Stepper Motor / Air Idle Control Valve : as easy as it sounds?

In theory:

- plug off the plug
- unscrew
- switch for the new one, grease the fitting a bit
- reconnect the plug

Is it that simple? In the world of Landy's I suspect that I'm being naive...

...it really is, probably will not even get your hands dirty!

Lets hope that is the problem though.

Neil

Seconded. It is that simple, but what problem are you trying to resolve?

It might be worthwhile taking off the plenum and ram housing and giving everything a thorough clean. Also, if you are having idle problems, check the state of the air filter.

Thanks guys, will give it a go, and also look at the air filter.

Hi again.

So, have now replaced the Idle Air Control Valve (IACV). Biggest challenge was finding a spanner small enough to fit the nut hidden behind some sort of oil pipe (didn't want to detach the pipe).

However, now I have the fast idle problem that I believe the garage was telling me about, which they think is to do with the ECU:

Before replacing IACV;
- unreliable start
- eratic idle speed, sometimes to low <300 rpm, stalled on gear change, sometimes 2,000 rpm
- ran OK when in gear

After replacing IACV;
- starts first time (but was warmer already)
- idle speed in Neutral or Park far too high. Initially 2,500 rpm and climbs towards 3,000 rpm.
- seems to run better when in gear

Most worryingly, the vehicle 'jolts' alarmingly when moving into gear (Drive or Reverse). I guess this is a result of the transmission coping with a sudden drop in the rev range, from 2,500+ at idle, to ~1,000 when in gear. Have to keep my foot hard on the brake during these transitions.

Obviously something is still not right. Another sensor? Or the ECU itself.

Any ideas appreciated.

R

classic kev said:

It might be worthwhile taking off the plenum and ram housing and giving everything a thorough clean.

Click to expand...

Hello Reg, Kevin did say - clean your plenum?

What has happened is, you have opened up the idle air channels but crud and/or friction in and around the intake mechanism is probably not allowing the throttle plate to close fully so now you have too much idle air and the ecu adds fuel to go with it. Result: a perfectly good but high idle speed.

This is known as "Idle speed hang-up"

The PDF available from here will amuse and help.- not the same car - but all the tricks are valid.

Rover SD1 Vitesse Engine Idling Problems

And for lots more Plenum stuff here

Rover SD1 Efi Plenum Chamber

"Look after your Plenum and your Plenum will look after you!" with apologies to Trigger from "Only Fools and Horses".

Ramon
Vintage Model Airplane and Rover SD1 3500cc Twin Plenum Vitesse

overexposure said:

Hi again.

So, have now replaced the Idle Air Control Valve (IACV). Biggest challenge was finding a spanner small enough to fit the nut hidden behind some sort of oil pipe (didn't want to detach the pipe).

However, now I have the fast idle problem that I believe the garage was telling me about, which they think is to do with the ECU:

Before replacing IACV;
- unreliable start
- eratic idle speed, sometimes to low <300 rpm, stalled on gear change, sometimes 2,000 rpm
- ran OK when in gear

After replacing IACV;
- starts first time (but was warmer already)
- idle speed in Neutral or Park far too high. Initially 2,500 rpm and climbs towards 3,000 rpm.
- seems to run better when in gear

Most worryingly, the vehicle 'jolts' alarmingly when moving into gear (Drive or Reverse). I guess this is a result of the transmission coping with a sudden drop in the rev range, from 2,500+ at idle, to ~1,000 when in gear. Have to keep my foot hard on the brake during these transitions.

Obviously something is still not right. Another sensor? Or the ECU itself.

Any ideas appreciated.

R

Click to expand...

Replacing the stepper motor will not cure your poor starting - all you need to do is get it on Testbook and reset the stored closed throttle voltage under Sub System Tests.The poor idle quality is probably more to do with air leaks,a failing MAF,Oxygen sensors.This is all assuming the plugs/leads are all in good shape.Gems Rover V8's need all this in place to work properly.

ramon.alban said:

"Look after your Plenum and your Plenum will look after you!" with apologies to Trigger from "Only Fools and Horses".

Ramon
Vintage Model Airplane and Rover SD1 3500cc Twin Plenum Vitesse

Click to expand...

Ramon, you couldn't be more right.

A little while ago a friend of mine asked me why his 3.5 V8 Disco kept cutting out on over-run and also why he had erratic idle. Part of the problem was a duff vacuum advance but I made him bring it over and strip the plenum having found I could make it stall by gently wiggling the throttle linkage.

It will be no surprise to you to learn the throttle plate was gummed up and the plenum was pretty awful too. After a thorough clean of the plenum and all pipes attached his Disco has a smooth steady idle, he gets better throttle response and he believes slightly better fuel economy (such as it is with a V8!!). The moral is as you say, look after your plenum.......

classic kev said:

The moral is as you say, look after your plenum.......

Click to expand...

Ooh Er Betty!




And inside?



Ramon

Hi Ramon,

thanks. Is the intake for the air idle the big vent between the windscreen and bonnet? If so then the plenum air filter is perished (previous owners(s) appear to have skipped various service elements), and this will obviously not have helped re gunk reaching the throttle control valve.

I didn't have time to strip of the plenum etc, will have to be next weekend's project!

R

Intake plenum filter replacement (Range Rover 4.0/4.6/p38)

eightinavee said:

Replacing the stepper motor will not cure your poor starting - all you need to do is get it on Testbook and reset the stored closed throttle voltage under Sub System Tests.The poor idle quality is probably more to do with air leaks,a failing MAF,Oxygen sensors.This is all assuming the plugs/leads are all in good shape.Gems Rover V8's need all this in place to work properly.

Click to expand...

[You'll tell from this post that I'm not a mechanic....]

Would failing MAF or Oxygen sensors be readily obvious to the garage with testbook connected? I'm minded to stay at the garage on Friday and see for myself exactly what is coming out now.

One thing I don't follow re Ramon's post on the Throttle Control Value is that, if the Throttle Control Valve were jammed open by some gunk, how come the rev's drop back when in gear. Does the accelerator pedal 'override' the throttle control value when the vehicle is in gear? i.e. that the throttle control vale only influences air flow when the vehicle is idling (park or neutral).

I don't know much about Gems, but eightinavee will hopefully put me right where necessary;

The Throttle Control Value is taken from a potentiometer on the throttle butterfly shaft; this is often referred to as the throttle pot. In more basic systems this is used to establish injection parameters at idle, no load.

When you drop the transmission into gear, the extra load will slow the engine down so the ECU will detect dropping rpm and then adjust it by activating the stepper motor (idle air bypass valve). However, if the engine rpm isn't dropping below the stored value, it wouldn't try to raise or lower the engine speed with the stepper motor because it wouldn't be necessary. The problem is that if the base/no load idle speed is too high you will get a jolt dropping into gear.

What eight has been saying is that if there was gunk on the throttle butterfly and this has now been removed, the position of the throttle pot has changed and as a result so has the input to the ECU. This means the engine is trying to "restore" the situation it was programmed to but the physical condition has now changed so your ECU has to be set to the new parameters.

The next bit you ask is whether or not the accelerator pedal overrides the throttle control value - yes I suppose it does because as the throttle moves, the resistance of the throttle pot changes and sends a larger or smaller voltage to the ECU throttle input. The speed of change and the resulting voltage tell the ECU what you are asking the engine to do. It then looks at other inputs and responds according to it's stored map.

MAF and Oxy sensors should be easily detected by test book.

Cleaning the plenum, throttle assembly and all the hoses is often all that is necessary unless there is a readily identified fault in one of the sensors. It also gives you the opportunity to make sure that all vacuum hoses are in good condition as they are known to perish, also, by taking off and cleaning the plenum you have the opportuinity to make sure there are no leaks when you reassemble it.

Anyway, hope that helps you and if I'm misinformed I'm sure eightinavee will correct me, he knows his onions, he does.

Sort of,the Gems ecu stores a closed throttle voltage,(usually .5 -.7v) when told to by Testbook.For Gems to operate idle control the feedback from the throttle position sensor must be equal to or less than the stored value.If its above the ecu thinks you are demanding power and will leave the IACV jacked open - hence the high idle.It does this so that it can bring down the engine speed at a steady rate as the car slows to a halt,but also be ready for sudden loads from power steering,EAS compressor etc.If the engine is healthy and making good power it does not take much extra air for it to rev up to 2000 odd rpm,but if it is tired or has bad ignition/air leaks etc then the IACV might not even be able to keep the engine running - let alone at the target speed.The Gems ecu likes to see 15-30 steps open,hot,all loads off when running at target idle speed.Off idle the stepper usually reads about 70 steps.
Its a good system,I really like it - had one in for MOT emissions failure last week,the garage told the owner it needed new cats.A set of plug leads later and it recorded 0.01 Co and zero Hc at its 2500 rpm fast idle test - not bad for an 11yr old 4.6 v8 !

Sorry Reg, you got me there, but I dont think its the same plenum. That name (plenum) also applies to any air reservoir and I reckon the one you identified is the one prior to the cabin air intake system.
I've got a cabin intake "plenum chamber" on my car too.
You need to get familiar with the various components of your engine and the RR Efi plenum chamber sits right on the top and in your case probably has a single round intake tunnel instead of the two tunnels shown on the prior porno image of my own engine.
Something like this:

overexposure said:

if the Throttle Control Valve were jammed open by some gunk, how come the rev's drop back when in gear. Does the accelerator pedal 'override' the throttle control value when the vehicle is in gear? i.e. that the throttle control vale only influences air flow when the vehicle is idling (park or neutral).

Click to expand...

An engine idling off load at excessive rpm when put into gear, has to start turning the transmission torque convertor which in turn wants to get the car to move. - not easy from a dead stop with all that tonnage requiring forward motion. so the engine speed drops until the TC can get going. in the meantime you also have your foot on the brake.

Result engine speed remains baulked.

Take your foot off the brake without touching the accelerator pedal, and the engine will try to get back up to its prior rpm depending upon, load, gradient and transmission power losses.

Kev, Eight, Ramon: thanks ... It's been helpfully educational, and I have a better idea now of what triggers what and what to do. Ultimately, next step is ECU reset, have a look at the Testbook output, and potentially clean up the plenum.