Thread: Discovery 1 1994 Idle issue

The fast idle problem is a result of emissions control strategy. High idle can be partly corrected by 1. screwing down the base idle screw, 2. removing the fibre washer between the stepper motor and its housing. However finding the road speed connection to the ECU and cutting it gets rid of the problem altogether. I once fitted a 14CUX system to a 110 County without speed input and it was great to drive without the incessant idle up phase.

Originally Posted by bee utey

The fast idle problem is a result of emissions control strategy. High idle can be partly corrected by 1. screwing down the base idle screw, 2. removing the fibre washer between the stepper motor and its housing. However finding the road speed connection to the ECU and cutting it gets rid of the problem altogether. I once fitted a 14CUX system to a 110 County without speed input and it was great to drive without the incessant idle up phase.

Ok Thanks.

Have I got this right.

The idle up during driving is linked to emissions control. Seems a bit high particularly when descending hills with the throttle closed.

When stationary the road speed input then brings back the idle speed to normal via the stepper motor.

No sure of the ECU type fitted to the 1994. I will to check I believe the 14CUX was fitted to later models.

My question is how do you cut the road speed input to the ECU there by preventing the idle up condition?

I have disconnected the plug to the steeper motor at normal operating temperature which is fine once the engine warms up however it does not allow for an increase in idle when cold or when the AC is working.

Originally Posted by right

Ok Thanks.

Have I got this right.

The idle up during driving is linked to emissions control. Seems a bit high particularly when descending hills with the throttle closed.

When stationary the road speed input then brings back the idle speed to normal via the stepper motor.

No sure of the ECU type fitted to the 1994. I will to check I believe the 14CUX was fitted to later models.

My question is how do you cut the road speed input to the ECU there by preventing the idle up condition?

I have disconnected the plug to the steeper motor at normal operating temperature which is fine once the engine warms up however it does not allow for an increase in idle when cold or when the AC is working.

The ECU fitted to every D1 is the 14CUX type, there may be minor variations but they are essentially the same thing from 1989 to 1998.

I can't tell you which wire is the idle speed input but it should be at the multi plugs on the cabin side of the large grommet in the firewall where the engine harness goes inside. Either a wiring diagram or a pulse tester should be able to find it.

I have been trawling the internet for info on the speed sensor input to ECU and noticed some comments referring to the impact of a dodgy speed sensor on the stepper motor / idle.

Although the speedo is working fine could the speed sensor output be faulty in a way that is corrupting the ECU input.

If I disconnect the speeded sensor then obviously the speedo would not work but it would simulate the disconnect of the ECU speed sensor input thus verifying the fix as described previously in this thread.

Any comments would be appreciated.

Road speed sensor disconnected at transfer case and idle issue resolved.

Next step is to isolate the road speed sensor input to ECU whilst maintaining speedo functionality.

Just inquiring if the speedo is working fine is the integrity of the signal to the ECU all good.

Also is it worth having the ECU checked for codes etc.

have you had a look at the potentiometer on the throttle body to make sure it is not sticking/faulty , carbon/dirt in the butterfly valve stopping it closing correctly.
probably not, but Also check your vaccum advance unit on the dissy, to make sure the rod is moving back and forth correctly on the plate and not stuck or sticking and advancing the timing.