Fast idle

[jamo1234]

G'day,
My 91 VN/VG 5 speed idles at 1700 rpm until I stop then the idle drops to normal, does the manual g/box have a sensor to ECU

 

[Immortality]

Yes, the vehicle speed sensor is mounted in the gearbox and tells the engine when to return to idle.

Any modifications?

Have you tried cleaning the throttle body?

 

[jamo1234]

Hey thanks for your reply, I thought the speed sensor must have been in the g/box, no mods, have cleaned throttle body and replaced throttle position sensor and Idle control valve, no worries, that's just the way it is.

 

[Immortality]

Was it doing this before replacing the TPS and IAC stepper motor?

Has the throttle stop screw on the throttle body been adjusted at all?

 

[MikeC]

Make sure the throttle butterfly returns quickly to the stop when you take your foot off the pedal. Could be a sticking throttle cable.

 

[Immortality]

The VN/VP TB's were of an earlier design without needle roller bearings so I like to (or rather did) give the butterfly shaft a little squirt of white lithium grease to keep the operation smooth as well.

 

[jamo1234]

Thanks for replies, the throttle is nice and snappy, in spite of fitting new bits to the throttle body the idle has always been the same 1700 rpm rolling to a stop, the idle speed drops back when complete stop, no matter as I can live with it.

 

[Immortality]

Always done it?

Is it a really early VN?

 

[jamo1234]

Well I've had it for a while and am restoring it, yes it has always done it, it's a VG 91 5 speed (original) It's not annoying to drive but I thought it would be good to fix it, obviously it's not a common fault, if it is a fault that is.

 

[Immortality]

I wonder if the butterfly throttle stop has been dicked with?

To set it properly you need to be able to log the ECU and see what the TPS voltage is and IAC steps, but a rough way would be to let the car idle and block off the IAC passage and then check the idle RPM. With the IAC passage blocked it should idle 100-200rpm lower than the target idle speed.

OR

Alternately, just turn the throttle stop screw out a 1/4 or 1/2 a turn and see what it does. You can always turn it back to where it was if it gets worse or makes no difference.

IIRC, on a unmolested throttle body there should a little green plastic cap over the butterfly stop adjustment screw, if that is missing chances are it has been adjusted before.