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MG MGB GT V8 Factory Originals Technical - Rover Fuel Injection - periodic glitch

The 3.9 has occasional fits of running too fast (1500 - 2000 rpm) - but the ECU does slow it down to 750 after the car comes to a halt. The stepper motor seemed the likely culpret but the valve is working as it should and seats tightly so far as I can tell.

Anyone else got any ideas before I get it on the test rig ?

Good wishes

Roger
RMW

I can offer some general suggestions, but there might be others that know the specifics around the -14CUX system better, but here goes-

In other conversions I've done (GM) that exhibit the same problem, I've wired the vehicle speed sensor back it and made the problem go away.

(Before I proceed, I must tell you I also had this speed problem on my 3.9L FI system and never got around to hooking up the speed sensor.)

When the ECU lacks a speed signal, it sometimes will do unexpected things with idle speed. Idle control is there for several reasons - correct for ambient conditions, idle-up to keep the power steering pressure up for low speed manuvering (in the original application)and airconditioning load. When the box doesn't see the signal, it may do things you didn't expect.

In some GM PCMs, the vehicle speed sensor is only used to command idle-up during low speed manuvering. The accuracy of the speed signal isn't critical, only the wave form that the computer reads is. For instance, it doesn't seem to make any difference if you use the teeth off of the reluctor wheel or the bolts on a CV joint.

Food for thought
Phil

Interesting Phil, and it sounds dead on. Any thoughts on what would cause injector pulse width to oscillate wildly at 2000 rpm? It's a ford system.
Jim
Jim Blackwood

Phil - thank you. I couldn't stand having a dead engine at lights after an over-run and so fitted a speed sensor. Since the speed reverted to normal when the car is stopped, I guess the sensor is working.

Maddeningly after a 65 mile fast run on Sunday, the problem is now obscured. It will return 'though.

The only other sensor which conforms with the symptoms, is the throttle sensor but so far as I know, the ECU takes a 'direct' and instantaneous reaction to its movement. With my problem, there was a short delay after stopping before the engine slowed - of about 1.5 - 2 secs.

I have heard it said that all ECUs induce involuntary actions and have a mind of their own. Embarassing if they make you sound like your Grandmother struggling to maintain control !

Thanks again

Roger
RMW

Yes, my 14CUX has taken to erratic idle...I set it very slow in the summer, and have recently wound up the air bleed screw to improve tick-over, and now its hunting from 800-1100rpm....it may be the stepper motor that's having a moody, I have a spare one so I'll do a swap this weekend.

mike
Michael barnfather

I've been tying to sort out the same fault on my v8 for the last month or so, running at 2000rpm when comming to a halt and it takes a few seconds to drop to about 800rpm. It also does this every time i start the engine. not good on a cold engine. (the speed sensor is fitted)

I've got the diagnostic equipment for the 14cux, which shows all the inputs, so I know the speed sensor is working ok. The only fault the Ecu is showing is 'Idle fault, stepper motor problem' However I have replaced my stepper motor (new). Having spoken to a few V8 specialits, it seems that if the 14cux ecu sees a idle fault, is will suggest the stepper motor because that is what contols the idle, when in fact it could be somthing completly different, ie a ignition fault?
I did try swopping the ecu for a known good one, which made no difference.
I havent fully cured the fault yet, but the idle did make a big improvment when I replaced the distributor cap & rotor?,
My next step is to check the rest of the ignition system.

Mike

Mike

As I recal, the stepper motor actually moves a quite large needle valve against a seat in the intake manifold. Unlike some GM stepper motors that actually move the throttle plate externally to step the idle, the Lucas/Rover design is subject to fouling because it is internal to the manifold and is essentially just a 'trim' to the fixed throttle plate idle position.

The situation you describe sounds a lot like hysteresis induced by the stepper motor's tapered air valve sticking in the seat.

I'm curious if cleaning out the air valve and setting the 'base idle' wouldn't go a long way to reducing this problem. I would think you might be able to tell if this was the root cause of the problem by disconnecting the electrical connector to the stepper motor when the car was idling properly and driving it to see if it exhibited the same problem. I guess it doesn't completely eliminate the posibilty the ECU is commanding the stepper motor to exibit this problem, but it is a clue.

There was a thread a few months ago on the subject of setting the base idle on the hot-wire system.

Phil

Yes, Phil.

It's a big conical valve on the end of a thread, a fairly crude arrangement, I will try unfastening the connector and seeing what happens.

Mike
Michael barnfather

The cold start 'valve' operated by the stepper motor is not easily accessible however it is prone to 'soot up' so that does leave a trace of how it seats. I did not think the valve surfaces were a problem although there was a little 'gum' on the stepper 'probe'.

Phil - your suggestion that we disconnect the motor will be the first thing I try when the problem recurs.
Thanks.

The Landrover team thought the motor or the valve were the most likely cause - I still incline to question the ECU design - that element which enriches and causes the engine to run faster on starup.

Mike - interesting you have a very similar experience - I wonder how the RV8 owners get on.

Good wishes

Roger
RMW

One intresting point I noticed is that when the engine/ efi was in my Range Rover I never had any idle problems, its only since its been tranferred to the MG.? Which is why I have started to look further afield than the efi system.

I found on one of the TVR forums, that they had a similar fault that was caused by voltage variations from a faulty alterntor ??? food for thought.

Mike
Mike

Sounds to me like what happens when the Idle isn't set properly. I wonder if resetting the Idle wouldn't cure the problem.

Justin

This thread was discussed between 01/02/2005 and 03/02/2005

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