So today we have my first call to you lovely lot as googling my symptons doesnt seem to return any results that match…

After a quite worrying phantom throttle incident last week (promptly fixed by re-attaching the cruise control apparatus which was just floating around in the bay), i seem to be struck by a very light involuntary throttle input while in drive.

Any other normal torque converter car in my experience will quite literally idle creep, however for “the duchess” as we shall remain calling her, this isn’t enough. The revs will actually pick up and the car will creep along until it breaks into a brisk walk and it will even upshift itself into second. All the while my foot is nowhere near the go pedal. Now the weird thing is that the car will idle very steadily at the 700rpm that it should be, yet as soon as drive is selected, it goes up to 1000 and acts as though i have ever so slightly teased the throttle. This immediately tells me my throttle cable is not under any more tension than it should be.

I plugged the nanocom in at the weekend when i was sorting my previous throttle (cruise control) issue out and was met with a myriad of old stored codes (none current), however just to be sure i reset the throttle adaptations to give it a clean slate with my fix.

I shall now open up the floor, any experience with this issue before? Any pointers as to where i should be looking, whether thats IACV, TPS or even adjusting throttle cable despite a correct idle? Would it be a calibration issue with the ECU? Is my TC simply locking up far too soon and prompting the ECU to apply a voltage to the throttle to save a stall situation?

Any advice is dearly welcomed :)

First off, it won't be a problem, just use the brake to control the speed. However, to help you get to the bottom of it, here's how it should work. Idle speed is controlled by a combination of the IACV and throttle position sensor. The TPS voltage at a closed throttle is stored and, as long as the voltage from the TPS is at or below this stored voltage, the IACV will adjust to keep the idle speed correct. As soon as you touch the throttle, the TPS voltage rises and that causes the IACV to open up to allow air in to raise the revs before the airflow through the throttle butterfly allows a lot more air in. So a raised idle is usually caused by a raised voltage from the TPS (so it thinks the throttle is being opened), a sticking IACV or an air leak at the IAVC hoses.

Using the Nanocom, look at the stored and actual TPS voltages, they should be the same or the stored voltage should be fractionally higher than the actual voltage. Look at the IACV opening, at normal idle it should be between 20 and 30 steps. If it is lower than this, that suggests an air leak so the valve is closing down to try to keep the revs correct. If the revs are rising when put in Drive, check the TPS voltage when you do that.

On the Thor, the cruise control cable needs to be adjusted correctly as that can cause issues too if it isn't. Check the process in RAVE and make sure that is correct too.

Gilbertd wrote:

First off, it won't be a problem, just use the brake to control the speed. However, to help you get to the bottom of it, here's how it should work. Idle speed is controlled by a combination of the IACV and throttle position sensor. The TPS voltage at a closed throttle is stored and, as long as the voltage from the TPS is at or below this stored voltage, the IACV will adjust to keep the idle speed correct. As soon as you touch the throttle, the TPS voltage rises and that causes the IACV to open up to allow air in to raise the revs before the airflow through the throttle butterfly allows a lot more air in. So a raised idle is usually caused by a raised voltage from the TPS (so it thinks the throttle is being opened), a sticking IACV or an air leak at the IAVC hoses.

Using the Nanocom, look at the stored and actual TPS voltages, they should be the same or the stored voltage should be fractionally higher than the actual voltage. Look at the IACV opening, at normal idle it should be between 20 and 30 steps. If it is lower than this, that suggests an air leak so the valve is closing down to try to keep the revs correct. If the revs are rising when put in Drive, check the TPS voltage when you do that.

On the Thor, the cruise control cable needs to be adjusted correctly as that can cause issues too if it isn't. Check the process in RAVE and make sure that is correct too.

Thanks yet again, Gilbert. I say i’ll come to the forum and open up the floor to everyone but really i know deep down i’m just waiting patiently for your well informed and incredibly insightful advice.

You are right, it is easily manageable by utilising the brakes a bit earlier, but it did spook my brother when he went to drive it the other day ( a range rover veteran including a 4.0 westminster) so thats enough to warrant me investigating. I was flicking through all of the input data at the weekend and i found the table values and headings a little confusing at first, so you may have helped clear up some confusion on what exactly i was looking at.

I shall get it plugged back in later and have a good look!

I agree it's gonna be the TPS, IACV or an air leak. Both mine stay at 700rpm from Neutral to Drive or Reverse. Also check throttle cable adjustment just in case.

Throttle

1. Ensure throttle lever is against stop in closed position.
2. Rotate thumb-wheel clockwise until all slack is removed from cable. (Throttle lever is about to lift from stop)
3. Back off thumb-wheel anti-clockwise by one quarter of a turn.
4. Check cruise control cable adjustment.

Cruise

1. Using light finger pressure only, push cruise control lever towards plenum chamber to remove all free play from cruise control cable.
2. Adjust cable outer length by turning plastic thumb screw to achieve a clearance of between 0.5 mm and 1.5 mm.

Pete12345 wrote:

I agree it's gonna be the TPS, IACV or an air leak. Both mine stay at 700rpm from Neutral to Drive or Reverse. Also check throttle cable adjustment just in case.

Throttle

1. Ensure throttle lever is against stop in closed position.
2. Rotate thumb-wheel clockwise until all slack is removed from cable. (Throttle lever is about to lift from stop)
3. Back off thumb-wheel anti-clockwise by one quarter of a turn.
4. Check cruise control cable adjustment.

Cruise

1. Using light finger pressure only, push cruise control lever towards plenum chamber to remove all free play from cruise control cable.
2. Adjust cable outer length by turning plastic thumb screw to achieve a clearance of between 0.5 mm and 1.5 mm.

Solid, i’ll have a good nosey at this this weekend then. Thanks Pete.

I assume a manifold blow could be classed as an air leak?