Engine Revs Increase Significantly Once Moving off Idle?

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  altomic on 19/07/2017 - 20:56

  +3

  what model?

  • 

    [Deactivated] on 20/07/2017 - 09:45

    I too think that this is a very important bit of missing info.

    Most every day cars with an auto will have a TCU that changes gears based on both RPM (high or low) and/or load (high). This function is in the computer and overrides where your shifter is physically in. It is just a manufacturer insisted protective layer.

    • 

      tunzafun001 on 20/07/2017 - 16:11

      I'm starting to think the issue is electrical / ecu related. As per below, VX commodore.

      It can't be the drop down solenoids (probably not technical name) in the gearbox, as the issue of revving up happens in every gear (1,2,3 even R). Once you hit above approx 15kph in D and then place it in say 1st or 2nd you will feel it surge up and down?

      • 

        [Deactivated] on 20/07/2017 - 22:58

        I'd be thinking the same too.

        Let us hope it is nothing to do with mapping config, otherwise you're going to be hard-pressed to justify the cost to investigate.

        At a lost.

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  Euphemistic on 19/07/2017 - 21:08

  The downhill thing sounds almost normal. Automatics will not engine brake very well and will often run away on a hill but it isn't using engine power to accelerate, just gravity. Dropping it back a gear does provide some initial deceleration, but that doesn't typically last long in my experience. If towing then the trailer is going to try to push you down the hill. If it is the engine doing the pushing (pulling) the trailer shouldn't make any difference.

  As for on the flat it does sound wierd that it will maintian higher revs. However depending on circumstances it can take a reasonable amount of time to slow down significantly. Does braking when on the flat make you slow down or is the engine really trying to hold the car at speed?

  • 

    tunzafun001 on 19/07/2017 - 21:41

    Engine is trying to hold the car at speed on the flat.

    As for the gear drop down, when it's working properly it gets slower and slower over time (with the revs sitting around 1600). When it isn't the revs quickly shoot up to 2100 range and then increase further and further as it is clearly accelerating / fast idling.

    Definitely not normal as I used to have the same model car and never had this issue.

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  Toons on 19/07/2017 - 21:57

  +1

  need to know model

  Were both cars identical age / range

  I've had cars that will Idle at different speeds depending on requirement / slight tap on accelerator will take it out of idle idle and move it up.

  does it only happen when loaded or does it react the same when not towing the boat?

  • 

    tunzafun001 on 20/07/2017 - 16:00

    Happens regardless of load, and yep have driven the same year VX commodore and it used to get slowed down perfectly by dropping it down into 3rd (for 80kph) or 2nd (for 60kph).

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  Mad Max on 20/07/2017 - 11:13

  Many cars have and ICV (idle control valve) controlledby the ECU (engine control unit).
  It opens/close to keep the idle stable when under load from A/C or power steering pump.

  • 

    robertbruce on 20/07/2017 - 12:46

    +1, + many other possible causes, leaky injector seals, auto fluid (re pressure re sensor re selector-controller) to name a couple

    • 

      tunzafun001 on 20/07/2017 - 16:19

      Injector seals would present with a rough idle though wouldn't they?

      Fluid is perfect, and changed the trans filter and fluid 15 000 km ago.

      The car has ran perfect for the 60 000km I have had it, except this over run going down hill issue.

      The guy I bought it from (brothers car apparently) said he thought it didnt have the original engine. So any possible issues with mismatched ECU's between Ecotec's?

      I've read there maybe an issue putting an auto ECU in a manual car as it will rev up when the clutch is pushed etc, but can't see this being an issue if in reverse, and mine is a standard auto.

  • 

    tunzafun001 on 20/07/2017 - 16:05

    Its a 2001 VX commodore. So yep, has an ICV. Idle (when not moving) is perfect(with all variables in drive, A/C on, turn the wheel etc). The mechanic said it wouldn't be the ICV as it would fail above tests.

    Just had the throttle body cleaned and TPS replaced as it was $30.

  • 

    tunzafun001 on 20/07/2017 - 16:33

    My suspects at this stage:

    1. A leaking vacuum hose? (but brakes, steering and still idle are all good). So has to be a vacuum line that isn't involved in any of the above scenarios? Don't know which that would be. Some off idle vacuum?

    2. ECU issue itself? Just plain old electrical problem / mismatched ECU?

    3. Throttle body itself?

    4. Some information received wrong from the gearbox. Ie, I can understand it is supposed to rev up when selecting a lower gear to make it smooth, but then should turn off, but that bit isn't shutting off once gear is engaged? No idea what that would be though?

    • 

      robertbruce on 20/07/2017 - 18:15

      The injector seals is only a possible and they don't necessarily run rough if leaking. If leaking they will suck in more air causing a high idle by confusing the ecu. Also, if it is the seals you'll occasionally smell fuel.

      There most definitely a issue putting a manual ecu into an auto. Normally when you shift to D the ecu ups the idle a little to overcome the drag on the auto, a manual ecu would not have this function.

      Mismatched ECU's between ecotechs, that's a toughy… if it was a Duratech (ford) you would have had to also change over the fuel injectors, starter motor, ignition locks and other stuff, but that's a ford, surely GM wouldn't do that too….

      I would start with checking if indeed it has had a engine change and double check all the things that should be done in this scenario….

      My next step would be to test the auto shifting sensor and anything that attachs to that….

      Double check all grounds…

      Another possible is exotic devices like solarpanel controllers, rust controllers and battery charger systems. If something has been added that uses a positive ground, theoretically it stuffs up all the non-returning grounds…but im not real sure on that apart from having to wire the grounds on my bus to a negative source rather than just the body…

      My moneys between the auto shifter and the ecu, usually the sensor on the shifter gets dodgy….

      Im a little slow on the post…..to answer your question in point 4… No, the function would not turn off once your underway… the lift in the revs should only be by about 100rpm, so it should be still idleing in gear at 750.

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  JIMB0 on 20/07/2017 - 16:03

  Is it a Falcon?

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  Toons on 20/07/2017 - 16:05

  +1

  Until you state exactly what car you have everyone is only going to take a punt on possibilities

  That will be the prompt someone reading this might click and go oh yeah that happened to…………..ask about that.

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    tunzafun001 on 20/07/2017 - 16:29

    It's a 2001 VX commodore. Standard V6 Ecotec that I think was used from the VS to VY.

    Also, it's a factory LPG model, so dual fuel. Might be relevant / differences in vacuum hoses?

    Problem presents itself on both fuels, but I used to drop back through the gears on petrol on my previous VX and it used to slow it down well. Never really worked well on either via LPG. Assume the injectors shut off on petrol, but LPG still keeps flowing at idle rate.

    • 

      Euphemistic on 21/07/2017 - 16:44

      More late info with the LPG thing. For all of these sorts of questions you should be stating the model, motor and any modifications up front, and in mods I mean like dual fuel even if factory as everyone here would assume petrol. It is just to hard to diagnose anything without this information, and if you have a common car then it entirely possible that a forum member has experience with that model.