Disco II TD5 - Why wait 15sec before switch off ?

That is normal operating procedure for turbocharged engines to allow the turbocharger to cool before shutdown. Failing to do so can result in damage to the turbo premature wear especially in the bearings due to the high temperatures produced in operation . have a look at a very extreme example actually a petrol engine

Typically it's to allow the Turbo to cool down whilst still being lubricated by Oil from the running engine. This is especially important when the engine has being worked very hard prior to the intended shut down.

Look at this to see whats happening under the bonnet! (OK maybe not that hot on a TD5 but you get the idea)

To give the turbo a chance to cool down and stop spinning fast before you shut off the oil feed, it's standard advice on any turbo engine although it's usually quoted as 1-2 minutes not 15 seconds! Personally I never bother, because I don't see that this would be an issue anyhow unless you cane the s**te out of the engine then pull over to the side of the road and turn off, the turbo doesn't have a lot of momentum so will have slowed down a lot anyway unless you're really trying hard to ruin it. If you don't pull up with a screetching of tyres and a hard-revving engine then shut off the engine immediately then forget it.

So in short, just don't rev the engine hard then shut down.

If you do a search around the net you'll find that people advise this all the time and say things like "I've always done this and have never had a turbo failure" which proves nothing, few people seem to actually do it from what I've seen, and turbo failure is not endemic so if it was that important then I reckon we'd see a lot more turbo failures than we do.

I note that you're the only one to mention 'spinning fast' as well as cooling. The turbo can be turning at over 100,000rpm when stopping immediately after hard use and therefore it's important to keep the oil supply maintained until it has slowed appreciably, that's one of the main reasons for the wait. The oil supply also keeps the bearings cool until the turbo cools down. As for the 'proves nothing' in the above paragraph it's not a bad habit to get into, it would save damage if just for once you had 'caned' it just before stopping and forgot. In practice though, most drivers would run at almost tickover whilst negotiating a parking spot or their drive for instance and the turbo would have already had time to cool and slow down.

Martin

Aye. By the time you have parked up the turbo will have had time to spin down and cool even when stopping at a motorway services there is enough time for the turbo to cool as you drive up the slip road and around the car park. I also suspect that they only get *really* hot when you have your foot flat on the floor for extended periods of time.

From previous reading up on this, I think the advice about letting them cool down on idle for a few minutes has come from the track racing fraternity especially with after-market or home-made turbo installations, where the cars are caned like mad for long periods and aren't designed with good enough cooling to cope. Also some hints of it coming from any industry where a large turbo diesel is worked hard for long periods, however in those industries I've also seen advice that even non-turbo engines should be allowed to slowly cool down rather than just going from full whack to being turned off. It looks like the advice has crossed over to users for whom it's not really relevant.

For most users, the usage patterns don't look like they need special cooling-down cycles, due to either not working the engine very hard, or not being likely to *regularly* go suddenly from full beans to turned-off. If it was an issue worth worrying about for most users, turbo engines would have a woeful reliability record as they'd be scorching their turbos left right and centre.

I've also not seen my 300TDi turbo installation ever glow orange-red, something I'd see regularly on my old Lotus when caning it, on the lotus I did use to allow the engine to relax for the last few miles towards where I was stopping, mostly out of sympathy more than anything!

Oily wrote: In practice though, most

And this is why when I stop on the motorway, I always wait for a couple of minutes before killing the engine because I will typically have park without letting the engine much chance to tickover.

But you're right that on normal drive, the getting into my drive or parking slot is usually more than enough, IMHO.

Fred

It depends on the design of the motorway services. On French motorways, the small "aire de repos", which are not service places, are often such that you can park from running full speed in a matter of seconds (if it's not too crowded).

Fred

How often can you hear an electric fan go on a car which had its engine not running for a while (at least in hot places or on the motorway)? This clearly indicates that the engine's too hot (at least the coolant in the radiator) and yet is not properly cooled since there is no water circulation.

I was adviced by a mechanics that after long drives in hot weather and/or on motorway, the engine should be left running for a short while before killing it, even on non-turbo engines. So I do it.

Just my 2p worth.

Fred

A fair point but unless you are hammering down the motorway with your foot on the floor and the revs at or near the red line for several minutes the engine isn't working that hard. A decent sized modern car will probably only have 25% throttle at 70mph and thus won't be seriously hot.

Indeed.

Perfectly sensible, I do the same.

I'm puzzled by your post though, did you think I said something I didn't say, or were you agreeing with something?

Thank you all for the information. Most of the time, i will do a relaxed driving, but from now on i will count a short time before shutting off to give my Disco time to relax und cool down.

Again, thank you for your help. Kind regards from Austria Reinhard

As other people have said, just taking it easy as you get close to home will help.

In my turbo car i always get off the boost for the last mile/half mile. its fairly built up anyhow so you cant really use the turbo much then!

Does the TD5 not have a water cooled turbo? saab turbos changed from oil cooled to watercooled in the mid 80's as it increases turbo life and stops you having to worry about letting it cool when stopping.

Sorry, no I was just adding some more to it, just to reinforce ;-).

Fred