Idle slow to drop to 950 rpm

[blaze_away]

My 4.6 Chim running 14CUX

When lifting off the throttle my engine stays at a higher revs ~ 1700 for a few seconds before settling to a perfect 950 rpm if the car is stationary eg at a junction.

With all the work thats been done recently to solve the little rattle (it was a slightly worn big end shell) I am a bit puzzled by whats causing this high revs issue.
Stepper is genuine been cleaned greased checked all good even swapped out and still the same.
Base idle been set at 600rpm
All new ignition components (swapped back to all old stuff still does it)
Swapped out the whole 14CUX unit too.
No vacuum leaks found by spraying easy start around intake area.

Over to the collective wiser ones, any ideas ?

Edited by blaze_away on Sunday 1st September 11:56

[colin mee]

May sound silly. I had that on my wedge. A jubilee clip was catching the cable

[Classic Chim]

Whats it doing with stepper disconnected Frank.

Symptom sounds like stepper or inlet leak but that’s just stating the obvious.

[Classic Chim]

colin mee said:

May sound silly. I had that on my wedge. A jubilee clip was catching the cable

Check carpet is tucked up properly by throttle pedal, I had that once.

[asd2001]

Frank
I had the same earlier this year. I stripped and cleaned the stepper a second time (and the housing) and it seemed to do the trick. Good luck.

[ric355]

Try putting your toe underneath the throttle pedal and lifting it to see if the revs drop. Could be a sticking cable.

[Steve_D]

Have you checked RoverGauge to see if the speed is dropping to zero.
If coming down to a junction the revs will hold for a few seconds after coming to a standstill. Designed that way.

Steve

[blitzracing]

Download and read Steve Heaths ECUmate manual- really good insight on stepper / idle control

http://www.shengltd.com/tvrextras/docs/Ecumate%20i...

Otherwise check the throttle pot voltage values are correct with the throttle shut.

The following voltages are within the units self calibration range.

Voltage measured between the red wire (+ve ) and green (-ve)

Throttle closed .085 - .545 volts

Edited by blitzracing on Sunday 1st September 18:41

[ChimpOnGas]

Steve_D said:

Have you checked RoverGauge to see if the speed is dropping to zero.
If coming down to a junction the revs will hold for a few seconds after coming to a standstill. Designed that way.

Steve

Sound advice, the Lucas 14CUX system definitely needs that road speed signal for correct idle valve (stepper motor) function!

Years ago when I was still on the 14CUX system I had a stalling issue, the car would stall if I approached and stopped at a junction too rapidly which I traced to a lose wire at the back of the speedo, as soon as I reconnected the wire normal service was resumed. It sounds like the OP has the opposite symptom but this could still easily be a road speed signal issue, if the ECU still thinks the car is moving it will instruct the stepper motor to bleed air above it's idle setting and the engine will race.

Usefully Rover Gauge will show both stepper motor behaviour and road speed so would make a good diagnostic tool in this case, but an even easier starting point would be to clamp the stepper motor feed hose when the engine is racing, if the engine speed falls you'll at least know for sure it's caused by excessive air passing through the stepper motor.

It's all too easy to condemn the stepper motor itself, but its equally important to consider what's controlling it, a perfectly performing stepper motor that's being told by the ECU to open when it shouldn't is not a stepper motor fault it's an ECU fault or more likely false road speed information being fed to the ECU.

Keep in mind the road speed signal is sent to the 14CUX ECU via TVR's own road speed pulse wave converter box that sits under the dash, this box of electronics was needed as TVR used a different speedo to those fitted to Range Rovers & Discoveries the injection system was designed for. TVR's little box of electronics just modifies the wave form output from the Caerbont speedo into something the 14CUX can work with, the trouble is the standard of TVR's control modules during the 90's was rather poor, ask any Cerbera owner as they have lots more of these unreliable little TVR made black boxes.

If Rover Gauge shows you're still moving at say 10 mph when you've actually come to a stop I'd suspect the TVR wave form converter box, here's a photo of it's location.



With the dash top off you'll find it roughly in the centre behind the heater controls and towards the base of the windscreen



If its the source of the OP's problem Jody at Python Racing may be able to help as he seems to be the go to man for fixing TVR control modules.

http://www.pythonracing.co.uk/electrical/

Hope this helps?

Dave.

[blaze_away]

Thanks to you all for very helpful posts.

Started checking and quickly found the blue wire on the connector of the TPS was loose. Just fell out when I touched it. Luckily I had an old TPS so have now soldered that into place. Will go for a run later to log Rovergauge data and see what the stepper and road speed relations are......I'll be back

[Steve_D]

Was just about to post that Marks Finger point at TPS was as valid as my road speed answer.
I suspect all will be fine now on your road test.

Steve

[blaze_away]

Sorted.

Been for a run and its all behaving itself properly now. Looks like the loose blue wire on the TPS was the root cause.

Thanks folks all good pointers as always.