RPM is slow to return to idle rpm range.

[nsxspdfreak]

When I let off of the throttle the RPM remains at around 1300 rpm and then the rpm slowly drops over a period of about 10-15sec to approximately a normal range of around 800rpm. This happens parked in nuetral when the throttle is blipped or in gear. The rpm dropping to idle speed is slow enough where if I leave it in gear as the car is slowing down the engine will lug as the car slows to below the engines rpm speed. The engine idles smoothly. I noticed this happening intermittently for while now, but now it's happening consistently. I just had a camera in the throttle body while checking for loose butterfly screws and it looked clean, but I plan on taking it off this weekend to make sure it is clean.

 

[Old Guy]

There is a chance that your ECU is not going into idle control mode. On both the pre and post drive by wire cars, once the ECU enters idle control mode the ECU monitors engine speed and operates the Electronic Air Control Valve (pre DBW) or the throttle plate actuator (DBW cars) to hit the target idle speed. Depending on the car pre-DBW / post-DBW, auto / manual the ECU gets signals for transmission in neutral, clutch pressed in and I think brake pressed. Get the service manual for your model year and in the Fuel Section you will find Idle Control section with a description of the inputs to the ECU. Check them using the service manual procedure to make sure that ECU is getting the correct signals.

If you have a pre DBW car, two additional possibilities:
- your EACV is gummy and is slow to respond. Normally; a dirty EACV results in too low idle / stalling; but, it might be the cause of the problem. A clean EACV is always a good thing.
- the engine speed needs to drop below a certain value before entering idle control mode. I don't know what that value is on the NSX ECU; but, if you have a sticky or worn throttle cable (or something on the pedal which is causing a hang up) that is preventing the throttle plate from closing completely it may initially be stuck above the idle control setting. Check to make sure your throttle plat is closing completely.

 

[nsxspdfreak]

There is a chance that your ECU is not going into idle control mode. On both the pre and post drive by wire cars, once the ECU enters idle control mode the ECU monitors engine speed and operates the Electronic Air Control Valve (pre DBW) or the throttle plate actuator (DBW cars) to hit the target idle speed. Depending on the car pre-DBW / post-DBW, auto / manual the ECU gets signals for transmission in neutral, clutch pressed in and I think brake pressed. Get the service manual for your model year and in the Fuel Section you will find Idle Control section with a description of the inputs to the ECU. Check them using the service manual procedure to make sure that ECU is getting the correct signals.

If you have a pre DBW car, two additional possibilities:
- your EACV is gummy and is slow to respond. Normally; a dirty EACV results in too low idle / stalling; but, it might be the cause of the problem. A clean EACV is always a good thing.
- the engine speed needs to drop below a certain value before entering idle control mode. I don't know what that value is on the NSX ECU; but, if you have a sticky or worn throttle cable (or something on the pedal which is causing a hang up) that is preventing the throttle plate from closing completely it may initially be stuck above the idle control setting. Check to make sure your throttle plat is closing completely.

Thanks for the advice. It's a 91. I'll check the EACV and take a look at the idle control section of the manual. Since this started as an intermittent issue last year and has been slowly growing into a more persistent problem I have a feeling it's an accumulation of crap building up. The throttle cable is not hanging up.
Joe

 

[Old Guy]

If this problem has developed and become worse over a period of time, then I would focus on the EACV and the associated passages in the throttle body. Loss of the signals from the brake / transmission / clutch tend to be more immediate rather than evolving and getting worse.

 

[nsxspdfreak]

If this problem has developed and become worse over a period of time, then I would focus on the EACV and the associated passages in the throttle body. Loss of the signals from the brake / transmission / clutch tend to be more immediate rather than evolving and getting worse.

I agree. Thanks