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Can't 'blip' the throttle - revs stay 'high' for a second or two before dropping...

Discussion in 'A4/S4 forum(B5 Chassis)' started by impster, Aug 10, 2009.

  1. impster
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    impster Member

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    As above, had a slight problem the other day when the turbo didn't 'come on'. It seems fine now - I put it down to a 'sticky' variable vane turbo actuator rod which i've unstuck.

    The problem i think I have now is this - when I 'blip' the throttle, the revs stay high (i.e. at the top of the 'blip') for a second or two before coming down - it does this whatever the engine temperature - whether cold or running temperature.

    Is this normal? The car is a 1.9TDi SE with the 85kw / 115hp engine.

    Any help greatfully received as always.

    many thanks

    Impster

    P.S.:
    Last week had the codes read as follows:

    19464 Cam Position Sensor G-40: Not Plausible Signal (I put this down to it being disconnected and reconnected during cambelt change 10K miles ago)

    17664 Coolant Temp Sensor G-62 Open / Shut (I've noticed an occasional lack of temerature gauge reading - now it seems to want to stay at 80C and not go anywhere near 90C on the gauge!)

    17965 Turbo Pressure COntrol Limit Exceeded P1557 (this is what caused the lack of turbo last week).
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  2. impster
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    impster Member

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    anyone?
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  3. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    Well you should start by fixing the faults.

    Change the CTS first of all, and reset the fault codes and take it for a drive.

    Then see if any of them return.

    The Cam sensor shouldnt have come up if it was disconnected with the engine off, so taht means its been faulty at some point when the engine was running, you will need to clear the codes and have a drive to see if it comes back, if it does its still broken.

    Being a diesel, it really has to be an electronic fault. A petrol could have a throttle cable sticking or an airleak, but a derv needs fuel to rev, and only the ECU can add that fuel.
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  4. impster
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    impster Member

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    Thanks for that - a really do appreciate your help as I'm not a 'diesel' expert at all - in fact, i'm no expert on any car that has an electronic brain. Far better to have points, distributor, and 2 great big Webers imo...

    Can you clarify then, when 'blipping' the throttle from idle up to about 2,000rpm, should the revs come straight back down to idle on a diesel if the turbo's kicked in - or would the turbo 'charge' mean that the revs don't drop immediatelly but stay up for a second (as happens on mine?).

    I had though that there may have been a leaky 'air pipe' somewhere that was leaking 'air'(?), and that the ecu was compensating for this by adding a bit more fuel (thereby causing the revs to stick for a second)?

    Can you clarify what the CTS does - is it simply a sender for the fuel gauge or does it send temperature information to the ecu?

    Thanks!

    Impster
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  5. impster
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    impster Member

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    Quick update - just noticed that the clutch pedal has an impact on this revving thing.

    Clutch pedal out (i.e. not pressed in) = revs stay high for a second or two when i blip the throttle.

    Clutch pedal pushed in = revs drop stright away upon releasing throttle pedal.

    So - does this help 'diagnose' something - or is it normal.

    Vag-com'd it again and no codes stored - although temp gauge is still a bit drunk...

    Thanks

    Impster
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  6. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    It sounds like you have a faulty CTS for a start ("drunk" temp guage) This will affect the compensation the ECU applies (if the engines cold you need more fuel, add more fuel when its hot and weird stuff happens)

    Clutch thing is strange, not really sure whats going on there.

    It seems a long shot, but stick a new CTS in and go from there, they're only about 20quid and it will probably save you some fuel too.
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  7. jcb
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    jcb Active Member

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    hydraulic clutch?
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  8. impster
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    impster Member

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    Not sure - why do you ask - could that have a bearing on the 'throttle blip situation'?

    I believe the clutch has it's own sensor at the pedal as well? Maybe that needs a clean?

    Impster
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  9. impster
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    impster Member

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    On the drive home, I noticed that the clutch pedal only needs to be marginally pushed down (i.e. no more than 1/4 inch - before there's any noticeable pressure on the clutch plate) for the effect to occur.

    So, to recap:

    Clutch pedal not pressed - revs stay high when throttle blipped momentarily
    Clutch pedal pressed down (1/4 inch or more) - revs come down immediatelly following a 'blip' of the throttle pedal.

    Is there a clutch pedal sensor on these cars - and what does it do?

    MY2000 Audi A4 1.9 TDi with the PD engine (85kw / 115hp).
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  10. jcb
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    jcb Active Member

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    same reason if you turn the wheel hard at standstill the revs drop. teh power steering pump draws more power when under load.
    if there is a problem with the hydraulics or swithc it could affect revs.
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  11. impster
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    impster Member

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    just found this thread on another audi forum:

    http://forums.audiworld.com/showthread.php?t=1512421

    I've adjusted it tonight as per the thread - seems ok so far but will find out tomorrow on way to work.

    Also adjusted the brake pedal switch so that the brake light comes on a bit sooner.

    Fingers crossed.

    Thanks.

    Impster
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  12. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    The hydraulic clutch system isnt driven by engine power at all...

    It just has a master cylinder at one end and a slave at the other, instead of a cable. All the force comes from your foot. Think of it like a braking system without a servo.

    Theres a switch on the clutch pedal on DBW cars, which the management uses for "stuff" (i have no clue whatsoever what its for, perhaps idle control or something related) clearly thats having an effect on whatever is holding the RPM's high.
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  13. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    Thinking about it, its going to be for a mild anti stall isnt it?

    If the clutch is down then it will try harder to keep the revs up to stop a new driver stalling it.
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  14. impster
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    impster Member

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    Thanks for that - I was wondering how the hydraulic system would be involved (no offence jcb - all responses greatfully received).

    Anyway, just like the thread link I posted, with the clutch pedal disengaged, the plunger on the switch wasn't properly 'home', so I've adjusted it so that it sits 'home'.

    I'll know within a few miles tomorrow what the score is.

    I'll see what it's like tomorrow, and report back.

    Many thanks once again.

    Impster
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  15. impster
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    impster Member

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    Ok, car seems much better with this revving and throttle blip situation though not cured. So going to change the Coolant Temperature Sender to see if that makes a diference.

    Popped down to local VAG dealer and they offered me a choice of 2 diferent sensors - both green, one a 'square' shaped plug, and the other a 'arch window' shape plug. Couldn't for the life of me work out which it was without taking some air hoses off, so will check tonight and get the right one.

    Ok - here's the niggle.

    I worked out how to display the engine coolant temperature via the Climate Control panel, and once engine is warmed up it shows a steady 90C with no deviation. Thing is, the tem gauge on the dash will vary between 60 and 85 with a mind of it's own.

    Does this deviation (90C on the Climate Control display and 60-85C on the gauge) point to a CTS problem or something else?

    Many thanks

    Impster
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  16. Broken Byzan
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    Broken Byzan Photographic Moderator Staff Member Moderator

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    The climate control is using the pick up from the ECU control circuit, the dash uses the temp guage dispaly section. Effectively the sender is 2 in 1 unit and at MY2000 i would assume the D shaped pluggy in thing.

    When my mates car was playing up it was a combination of leaks causing problems, lots of the Vac pipes were broken/perished and some if the short hoses that join the large metal boost pipes were split too
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  17. impster
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    impster Member

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    Just checked this evening, and yes it is the 'D' shaped sensor I need. Will pick one up tomorrow.

    Impster
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  18. impster
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    impster Member

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    Ok, replaced the CTS this afternoon - surprisingly easy actually.

    Initial test drives seem ok - temp gauge at a steady 90C.

    however on the on-board climate control diagnosis - on channel 49 (not 51) which shows unadjusted engine coolant temp, the reading seems to stay between 80C and 86C. Mind you - haven't given it a proper drive just yet.

    Throttle 'blip' situation still seems to be there though - and seems to be connected to whether the clutch switch is actuated, rather than any significant hydraulic pressure applied by the pedal (i.e., pedal only needs to be pressed around 1/4 inch for the throttle 'blip' thingy to dissappear).

    For those struggling with reaching the CTS on the 85kw (115hp) PD engine'd cars, you need to remove the engine cover, and the cover over the MAF sensor (big triangular bit of plastic about 10" across) to allow good access to the CTS. You need to pull the clip on the sensor harness away from the sensor for it to release.

    I lost around a pint of coolant, that's all.
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  19. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    80 and 86 seems about right, especially for an idling derv, they tend to cool off when not under load.

    The guage should show 90 at any temperature between about 80 and 105 as thats the "normal" range.

    Maybe there is another faulty switch?

    Have you tried comparing the vagcom output for the clutch/brake switch positions to what they should be? The link up there shows the channel and what it should be showing.
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  20. impster
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    impster Member

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    Thank aragorn.

    That's next on my list of things to check I'll hopefully get round to it next week.

    I'll get to the bottom of it somehow...
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  21. impster
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    impster Member

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    Another question - sorry if I seem ignorant but I really am not familiar with turbo or diesel engines.

    Does my car have a dump valve? And if so, could a faulty one be the cause of the odd revving issue?

    Hopefuly get it on vag-com next week.
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  22. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator

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    Diesels dont have a throttle plate, so they dont have a dump valve as there is never any need to dump anything.

    On a petrol you limit the air flowing into the engine, and add the correct amount of fuel for the air thats gone in. This limiting is done with the throttle plate, and when you lift off the gas pedal the plate closes. The turbine will still have inertia so will continue pumping air in, against this closed plate, which causes a pressure spike and then compressor stall, as the air builds up to a point it starts forcing its way back out past the spinning compressor wheel. This is bad for the turbo, hence they install a dump valve to relieve this pressure.

    On a diesel, the inlet tract has no restrictions, the engine always takes in a full cylinders worth of air. The amount of fuel injected, governs how much power is created meaning a diesel runs lean most of the time, but due to dervs direct injection it burns properly, unlike a lean petrol engine, where the air fuel mix would simply not ignite properly. Anyway, the lack of a restriction means that when you lift off the throttle pedal, the ecu simply stops injecting fuel. The inertia in the turbo just blows harmlessly thru the engine as it would while accellerating. Hence no pressure build up and no need for a dump valve!
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  23. impster
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    impster Member

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    Been suggested on another non car sprcific forum that it could be Clutch drag causing this 'blip' thing?

    Plausible?
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