Rough idle - discovery 1

discool said:

Simples first, so remove the crankcase breather (flame trap) and its hose to the plenum chamber and soak in petrol to clean it, then refit,

If you still have an issue carefully remove the throttle control valve and clean and replace and then check the base idle which was set in the factory, info on the procedure and how to do that is in workshop manuals.

There’s not much you can do with the throttle position sensor, its just a variable resistor, so don’t fiddle with it as they are now obsolete… so you will have to find a replacement or make a replacement item

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hi_arshard said:

Hello guys, good day …!

I am having an issue with idle. The RPM goes up and down. In cold start By Default the RPM idle on 900 to 1100 RPM. When stopped in traffic or signal light the RPM goes up and down and the engine stalls. when the car comes to a complete stop on Gear D this happens. In neutral the idle is up and down for 1800 and down to 600RPM but it doesn’t stall the engine.

Only last change done was replaced the vacuum advance unit on the distributor.

Appreciate your support on this as usual.

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classic kev said:

Check ignition timing and vac advance. What is the base voltage on the throttle pot?

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classic kev said:

To check ignition timing you need a timing light.

Remove the idle air bypass pipe and check it carefully - they perish and split. I also assume yu are happy the stepper motor is sealed against the plenum? The gasket, if original likely fell to bits, I found a smear of RTV silicon was effective.

600RPM is a bit high for base idle, should be 525+/- 25rpm although I'm not convinced that would prevent the ecu being able to maintain the correct idle speed.

According to my Range Rover workshop manual the following settings apply to the 3.9 engine.
Idle all loads off in neutral should be 665 to 735rpm.
Auto, in gear and AC on 650 +/-28 rpm
Auto, in gear AC off 600 +/- 28 rpm

Manual box, 700 +/- 28rpm
Manual box AC on 750 +/- 28 rpm

Double check you don't have an inlet leak and the EFI is trying to compensate although this typically presents as an unstable idle and stalling. However, this is exactly what you describe.."When stopped in traffic or signal light the RPM goes up and down and the engine stalls".

You should check all inputs to the ecu that affect idle stabilisation:

Throttle potentiometer base voltage.
Speed sensor.
Air conditioning.
If auto, gear selector switch.
Also, a bit left field, check your alternator and battery.

I also assume you are checking/setting things with a warm engine?

I can't say I recall the idle being at 600rpm but I worked on the basis I was happy with it being anything under 800rpm. When cold, the idle is higher due to mixture enrichment, by increasing the injector pulse duration IIRC.

You don't say if this is cat or non-cat, I assume it is cat tune, in which case CO Trim should be 1.8v IIRC, if non-cat it will probably be lower, I an't remember where I used to run mine, less than 1.0v though, but mine was non-cat. If catalyst, then maybe check lambda switching and that the O2 sensors are working correctly, Lambda control alters fuelling to maintain the correct lambda value especially at idle and part throttle. Lambda is ignored at WOT. Accordingly, if there is an inlet leak, the ECU would try to make the mixture richer to balance lambda, it then goes rich so it backs it off and this continues until essentially the ecu can't get the mixture under control and the engine dies although I would expect the EFI warning light to come on - assuming the D1 had an EFI warning light. I'm sure Discool can confirm.

HTH

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classic kev said:

To alter CO trim, just remove the sealing plug then use the appropriately sized allen key to adjust. For various inputs etc, do a search in "Engines, it's a V8 mate" and you'll find lots of info, you might have to think about the search terms though.

OK, if you go to the V8 section as suggested and do two searches:

1: Base idle and 2: CO trim, you should get a number of pages of references, just have a breeze through some of those old threads and I'm sure you'll find what you need.

You might also consider a search for throttle pot testing and ignition checks as these may provide some helpful details.

One other thing to maybe do - Remove and clean the plenum/ram housing and inlet trumpets. Also, check, clean and set the throttle butterfly. Trust me on this!

If I just tell you everything, you don't learn as much as searching and reading, we all had to do it that way, it's kind of an unwritten law in here. Anyway, good luck hope that helps, let us know how you get on.

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