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how long should i let my vw tdi idle after a motorway run? secondly, at what point does an engine reach maximum temperature; does a 3 hour motorway blast make the turbo significantly hotter than 30 mins?
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Personally speaking, my feeling has been prolonged usage warrants a few minutes at journey's end to give the turbo oil a chance to cool especially if it's straight off the motorway to a stop at services.
If there's some town usage afterwards, my attitude is a few miles poodling about should be enough to give the turbo a chance to cool. The thing about a motorway blast is higher engine revs and also turbo revs.
There has been a thing with turbo's cooking their bearings if stopped hot without the oil to cool things down.
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For most of the time when I am driving my C5 round town, I barely touch 2000 revs. Crusing at 30 or 40 is done at around 1400 rpm in 4th or 5th. Note the very torquey low down 8v HDI engine with PowerBox.
Compare that with motorway where 75mph would be 2500 rpm and that would be a constant speed for an extended period.
The motorway driving takes the oil temperature guage much higher than poodling around town, but after a few miles in town and traffic light stops the oil is back closer to normal town driving.
At a motorway service station, if driving with passengers and one doesnt want the loo for example, I would leave the engine running while I pop inside. I would never dream of stopping the engine within seconds of doing 70.
Also, HJ recommends fully synth oil changed frequently, I feel safe with that too.
Ben (wouldnt consider a petrol)
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What does your handbook say about this? My Vw manual (1993 Polo)states that:
"whan the engine has been running fast for a long time, let it idle for 2 minutes so that it can cool slightly before being turned off"
Have a look in yours, it'll probably be near the front in the area of starting and stopping your engine.
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have a look at
www.honestjohn.co.uk/forum/post/index.htm?t=10868&...f
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I always found the advice about allowing the engine to idle etc.. a little strange. In my experience, the sliproads off UK motorways are usually long enough or crowded enough to allow the engine to run under 2,000 rpm for at least 30 secs or more after leaving the motorway.
Some exceptions on M1 and M6 but how often (except at night) can you enter the sliproad and park in under 30seconds? Not often IF you keep to the sliproad speed limit.. imo.
madf
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Nothing in my Xsara 1.9TD handbook about cooling off, but it runs at 80C no matter what I'm doing - motorways are conducted at 3000RPM at 75MPH. It's rather high, but bang in the middle of the turbo range. Presumably, it develops most torque at this engine speed.
I usually allow it to idle for ten seconds before switching off - the manual does say not to blip the throttle before switching off because the turbo bearings can then run dry as it spins down with no oil.
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Remember that oil temperature is different to water temperature, and when on the move, they can move almost independantly of each other.
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That's why all Ford turbos-petrol & diesel-are fitted with oil coolers;usually Modine(water/oil).
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Diesel engine exhaust gas is considerably cooler than that from petrol engines so idling to allow them to cool after a long run isn't such a big issue. A petrol turbo needs idling for a few minutes after hard driving.. 30 seconds should be enough with a diesel IMHO.
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Thats not what the wifes Toyota Corolla manual states. It talks of a minimum of 30 sec & up to 2 minutes after a long fast run and even gives example speeds, this is for the D4D.
As it is having spent £ 600 on a new turbo for the Alfa JTD and the fact I drive it quite hard I always give it 1-2 minutes before turning off and I'll have driven very gently for the last mile or so. This includes short shifting so the revs stay low. I DON'T intend changing this one!
Also as someone pointed out water temp and Oil temp are two totally different things and one can be high and the other low, so the water temp gauge is irelavent.
Jim
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