Hi all,
Just got the excellent rovergauge software working with my 3.9 disco,
Just a few points of note.
I have the correct base idle (set using tacho in rovergauge). But when the engine is idling warm I only see Stepper motor values of 3% to keep it at its target of 700ish, is this normal or wrong ( I would assume it ahould maybe be a bit higher?
Also fuel temp always shows 0deg but no fault code showing, it is a new sensor, maybe I will have to bell the wiring?

any help much appreciated,
Olly

The volume of fuel/air required at tickover is pretty small, so 3% seems OK.

Peter

Hi peter,
Thanks for reply, One of the issues of having the Stepper motor at only 3% means it has little room to compensate for temperature etc, and it meant it raced to 2000rpm when started, this is due to ECU fully opening the stepper motor (100%) when you stop to aid starting, it then quickly cuts it down to a stable idle speed in steps, using its learned idle positions.

Anyhow I took off my plenum, cleaned it out, changed a rocker cover gasket, replaced dipstick seal, replaced stepper (only because I broke the old one :-( ). Re-set the base idle And it now only races to 1500rpm when started, with the stepper open about 9% when warm, this seems a bit better, I'm still thinking the MAF is on its way out though due to its 27% reading, maybe I'm just being to picky!

Olly

somersetsparks said:

due to ECU fully opening the stepper motor (100%) when you stop to aid starting, it then quickly cuts it down to a stable idle speed in steps, using its learned idle positions.

Re-set the base idle And it now only races to 1500rpm when started, with the stepper open about 9% when warm, this seems a bit better, I'm still thinking the MAF is on its way out though due to its 27% reading, maybe I'm just being to picky!

Olly

Click to expand...

14 CUX ecu doesn't really "learn".

It is true the stepper winds out to fully open when you turn off the ignition. When you start the engine, the ECU sends a longer pulse to the injectors giving enough fuel for start up; once the engine has fired, air flow then takes over - the air flow meter sends the flow volume to the ecu which then winds the stepper in to give an idle speed of around 800rpm.

Unstable idle is often a symptom of a sticky stepper, essentially, if you just take the stepper out and start the engine it will "idle" at about 2000 rpm.

For the ecu to regulate the idle speed, it has to switch on the idle control. Idle control in turn takes inputs from the speed sensor, the throttle potentiometer and engine speed (from neg side of coil).

If you have cats then the control is slightly different. ASFAIK the ecu ignores the air flow meter and looks to the lambda readings to adjust the fuelling depending on whether or not there is a rich or lean condition. The ecu reverts to open loop when the throttle is more than 75% or the engine speed is above about 3400/3500 rpm.

You say you removed and cleaned the plenum (and I assume ram housing?) did you check the setting of the throttle disc? - this needs to be right or you will struggle to get a smooth idle at the right speed because there may be too much air flow if the gap is too big, it may also be that the throttle pot base setting is wrong because the throttle pot is attached to the throttle spindle if you see what I mean.

Obviously things like air leaks and ignition timing can affect idle as can the temperature thermistor. Is it running rich?

Anyway,

HTH.

I'm trying to remember the values I had on my old disco 1 using Faultmate. I think 3% is a bit small tbh. As you've had the inlet apart I'd do a double check for air leaks.

Thanks for replies,
After I took the plenum off and cleaned it (especially around throttle butterfly), and replaced the stepper motor, and reset base idle, the stepper is now 9% open on normal tickover, this has improved the engine racing on startup, it now only revs to 1500rpm then settles down to normal very quickly! I checked the throttle butterfly was sealing with a little petrol and it was perfect after cleaning.
My throttle Pot, shows 5% at idle which is apparently good,
Lambdas show good values when engine is warm, and it accelerates well, its difficult to get it much over 80deg this time of year though with the stat. open and a viscous fan, maybe I have an 82deg stat fitted and should be 88 I assume?
Just the MAF showing a lower then expected reading on the 'direct scale' also when measuring with a voltmeter, one of the readings was low.

I didn't remove the Ram housing as it was pretty clean.

The clearance on the throttle disc should be 0.05mm.

Forget talking about percentages - it means nothing to the ECU. What is the base voltage for the throttle pot, how does it scale and what voltage does it show at WOT.

You'd have to look up what the correct value is for your vehicle, my '91 RRC non-cat has a base voltage of something like 0.3v which is 11% at idle. If your throttle pot is at 5% the base voltage could be too low so the ECU may not be controlling the idle.

classic kev said:

The clearance on the throttle disc should be 0.05mm.

Forget talking about percentages - it means nothing to the ECU. What is the base voltage for the throttle pot, how does it scale and what voltage does it show at WOT.

You'd have to look up what the correct value is for your vehicle, my '91 RRC non-cat has a base voltage of something like 0.3v which is 11% at idle. If your throttle pot is at 5% the base voltage could be too low so the ECU may not be controlling the idle.

Click to expand...

Thanks Kev, have been advised that it should be anywhere between 3-6% when at idle, ideally 4%, but mines not slotted, I suppose the specs for a 1999 3.9 with cats are a little different.
Like I say though, seen a Big improvement in startup rpm after cleaning plenum and resetting base idle, don't sound like such a moron when starting car It used to rev to 2000rpm for a few seconds!

somersetsparks said:

between 3-6% when at idle, ideally 4%, but mines not slotted, I suppose the specs for a 1999 3.9 with cats are a little different.

Click to expand...

I still say check the voltage - pretty sure I've posted how to do it before so maybe do a search.

Spec on later model may be slightly different but not much, it's still the same ECU with the same internals. My throttle pot isn't slotted either but there is a small (very small) amount of play in the thing but a round or half round needle file can soon provide a bit of extra tolerance to enable some adjustment to be made. If I remember well, Discool posted about this not all that long ago.

You said something about your air flow meter - what is the CO trim value? For a cat cat I think it needs to be something like 1.7v or 1.8v whereas for a non-cat car it is anything between 1.0v and 1.5v.

Thanks Kev, voltage checked and in range, the CO trim is showing as 1.83v on my cheapo voltmeter, so likely in range, anyhow the other reading for the airflow is showing it to be failing something like 1.39 where, between 1.6 and 1.75 would be expected, I'm gonna give the MAF a good old clean anyways, can't hurt. Anyhow I did a 420 mile run at the weekend and used £86 of gas, so not running to badly, maybe i'm just being to fussy!

The throttle pot is non adjustable as ecu learns the values.

As long as you have smooth voltage curve it is just fine.

Has anyone ever considered removing lambdas and changing to non cat tune resistor to achieve better running?

Tuning to 14.7 all the time is not always ideal!