I read HJ's comment on the need to let the engine idle on a Audi 2.5d. in the Telegraph today. How long should you do this for to prevent turbo wear? I am now sold on diesels after 14 months with a Xantia 1.9 turbo. I hanker after an Audi A6 2.5 diesel Avant Would I be better of with a Mercedes diesel if I were to upgrade to the "prestige" class. Trouble is the price of the 300 and 320 diesel estates may me wince compared with the alteratives.
Peter
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Hi Peter - It really depends on how hard the turbo has been working, and how long you spend slowing down. When it is working it turns at a very high speed, and as the shaft floats on a cushion of oil, it's necessary to keep the oil under pressure until the shaft stops. When the engine runs at less than 2000rpm, the turbo is not working.
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Diesel turbochargers run much much cooler than their petrol counterparts. Therefore the spin/cool down time is not as critical.
Turbos on todays diesel vehicles are indeed working as low down as 1400 RPM to give the required ooomph from a standstill.
But, if I had driven the car hard, I would allow the car to idle for 30 secs or so to let things slow down and stabilise.
Peter - go for the 2.5TDi Audi - just make sure you but a late one (V reg onwards) as the early ones suffered from some sort of major engine problem.
I just don't relish paying for/fighting for warranty repairs to this complex engine!!
Great cars to drive although the economy isn't altogether great (35MPG)
The 6 speed gearbox - you'll either love it or hate it. Auto transmission is nice with this smooth engine.
Both the build quality in the Mercedes and Audi are not what they used to be years ago IMHO. Corners have been cut to make the cars more affordable I guess.
Kind regards
David
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Agree with David Lacey's comments here. The service conditions are so hard turbos use fully floating bearings to cope with the ultimate shaft speeds and temperatures. Again as David says diesel turbos run slower and cooler than petrol.
Having used charge cooled diesels as everyday transport for about 10 years now with no trouble in the turbo region, I probably allow slightly longer idling than David says, but in the worst case scenario an idle time of two to three minutes seems to suffice. For definition of worst case scenario read hot day, long fast motorway run and straight into service area car park. That time is reduced only if the previous few miles have been at light loading and low speeds or, perhaps, if the desperation for the loo is uppermost.
Having seen hard worked petrol tubos at cherry red temperatures, ie approaching 800 C ish, I think 2 to 3 minutes in this case is hardly long enough. If the casing is glowing cherry red what temperature is the rotor at? Mind you I think most turbos today are either oil or water cooled.
I recall two systems designed to combat this problem with keeping oil pressure at the bearings during spool down. Cannot remember now whether OEM or aftermarket fit. One used a small electric pump and timer, the other used a small accumulator bottle.
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Julian
You hit the nail on the head there!
Clean engine oil is they key to a happy engine, especially Diesels.
I'm not aware of the onboard service indicator system on the Rover 75 having any leeway for diesel turbos, only the service interval is shorter (12K instead of the petrol 15K)
Audi shoddiness began with the A4, I think. My old 80TDi was great. The A4 TDi I have now, is not so great.
A Mercedes A Class I used for a day or two - AWFUL!!
Rgds
David
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I agree about the Audis. Our 80 seems much more solid than our A4, especially the interior.
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You can still get these from Demon Tweeks.
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I totally agree with David Lacey regarding Mercedes, they most certainly are not what they used to be.The dreaded cost cutting syndrome bites!
No offence meant but I have always considered Audis to be VW in Drag!
The Diesel Magazine is a worthwhile read to assist in choice.
regards
Simon
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Mitsubishi GTO/3000GT has one of these, possibly some other Japs too?
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a lot of good car alarm/immbolisers these days have the option of a turbo timer which will keep the engine running for a pre determined time after you turn the ignition off and lock the doors. You can walk away and leave the car to turn off on its own.
why dont OEM sell cars with features such as this?
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Probably because I believe it's illegal to leave a car unattended in a public place with the engine running.
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The vectra TD Ecotec engine has a water cooled turbo and no mention is made in the handbook regarding the above.
However my wife's Maestro turbo diesel handbook gives instructions saying that upon starting the engine allow it idle for about 10 seconds to allow oil to reach the turbo and I think it says about 14 seconds to allow the turbo to slow down before switching off.
In the diesel car magazine there was a two page article by the head of a company who is the leading stockist and repair outfit and his advice was that there is little need to allow it to idle on shutdown unless you had been at high speed for a long period. As he said if your'e running down a motorway and turn into a service area it takes a couple of minutes before your'e actually parked which does the same thing.
However I always let the cars tick over for 20 seconds or so before driving away, and on return its always done the same as I reverse up the drive which allows it to slow down.
Alvin
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Not the first time I've heard of recent Merc/Audi shoddiness. When did quality start to decline? This could be vital when considering buying used.
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David,
All good stuff here.
One point that I would like to raise is the subject of oil degradation and its relationship with the Turbo seal failure. Diesel units are past masters at carborising their oil quickly, and I would imagine this would exacerbate the problem, particularly with bods who carefully follow the computer generated sevice intervals on their cars. Perhaps diesel engined cars have this issue configured into their onboard systems to allow more frequent oil changes, I'm not sure of my facts here.
Regards,
Julian
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Hi Julian - How right you are on the subject of diesel oil degradation. We have four diesel engines in the family and never do they go more than 5000miles without new oil and filters.
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Thanks everyone. Having recently become the proud owner of a my first turbo diesel I knew that you have to let the oil cool. The question was - how long ? I'm pleasantly surprised at how short a period of time it is. Any long, high speed runs tend to be followed by bumbling through town, so as long as I can avoid motoway service stations it sounds like there's nothing to worry about. I'd just like to add how useful and helpful this "phorum" is.
Regards,
John
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