oil to circulate. It warns of severe damage if this procedure is not carried out.
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Cyd is completely correct. Comments in car manuals refer back to the days when the practice was to leave the car running on fast idle for 5 to 10 minutes to allow it to warm up. It is regarded these days that there is no need whatsoever to do this as all it serves to do is waste fuel. Car injection management systems and modern day carburettors do not suffer problems when driving the engine from cold. You should always leave an engine 5 seconds minimum before you drive away. Why? Because overnight all the oil in the engine drains down into the sump and it takes a few seconds for the pump to pressurise the system and cover everything fully in oil once you start the engine. Sure, the car will not seize if you immediately thrash the car away, however you're hardly doing anything to promote long engine life by doing this.
Always makes me laugh when you see road testers, for example on Top Gear, start the car and immediately go screaming off it it...
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and immediately go screaming off it it...
Which is the other way of killing an engine quickly. Go gently until it's properly warm.
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For the first ten seconds the engine will be on fast idle anyway-after that drive as normal.
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What the old manuals are refering to is letting your car stand for 5 to 15 minutes whilst the engine warms from cold. The effect this has is to allow the excess fuel in the enriched mixture to wash down the bores, in turn washing the oil away causing excess wear to occur whilst the engine is stood idling. The oil in the sump is also gradually thinned and degraded by this process if regularly repeated, which in turn leads to more wear. This problem is less pronounced with modern injection systems, since the mixture is much better controlled during the warm up phase and oils are much better at resisting this damage. Nevertheless, allowing the engine to warm up whilst idling is still generally accepted as bad practice.
This is not the same, however as allowing the engine to idle for 5 to 10 seconds after start up to allow the oil to circulate. After a hot engine has been shut off, pretty much every last drop of warm oil sinks back into the sump over a period of just half an hour or so. The instant you turn the key in the morning and the engine fires into life, all that oil is still in the sump. It takes the pump a good few seconds to fill all the oilways and get a good supply up into the cam/rocker area at the top of the engine and also to get a good spray going into the bores. If you put the engine under load the instant she fires, then the cam, valves, bores, rings & other parts are under load with no oil to protect & cool the loaded surfaces.
One major component of many modern engines that is especially sensitive to this type of abuse is the turbocharger. If you look in the manual of any turbo car it will advise you to allow the engine to idle for 10 secs before driving off. It is vitally important that the turbo bearings have a good supply of oil under pressure before the turbine speed rises above idle because under normal running conditions the shaft runs in an oil film no thicker than a hair. If the shaft speed is allowed to rise rapidly without this film present, then massive wear occurs.
It is also good practice to drive gently when the engine is cold to allow it time to warm before being put under load. This is primarily to do with the change of viscosity of the oil and expansion of metal parts as the engine warms. The engine is designed to run properly and be properly lubricated at a temp of 80C (ish). It takes a good few miles for stable running conditions to be reached, after which the wear rate is so small as to be virtually zero. Untill this point is reached the wear rate drops down a curve as the temp rises. Have you ever noticed how much better she runs down those lanes after a moderate run? This is why taxis, buses and marine engines last for yonks, whilst the car you use to go back and forth to work in is knackered after just 100k or so. Engines that are rarely allowed to cool to cold just run for ever.
The same considerations for oil circulation in the first 10 seconds also apply to the gearbox and PAS.
Phew! Hope this helps.
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With an autobox, give it a little while (10+ seconds) for the oil pump to pressurise before selecting a gear.
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Thanks, Cyd ? excellent response.
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Cyd - is the rule the same for diesel engines, where presumably the rinsing behaviour on cylinder walls may be different?
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>>Cyd: all that oil is still in the sump.
Presumably not all of it. For after changing the oil in the engine, one is particularly exhorted not to rev the engine until it has been running for a bit to let the oil circulate - whereas when starting an engine it is not so important.
I presume that Cliff's non-return valve in the oil filter is coming to play here? (Which is why the filters where you only change the paper bit, and then soak the replacement paper bit with new oil, like on a W123 MB seemed to me like such a sensible way to build an oil filter.)
Personally, I like to start the car, and then put on seatbelt, check mirrors and radio and then move off (or whatever you need to do to occupy 10 seconds). It guarantees those few seconds to get the moving parts lubricated.
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Oh and thanks Aprilia - I noticed when I selected drive too quickly that it was a horrid experience, never again - that's why.
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Okay, so some of the oil is in fact held in the oilways by the non return valve. I tried to give a Plain English, easily digested answer that could be read by the non-technical with a decent degree of understanding. If one wanted to, one could have gone on for pages I'm sure.
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I'm not really a materials engineer and so cannot go into detail about how different fuels react with oil. But in general, I think it is safe to say that petrol will desolve/thin oil whereas diesel will mix with it. Therefore the immediate effects of fuel wash on the bores during cold running are likely to be less severe with diesel than petrol. I do know that desolved petrol breaks down the polymer chains that make up the oil, hence the degradation over time. Whether this effect is less or more with desolved diesel I'm afraid I don't know.
I would expect, however, that the benefits of idling for 10 seconds are the same for the rest of the engine whatever the fuel used.
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Everything you say Cyd makes sense, and fits with my own long understanding and practice. The only puzzle is why Yanmar should be so insistent that their marine diesels must be idled for 5 minutes, on pain of severe damage. It can't take that long for the oil to circulate?
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>>Yanmar should be so insistent that their marine diesels must be idled for 5 minutes, on pain of severe damage
Aren't Yanmar Japaneese...I suspect the handbook for this engine was written using Japaneese English and they mean idle for a few seconds.
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The air to fuel ratio at idle in a cold diesel engine is maybe 60:1 compared to perhaps 12:1 in a modern petrol engine at cold start so there is much less fuel available to wash off the bores. Diesel fuel also has a far greater lubricity than petrol and the spontaneous compression ignition process ensures that virtually all of the fuel injected is burned.
I remember that until recently Peugeot used to recommend allowing their diesel engines to "idle for a few moments" before setting off.
The fuel flow rate on an old car with carburetor and choke was typically about 6 times that under hot idling conditions because so much fuel would condense on the cold inlet manifold walls and not evaporate and form a combustible mixture which then washed oil from the bores. Same process happens with a modern port fuel injected engine when fuel droplets form on the inlet valve and port walls, but the amount of fuel caught up in this way is much less.
So bore wash isn't an issue with diesel engines or to a lesser extent modern petrol engines like it used to be with petrol engines of 20 years ago.
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