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Thread: Increased idle rpm

  1. Default Increased idle rpm

    My NSX usually idles at about 1000rpm when warmed up.

    Recently it has started to sometimes idle at higher rpm even when warm, at approx 1300rpm. There doesn't seem to be any adverse effect from this, but I'm curious as to why it might be doing it.

    Interestingly, if it is idling at 1300rpm and I switch on the headlights, the idle speed drops back to 1000rpm and stays there, even after switching off the headlights. However, if I then raise the revs by blipping the throttle the idle rpm settles back to 1300rpm again.

    Any thoughts?

  2. #2
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    Default

    Maybe throttle body (cleaning) or wrong air (searching).

  3. #3
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    The no load idle should be around 800rpm so clearly, it's not normal.
    You will notice changes in fuel efficiency if it happened just recently.

    There are many factors and unfortunately, if you try addressing it without first checking any other issues, you could introduce additional factors that may change the situation with higher rpm or even lower one ending up stalling the engine when suddenly lifting the TH pedal.

    I don't know the spec of your NSX so first, let us know at least the basic spec such as the year model, country spec, MT/AT, OBD2 or pre-OBD2, etc.
    It will help if you list up any modifications around the Eng especially the intake/exh area, air cleaner element (aftermarket air filter), aftermarket ECU, etc.

    I don't like the term 'year model' so if you know what DBW (Drive By Wire) is, it will help if you can tell whether you have DBW or non-DBW model.
    Idle on DBW is controlled directly by the stepping motor with ECU.
    With non-DBW, it's the EACV mounted at the side of intake manifold controlled by the ECU.
    Both relies on the idle air adjust screw for the base air flow.

    If non-DBW, then the obvious first check is to confirm the TH cable free play and look around the linkage area.

    Don't know how long you owned it but did the idle rpm always stay at about 1,000rpm since Day1 of your ownership?
    Or, did it happen after major services?
    You could be chasing something introduced by the previous owners/services.
    Someone may have opened the idle air adjust screw because of the hunting idle rpm issue without first washing the TH body thoroughly, etc.

    If non-DBW, there are several reports of Fast Idle Valve randomly letting extra air into the intake when it should not but you will notice this during the cold start up and while warming up as hunting warm up rpm.

    There are 3 x coolant temperature related sensors on our NSX.
    The TW sensor mounted at the front bank below the EGR valve is the one for the EFI control.
    Several NSX ended up with higher idle rpm due to wrong temperature info sent from this TW sensor.
    Please note that the temp gauge on the dash is using the output from another sensor so even if your gauge is showing fully warmed up status, doesn't mean the ECU is getting the same info.

    There are many other factors and as you can see, you could be chasing multiple factors that could be the result of something changed previously but first thing first, let us know the info of your NSX.


    Kaz
    Last edited by Kaz-kzukNA1; 29-10-2019 at 12:56 PM. Reason: extra info

  4. Default

    Cheers guys.

    My NSX is a UK model NSX-T manual first registered in 1996, though I believe it was built in 1995. It is a DBW model. There are no modifications to the engine, ECU, or intake system. The car has done 131,000 miles and is regularly used.

    I don't honestly think I would have noticed a change from a base idle of 800 to 1000 rpm so cannot comment on when this may have changed. The higher idle of 1300rpm is more noticeable, and this has happened recently.

    My car has recently had an MOT at which it struggled to pass emissions, though I believe it was not idling at 1300rpm prior to this. The garage did <things> and then the car passed the emissions test; I recall a vague comment about cleaning sensors but cannot confirm exactly what was done. It is very possible that intake manifold sensors were disturbed during efforts to pass emissions, but the recent change in idle speed might have been entirely coincidental.

    My car has previously had the throttle body and bypass passage cleaned to eliminate the revs dropping below the idle point and nearly stalling; this last clean may have been 12-18 months ago though so it's possible it has got dirty again.

  5. #5
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    So, it's DBW and thus, no EACV or FIV.
    ECU will force the TH to the targeted idle rpm of 750-850rpm for the MT model.
    There is no dashpot so ECU will also control how quickly the TH closes when you lift the TH pedal.

    First, please check the accuracy of the gauge reading.
    There is a small NE (rpm signal) test point connector mounted on the side of the INJ resistor block at the right edge of the eng bay.
    Hope you have a multimeter that can read the frequency.
    It's 6cyl Eng so 3 pulses per rotation and at 800rpm, it's 40Hz.

    By now, many NSX would have offset on the rpm gauge and although it's not popular as the main relay failure, already seen well over 300rpm offset on some of them.


    If the TH body was cleaned in the past, I would check the state of base idle air adjustment.

    It is possible that it was opened too much before cleaning the TH body and then after being cleaned, it now got too much air into the TH that ECU could no longer lower the idle rpm even the TH at the fully closed position.
    The idle air adjust screw was locked using grey coloured lock paint at the factory with tamper-proof sticker over it.

    By now, the sticker would be gone.
    If you look at the side of the TH body from the direction of the coolant tank, you should be able to spot it.
    If the lock paint gone, the adjusting screw was touched in the past.

    While in neutral, if you move the shift lever side way (no need to shift into any gears, just keep it in neutral and move the shift lever left <-> right several times), does the eng rpm change?


    You may need checking the TW, IAT, O2, EGR, VBatt, AC CL, etc but I'll leave them for now.


    Kaz
    Last edited by Kaz-kzukNA1; 30-10-2019 at 02:57 PM. Reason: typo, extra info

  6. Default

    Thank you Kaz

    I have a multimeter, but it is of basic function and cannot check frequency. I will pass this recommendation to my mechanic.

    Please note that it is possible to reduce the idle speed from an indicated 1300rpm to an indicated 1000rpm by applying load to the engine, whether electrical load (e.g. switching on headlights) or mechanical load (selecting a gear and gently raising the clutch to the biting point). Once the idle rpm has reduced to an indicated 1000rpm it will remain at that level if the load is removed, but if electrical load is maintained and I blip the throttle the idle rpm will settle back to an indicated 1300rpm. The reduction in idle speed from an indicated 1300rpm to an indicated 1000rpm is accompanied by an audible reduction in engine rpm.

    There is very possibly an issue with drift/offset in the rpm gauge, but considering the above I do not think that the indicated 1300rpm idle is entirely explained by a measurement error from the normal base idle; there is likely something else as well.

    I will review the condition of the throttle body to see if the idle air adjust screw lock paint is still visible and report back.

    Additional information:

    Moving the shift lever from side to side in neutral does not change the engine rpm.

    My car had a new replacement IAT sensor in about May this year, following which there were no issues with idle. It is of course possible that the new sensor has become dirty.

    During the warm-up process my car idles roughly (I believe this normal for the NSX), but on reaching the target water temperature the idle immediately smooths out. This is consistent with the water temp display on the dash reaching the middle of the gauge. I have not noted any change in roughness when the idle is at an indicated 1300rpm, which suggests to me that the ECU WT sensor is not putting the car back into warm-up mode.

    I hope that this provisionally eliminates a couple of possibilities.

  7. Default

    Update: the grey lock paint on the throttle body idle air adjust screw has been removed; the head of the screw is fully visible.

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