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VW Jetta TDI

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  • alltorquealltorque Posts: 535
    edited August 2010
    Nothing to do with the TDi engine, specifically, just good practise for any turbocharged engine. Pulling into a service area, say, after a run down the motorway, (Interstate), the turbo will be very hot, (I've seen turbos on test-bed engines get to pale yellow and certainly beyond cherry red). The bearings are very small and only contain a minute amount of lube oil. They rely on a flow of oil to keep them cool, (relatively speaking). Just switching off the engine stops the oil flow and even the best of lube oils will eventually form coke, choke the bearings and .......... BANG ...... goodbye turbo. Allowing the engine to idle for 1 - 2 minutes allows the turbo to cool and fresh "cool" oil to flow through the bearings, so no coke formation.

    If, prior to reaching your destination you have a period of gentle running then the turbo will already have cooled sufficiently so this idling period is far less important.

    Idling before switch-off is a good habit to get into to help ensure your turbo enjoys a long life. The effects of abuse won't show up for quite some time but they will come and bite you - or the next owner - one day.

    AFAIK this idling procedure is strongly recommended, here in Europe, by all the manufacturers using turbos, whether gasser or diesel. VW, Audi, Skoda and SEAT certainly recommend it and everyone I know with a turbo engine uses it............and the vast majority of them drive diesels; Audi, VW, Vauxhall (GM), Ford, SAAB, BMW et al.

    Just my two penn'orth.

    ETA : I was writing my post whilst Colin was posting 4252. His is a better tech post but I think we're both singing from the same hymn sheet. Amen to the use of synthetics. The VW TDi's are fussy about lube but for reasons other than the turbo.
  • bpeeblesbpeebles Posts: 4,085
    You are mistaken! VW does not use "watercooled" turbocharger on TDI. This is one reason that synthetic oil is MANDATED for all VW turbocharged engines.

    Allowing turbocharger to cool down before turning off engine should be standard procedure.

    Note: This 'cooldown' can be as simple as driving gently for a mile or so. This is how most people drive anyway before arriving at their destination. The real concern would be pulling into a 'rest area' immedeatly after traveling 65+ MPH... this is when a specific cooldown should be performed.
  • m6userm6user Posts: 2,952
    This could be a dumb question but isn't traveling at a steady speed on the interstate at a low rpm somewhat running the engine gently? Is the turbo fully engaged when rolling down the interstate? I thought it was for boost under fairly heavy throttle.
  • bpeeblesbpeebles Posts: 4,085
    edited August 2010
    You are correct that the turbocharger gets hottest while under 'boost' However, NO ONE drives under 'boost' for more than a few seconds at a time. (boost = accellerating)

    I have a turbocharged Subaru... if I keep it under 'boost' for more than 12 seconds, I would be traveling over 90MPH. (I love the look on my wifes face when I pin her to the seat with raw accelleration)

    The heat in the turbocharger is directly related to the amount of exhaust-gas flowing thru it. While traviling on hiway at 65+ MPH, there is a lot of hot exhaust rushing thru the turbocharger. The oil-flow over the bearingis the only thing keeping the bearings alive. If you pull into a restarea and immedeatly shut off the engine. A phenomanon known as "heat soak" occours. You have stopped the flow of cooling-oil over the bearings while the turbocharger housing is still VERY hot. This latent heat can literally cook the stagnant oil which is sitting in the bearing-housing. This is called 'oil coking'

    On the other hand, if you get off that very same highway and drive a few miles to your destination... this allows enough oil to flow thru the turbocharger housing to carry the heat away. There is much less latent heat to 'soak' into the bearings.

    All VWs (turbocharged or not) have an oil/water heat-exchanger. This allows the hot oil to 'dump' its latent heat into the antifreeze... which then carries the heat to the radiator so it can be eliminated.

    VW uses OIL COOLED VNT (Variable Nozzle Turbocharger).... this type of turbocharger does not have a "wastegate" to route the hot exhaust gasses around the turbine. Instead, the VNT flows ALL the exhaust thru the turbine at all times. This is more efficent... but creates more heat too.

    Hence - I ALWAYS idle my TDI for 2 minutes if I pull off highway into a restarea

    Does this answer your question?
  • m6userm6user Posts: 2,952
    Yes, nice explanation. Is this somehting that VW points out in their owner's manual to do if it is intergral to engine or turbo life?

    Also, this is something that I wouldn't want to worry about everytime I pull off the interstate to a rest area or to get a bite to eat. It seems like a small thing but kind of a pain. I like to pull in and jump out especially if I have to use the john. I know it's a small price to pay for great mpg but those small prices start to add up...higher fuel cost per gallon, longer warm up for interior heat, loss of heat in stop/go traffic, oily hands at fueling, synthetic oil mandatory, hunting for stations that carry diesel(many don't in metro areas), higher intitial cost for vehicle. These things do add up to an overall experience which may or may not be fully offset by the money saved on fuel depending on the person. Just my thoughts.
  • fho2008fho2008 Posts: 393
    You could carry a second car key so it can idle and be locked. I'd love a TDI, hopefully someday, and there are two stations in my town I know that sell diesel, and a third that might, I rarely go to em.
  • eliaselias Posts: 1,900
    peebs, i'm pretty sure that for my x-country TDI cruises, the turbo was boosting for hours/days at a time. I think boost is necessary to go at highway speeds in desolate areas of I-80 (~100 mph).
    in these sorts of drives, ABSOLUTELY i would let the engine idle for maybe 30 seconds or a minute at most to help turbo cool down at a rest area after hundreds of miles of highspeed driving.
    I don't bother with a cool-down delay for the turbo on east coast highways since the speeds are closer to 75 than 100.
    ps - Those rest areas (outhouses) in the I-80 nevada desert are no prize.
  • fho2008fho2008 Posts: 393
    If it came with a boost guage (or you put one in yourself) we would know for sure, but as far as I know you use the turbo only when accelerating, not going along at a steady speed.....even 100mph or more.
  • eliaselias Posts: 1,900
    edited August 2010
    fho2k8 ,
    at each increasing steady-speed point, once a certain road-hp level is required of the engine, the turbo will be boosting from that speed all the way up to top steady-state speed.
    if that's not the case, please elaborate, and i'll make a note of it and will wake up Isaac Newton to let him know! :shades:
  • colin_lcolin_l Posts: 591
    Are you certain of that? What model number is the turbo... I seriously doubt a variable vane modern turbo would not have a watercooled center section.
  • andys120andys120 Loudon NHPosts: 16,593
    If it came with a boost guage (or you put one in yourself) we would know for sure, but as far as I know you use the turbo only when accelerating, not going along at a steady speed.....even 100mph or more.

    Very little power is required to maintain steady speeds on level roads which is why this type of cruising delivers high mileage even at high speeds. I can tell you after 100,00+ miles of watching the boost gauges on Saab Turbos that they only deliver boost above 3000RPM or so, way more than you'd cruise at.

    I can't recall whether the TDI Sportwagen I drove had a boost gauge but there was so much low RPM torque that I'd be surprised if the turbo was on boost except when you accelearate.

    2000 BMW 528i, 2001 BMW 330CiC

  • fho2008fho2008 Posts: 393
    We basically said the same thing.......the turbo is used for acceleration (and fast acceleration at that, not gradual, unless going uphill), not when traveling at a constant speed, even if you are cruising at 80 or 100mph.
  • dwpcdwpc Posts: 159
    edited August 2010
    The potential result of a hot-turbo shut down is "coking"; oil flow stops in a near red-hot turbo and the oil bakes to a solid in the turbo's oil passage. Eventually, no oil gets through and the turbo welds itself together. At turbo speeds of 100,000 RPM, failure is instantaneous and very expensive replace.

    I've driven turbo cars for 30 years and its in my driving routine to idle for a few seconds after starting before loading the engine, and for about 30 sec. when I shut down. I did with ancient MB turbodiesels and I did it with my bi-turbo gasser Audi too, though it had water-cooled turbos and syn oil, and even a after-run waterpump to cool hot turbos after the engine was turned off. Its just a prudent habit with worth-their-weight-in-gold turbos. Turbos have better technology now, but are also a lot more complex than old school simple turbos (on diesels) that would go forever without much worry if you watched out for coking; I put 450K miles on one in a Mercedes. I think a few seconds at idle before shutdown is a good investment.
  • fho2008fho2008 Posts: 393
    Agreed, but as as said before by others, with newer turbos, most people are not using the turbo the last few miles before they get home, so it has plenty of time to get oil circulated, "cool down" etc.
  • eliaselias Posts: 1,900
    ~100 mph steady-state in my VW TDI requires "wind-out" RPM - 3500 rpm or so (?) - seems like we agree plenty of turbo at that RPM.
    100 mph is a moot speed here on the east coast, but was fun travelling fast like that in the desert. again, in the desert, at 100 mph, i sure did let the turbo cool down after cruising for many hours. and i have no doubt the turbo was working all those hours except maybe on some downhills. air resistance is incredible at high speeds; required "road horsepower" from the engine increases exponentially as speed goes up.
  • kyfdxkyfdx Posts: 27,671
    I wouldn't assume that turbo use is a function of RPM..

    Once your car is up to speed, even 100 mph at 3500 rpm, your throttle position will be the determinant of how often the turbo is on... If you just need slight throttle pressure to maintain that speed, then the turbo will likely be taking a break..

    Of course, at that speed, even slight uphills will cause you to mash the throttle to maintain speed... but, on perfectly level ground? I think little turbo action...

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  • m6userm6user Posts: 2,952
    Is the turbo being used the only determinant in the amount of heat is present, hence a cool down period? Does VW recommend this cool down period in their owners manual? If not, why not if it is a proven method of increasing turbo longevity?
  • fho2008fho2008 Posts: 393
    No I do not agree, as our host said in the message after yours, "I wouldn't assume that turbo use is a function of RPM..

    Once your car is up to speed, even 100 mph at 3500 rpm, your throttle position will be the determinant of how often the turbo is on... If you just need slight throttle pressure to maintain that speed, then the turbo will likely be taking a break..

    Of course, at that speed, even slight uphills will cause you to mash the throttle to maintain speed... but, on perfectly level ground? I think little turbo action..."

    Just because your car has a turbo, you think you use it most of the time?
  • eliaselias Posts: 1,900
    hey there fho,
    100 mph is near the top speed of a 2006 TDI, and I continue to understand that the turbo is working/boosting plenty at that speed.
    Your final question is not so respectful, but the answer to it is "no".
    best regards , and let's find a new subject to discuss! :)
  • sebring95sebring95 Posts: 3,231
    edited August 2010
    How can 100mph be the top-end for an '06? My '00 would do 130 and don't ask how I know :P
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