High rev on start up

Discussion in 'K-Jetronic OEM injection' started by spyrock, Jul 2, 2015.

  1. spyrock Forum Member

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    Hi guys,

    Since a very long time I've been wondering if this is normal on my standard '88 GTI 16V (kr):
    When starting completely cold, the engines immediately revs to 3000 rpm just during the 1-2 seconds it fires up. Then it imediately return to it's normal idle rpm (1000rpm) and stays there smoothly.

    Nothing special you would say, but doesn't sound healthy to immediately rev the engine to 3000rpm when it's been sitting in a cold garage for a few weeks (car is not my daily)..
    My K-jet system is in perfect condition and I've tested it many times with the VAG pressure gauge, etc. Also the cold start injector does it's job, obviously..

    Could it be some other component? I remember reading something about a possible faulty ignition coil, but can't find it back.
     
  2. Paul 16v Forum Member

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    Mine does this too. I just had mine setup at stealth racing and he said it was one of the temp senders on the side of the head causing it. I've not had a look at it yet though.
     
  3. BennyC Paid Member Paid Member

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    Yes swap the blue/white wire with one of the others see if it changes!
     
  4. spyrock Forum Member

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  5. BennyC Paid Member Paid Member

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    H4 is oil temp and those other three can be swapped round either way although h2 on mine if you undo it there's not direct contact with the coolant as in I change this one with no leakage!! If you remember which way round the wires were when you had issues then swap it another way it doesn't matter. You might find a different sensor by using the yellow wires as this is for the temp gauge
     
  6. spyrock Forum Member

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    Thanks guys!
    Seems like it could be both the sender connected to the blue/white wire (going to ECU to control the ignition advance) or the green/red wire (controlling the ISV).
    If needed,I think I'll order a new sender at VW because I read the aftermarket ones are buggered most of the time!

    Found more info in this thread:http://www.clubgti.com/showthread.php?221844-Mk2-16v-poor-starting-when-cold/page2&highlight=049919501


    "If the sensor for ignition is open circuited then your spark will be advanced due to a default of -48 deg C, if shorted to earth then the ignition will be corrected towards retard conditons. The vehicle will still start and run.
    If the sensor for idle is open circuited then the ISCV will tend to be open more and lead to higher idle, pending the mechanical state of the base thorttle opening and bypass. The opposite will occur if this sensor is shorted to earth. Car will still start and pullaway, pending state of base settings and of course the state of the transient components. "
     
  7. spyrock Forum Member

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    Seems solved guys, the blue/white wire going to the ECU was damaged due to it's age!
     
  8. spyrock Forum Member

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    Well, I thought it was solved but it still isn't. The temp. senders and wires are fine.

    Something must be causing the K-Jet airflap to briefly shoot up but can't find what.
    I did some more fuel pressure measuring. When the fuel pump primes after a night of standstill, it pressurizes the system to 1 bar. This is also my cold control pressure when starting the car up (at 10°C ambient temp.), which is slightly below the Bosch spec. of 1,3-1,6 bar at that temp.


    Doe anybody knows what the fuel pressure is on their system when only priming the pump before start?
    Perhaps mine is too low, which makes the piston (and thus airflap) rise too much.
     
  9. rubjonny

    rubjonny Administrator Admin

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  10. spyrock Forum Member

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    I actually meant control pressure on pump priming, not system pressure, meaning gauge valve open. How much do you guys get on priminng?

    This is the one which will determine the piston upwards travel on startup and so the travel of the airflap and amount of fuel supplied to injectors.
    I have measured everything with genuine VAG1318 gauge.
    The fuel pressure regulator is in excellent condition and works perfectly, so no issue with system pressure.

    Engine off, ignition on:
    Prime pressure: 1 Bar (with gauge valve open)

    Engine idling:
    Cold control pressure (10°C): 1 Bar
    Hot control pressure: 3,5 Bar
    System pressure: 5 Bar
    Holding pressure: 2,4 Bar (after 30min.)

    So it's all fine, except for the cold control pressure which is slightly below spec. acc. to the Bosch graph. which shows 1,3-1,6 Bar at 10°C.
     
  11. rubjonny

    rubjonny Administrator Admin

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    ah ok, its the same deal the control pressure should pump straight up pretty much, no real delay. maybe your wur is bad as you say its slightly low, worth stripping it down and have a look inside there are a few guides out there for this and I've seen rebuild kits on ebay

    you could try tweaking the warm control pressure up a little bit, might help your control pressure a bit as well? 38psi/2.65bar seems to be the go to figure which in theory will raise cold control pressure up to 1.25bar:
    https://clubgti.com/forums/index.php?threads/how-to-set-system-wur-fuel-pressure.81658/
     
  12. spyrock Forum Member

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    The control pressure does pump straight up, there is no delay. Only the value to which it pumps up is slightly low compared to spec.
    The WUR is new old stock and in barely has a few thousand miles on it now, so I prefer not to strip it.

    I have to disagree on modifying the warm control pressure. This will not affect the cold control pressure as it is is only determined by the position of the bimetallic strip (unheated). So turning the allen bolt behind the brass plug will only affect warm control pressure, but not cold control pressure.

    By the way,to all, tweaking the warm control pressure using the allen bolt as mentioned in the sticky (and way too many how-to's) is not really the way to go as it will only affect the warm control pressure at idle and medium load (when plenty of vacuum is present in the manifold) when no real performance is needed.

    If you want more performance you need to tweak the warm control pressure without vacuum (= WOT/full load condition when the manifold pressure is higher) by adjusting the position of the brass plug. The more you pull out the plug, the lower the pressure, the more air/fuel at WOT. Turning the allen bolt has no effect on this full load condition..
     
  13. rajloyal New Member

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    @spyrock did you get this resolved eventually? I have 16v turbo technics with same issue and I was wondering if its warm up or 5th injector related.
     
  14. spyrock Forum Member

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    Although it revved less high on my latest start I don't think it's completely solved. I haven't driven it much last months so can't confirm 100%
    I'm checking everything from a to z to solve this and have now replaced the injectors by new ones (don't forget to balance the flow rates if you do this!), have rechecked all temp. sensors and wires (are fine).

    After this not much improvements so now I'm thiking there could be very difficult to find vacuum leak and/or ignition timing issue. My timing is set correctly but the timing mark is slightly jumping around when using the timing light. I've found a vacuum leak at the throttle body spindles and noticed the butterfly valve themselves are not airtight when closed either (see my timing mark thread) and fixed both. But I have to do couple of cold starts before I can confirm the issue is gone.

    My timing mark is still jumping around though. Let me know if you have other ideas!
     
  15. rajloyal New Member

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    thanks for the reply - easiest way to look for vacuum leaks? soapy water and bubbles? issue there is vaccum may not produce bubbles externally right ?
     
  16. spyrock Forum Member

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    Do not use soapy water on systems with vacuum pressure as it will suck the water into the engine indeed. Three ways to check it:
    1. - With small flow of propane through a tube and running engine. Rpm will go up if sucked in the engine via a leak. Be very careful, don't burn down your house ;)
    2. - With engine starter spray and running engine. Same as above but more messy and not as effective as propane.
    3. - With DIY smoke device (see Youtube tutorials). This allows you to leak test with engine at standstill by pumping smoke into the engine.

    I only tried nr.3 and worked well but will soon try n°1 in case I missed something.
     
  17. rajloyal New Member

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    :thumbup: did you take a vid of nr 3?
     
  18. spyrock Forum Member

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    Nope. But this guy did:
    That's the smoke machine I made. Works fine.
     
    Simon Peter Dodgson likes this.
  19. Simon Peter Dodgson Forum Member

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    That's brilliant and cheap. Unfortunately I've just replaced all my split vac hoses
     
  20. spyrock Forum Member

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    I'm starting to think I could have an undetected vacuum leak inside the brake booster. You can't detect it with above methods and I'm having a different pedal feel (even though I can't find hydraulic leaks, placed a new master cylinder and purged many times).
     

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