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Thread: Audi A6 2.5 TDi V6 engine brake and turbo lag

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  1. Audi A6 2.5 TDi V6 engine brake and turbo lag 
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    Hello community,

    I have the BFC 2.5 TDi 163 engine in a 2002 A6 and I find these two things a bit troublesome:

    1. The engine has huge engine brake. The 1.9Tdi (Golf) which I also have has almost none, but the 2.5Tdi has huge engine brake. I'd like to hear from other 2.5 V6 TDi engine owners to confirm it this is the case with their cars. What could be causing this? Its even worse at winter time, the cold engine would almost bring the car to a full stop on a big downhill in 2nd gear.

    2. Big turbo lag. I like to drive the car at the lower rpm range (1000-2000rpm), but then it takes forever to accelerate, like 3 seconds to start building boost. Once it builds boost it goes well. The matters become worse on hot weather, its not so terrible on a cold day. When I checked the car with a VCDS, I saw that the boost was building up to 2.5 bar (absolute), but it takes some time to build it, it was lagging behind a lot. In comparison the 1.9 TDI PD builds boost instantly and follows that boost curve almost instantly. I think other 2.5TDI owners have reported boost of 2.2 (not 2.5) for the 2.5 TDi so this might be a problem too. EGR and Cat are installed, everything is stock to my knowledge, no mods whatsoever. The turbo actuator is working, I'v seen the lever moving while I rev the engine (open hood), though I don't know if it works 100%. I know this can cause turbo lag if its stuck open, but it seems to be working.

    The car passes the emissions tests with flying colors though, which to me is an indication of a healthy engine. What should I start looking at first if I want to fix these issues? It's not that I cannot live with this small problems, but it would be nice if I could fix them. Can the EGR valve have something to do with this? I experience no stalling, no rough idling or rough acceleration, so I am assuming the EGR is fine.
     
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  2. Re: Audi A6 2.5 TDi V6 engine brake and turbo lag 
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    I am not sure how valid this is as a comparison as my car is an allroad with auto transmission and an AKE 2.5 TDI but I don't get unexpectedly high engine braking.

    Accelerating, if I have rpm in the 1000-2000 range as you describe and push my foot down suddenly, not much happens until the engine revs climb into the 2000-3000 range and then it suddenly accelerates quickly.
     
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  3. Re: Audi A6 2.5 TDi V6 engine brake and turbo lag 
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    Mine pulls fine even from 1400rpm once it starts pulling, but on a hot day if you put your foot down it would just do absolutely nothing for several seconds, and strangely enough it takes off better if you press the accelerator pedal only a little bit than to push it to the metal. The engine brake, where does it come from, its a mistery.
     
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  4. Re: Audi A6 2.5 TDi V6 engine brake and turbo lag 
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    UPDATE:

    we put the diagnostic tool and went for a ride.
    1) 3rd gear at 30kmh, pedal to the metal. The ECU asks for 1.8bar, but it takes more than 5 seconds to reach that boost. Its stuck at 1.2bar and has no power.
    2) Once going up the revs, the ECU asks for 2.25 bar, and the turbo spools up to 2.6 bar.

    I think its safe to say the actuator is not doing its job, its probably stuck somewhere at 40-80% working area. It cannot close the vanes enough at low rpm, it can not open the vanes fully and high rpm.
    Sticky vanes is the most probable cause, meaning I have ton of carbon build up in the vanes.

    -How big of a deal is to take the turbo out, disassemble it, clean it, then put it back together?
    -Can you mess up the actuator rod position in the process which can cause over/under boost?
     
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  5. Re: Audi A6 2.5 TDi V6 engine brake and turbo lag 
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    You have reminded me that that recently it occurred to me that the my TDI seems much less laggy that it was when I first replied. I don't remember a specific date on which it seemed to get better but there certainly has been a change.

    It's always hard to be sure with subjective changes, i.e. it can always be a case of adapting to the car, but it doesn't seem laggy now and the auto transmission changes seem smoother too. If there has been an improvement in torque at lower rpm that would make sense because how quickly the change is made will presumably depend on how much torque the engine should be generating as reported by the ECU, not how much it is actually generating.

    You obviously know more about the mechanics of the turbo than I do but if it is a case of being caked in soot I wonder if it was taking the car on holiday in the summer was responsible for the improvement, i.e. may longer journeys with the engine hot may have burnt off some soot?
     
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  6. Re: Audi A6 2.5 TDi V6 engine brake and turbo lag 
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    Quote Originally Posted by Amphitryon View Post
    You have reminded me that that recently it occurred to me that the my TDI seems much less laggy that it was when I first replied. I don't remember a specific date on which it seemed to get better but there certainly has been a change.

    It's always hard to be sure with subjective changes, i.e. it can always be a case of adapting to the car, but it doesn't seem laggy now and the auto transmission changes seem smoother too. If there has been an improvement in torque at lower rpm that would make sense because how quickly the change is made will presumably depend on how much torque the engine should be generating as reported by the ECU, not how much it is actually generating.

    You obviously know more about the mechanics of the turbo than I do but if it is a case of being caked in soot I wonder if it was taking the car on holiday in the summer was responsible for the improvement, i.e. may longer journeys with the engine hot may have burnt off some soot?
    Long trips with heavy load does great for the engine. But unfortunately this V6 is not getting nearly enough of that. Plus is old and has high millage. My Golf is my primary choice and that one gets thrashed on a regular basis to prevent soot related problems. If time and resource are enough, I will try to disassemble the turbo unit for a nice cleaning.
     
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  7. Re: Audi A6 2.5 TDi V6 engine brake and turbo lag 
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    I'v just realized that the turbo forced induction design is utterly flawed. The turbo unit runs on your exhaust gasses, so it will eventually become dirty, which will cause soot problems and sticky geometry. This happens with a healthy engine too if you only do short runs. Things gets much worse when your engine becomes worn out, the exhaust gasses become much more contaminated, so the turbo unit will get contaminated much faster. The compressor is much better design after all, and like most good things, the compressor (supercharger) is much less used.

    Btw, I think that the engine brake is also connected to the sticky vanes, when the pedal is released, the vanes should be fully open, which doesn't occur, thus the turbo keeps spinning which causes back pressure and engine brake.
     
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  8. Re: Audi A6 2.5 TDi V6 engine brake and turbo lag 
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    Quote Originally Posted by A6C5V6TDi View Post
    I'v just realized that the turbo forced induction design is utterly flawed. The turbo unit runs on your exhaust gasses, so it will eventually become dirty, which will cause soot problems and sticky geometry. This happens with a healthy engine too if you only do short runs. Things gets much worse when your engine becomes worn out, the exhaust gasses become much more contaminated, so the turbo unit will get contaminated much faster. The compressor is much better design after all, and like most good things, the compressor (supercharger) is much less used.
    Isn't engineering often a matter of compromise? Correctly if I am wrong but turbocharging and supercharging are both ways to drive extra air into the engine compared to what would be drawn in by natural aspiration but turbocharging achieves this by using what would otherwise be waste energy to do the pumping whereas supercharging requires extra energy from the engine. So turbocharging should give you the better fuel economy of the two.
     
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  9. Re: Audi A6 2.5 TDi V6 engine brake and turbo lag 
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    Quote Originally Posted by Amphitryon View Post
    Isn't engineering often a matter of compromise? Correctly if I am wrong but turbocharging and supercharging are both ways to drive extra air into the engine compared to what would be drawn in by natural aspiration but turbocharging achieves this by using what would otherwise be waste energy to do the pumping whereas supercharging requires extra energy from the engine. So turbocharging should give you the better fuel economy of the two.
    Turbo uses engine energy too, but is more efficient and smaller to install. Compressor is much cleaner as it has no business circulating exhaust gasses. Supercharger is also not dependent on oil. Theoretically, the compressor should never brake down too, as its a simple belt driven air compressor. Also the compressor engine cannot suffer diesel runaway. I see much more benefits for compressor against turbo and yet...
     
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  10. Re: Audi A6 2.5 TDi V6 engine brake and turbo lag 
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    Another interesting development. In looking into a water leak, I have been running for the last week with the plastic cover that goes over the top of the engine removed. The car seems to be driving particularly well with the least sign of turbo lag when pulling away. In my case the plastic cover has significant cracks in, especially over the turbo area - I wonder if that bit gets hottest and becomes brittle - and has been fixed together with sticky tape. I can't quite see how, but I am now wondering if the broken cover could be resting on something and hampering its movement.
     
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