ECU idle trim confusion

88 XJS V12 with the 16C ECU.

I have read the book and Bywater’s writings several times. It sounds like a higher monitor voltage = lower lambda reading = ecu compensates richer mixture. So anything over 1.9V = richer mixture, anything under 1.9V = leaner mixture. Correct?

I’ve read a lot of archives, and sometimes people say the opposite.


i just turn a knob , and if want to keep the readout % number , i program it in !
it stays that way, my ECU has closed loop/open loop , just a button on/off!center%20consol%20J%20-bond%20system%20002

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Ok, rechecked today, you get different readings depending on which direction the capacitor is! First way, the open loop reads 1.9V like it should, the second way it reads 1.8V. The closed loop is then about .7V lower for the second way compared to the first.

So I ASSUME choose the direction that gives 1.9V for open loop. Measuring the wrong way yesterday, I was at 2.6V for closed loop with the knob turned all the way rich! It did idle nice, though. I backed it off four clicks to 2.0V-2.1V. Don’t want to foul plugs.

Yes sounds good.

I would put it back to the 1.9V reading.
It’s safer if the base fuel map is a little on the rich side, to avoid lean running at WOT, and gives you a bit of a smoother idle.
And anyways, as soon as the O² sensors kick in the ECU will adjust accordingly.


This is the confusion. Reading the book and Bywater’s writings, a higher monitor voltage means it’s compensating for a lower lambda voltage, so it’s a richer setting? So by going to 2.1/2.0, I am slightly rich.

High monitor V = Low Lambda signal = ECU corrects in rich direction.

In other words
High monitor V = Lean

This is from Roger Bywater’s website:

High monitor V = Low Lambda signal = ECU corrects in rich direction.
Low monitor V = High Lambda signal = ECU corrects in weak direction.

I see it the other way. High monitor V = Rich

Also, at WOT, not only does the ECU go to open loop, but the full throttle switches kick in. So the idle trim does not make a difference for WOT.

I asked Sir Roger Bywater and got a response. He didn’t say a number is related to rich or lean.

He said:
When running in feedback you cannot set the fuelling to anything but the stoichiometric point. The ECU will always apply correction to achieve that.
The monitor outputs provide an indication of how much correction the ECU is applying when in feedback and the idle trim is used to set that to the mid-range point.
The purpose of the adjustment is to obtain a setting where the ECU does not have to make a large correction swing when the throttle is opened – which can cause a flat-spot.

So pretty much, I got them as close as I could to 1.9V, so should have a smooth transition from closed to open loop, and open to closed loop, which I guess is when shifting from P to D, D to P, giving it sudden throttle, and then going from full throttle to cruising.

I’m probably talking to myself :wink: but can someone explain to me what’s happening?

So on Monday I got the car warmed up to it’s normal 180F and adjusted ECU idle trim to about 1.9V during feedback closed loop operation.

Tuesday, when I got home after half an hour of heavy city driving, the coolant temp was more like 190-195F. I checked the ECU idle trim and they were at 2.1V. So I re-adjusted them back down to 1.9V.

Does the monitor voltage change depending on how warm the car is? Is it best to get the car as hot as possible before adjusting the trim?


Yes it would change because the way the fuel burns is related to the engine’s temperature.
And the Air and Water Temperature sensors will alter the fueling according to temperature.

That’s what I did.

And by the way:
Base Fuel Map Setting
CCW: Rich
CW: Lean


thanks, I’m understanding it better.

One thing I’ve noticed, which makes sense, now that I’ve got the idle trim adjusted to 1.9V when nice and hot, the car’s throttle response when not fully warmed up first 5 minutes of driving (under 180F) is not great when going from idle to flooring it. There is a slight pause. This I assume is because the idle trim is very far from 1.9V now when the engine is cold, so the ECU must change trim quite a bit when going from closed loop (idle) to open loop (flooring it).

But I guess the trade off, it seems to idle a bit smoother at normal hot temperatures, and throttle response is perfect.

Moral, don’t floor it until it’s fully warmed up :slight_smile:

Yes mine does the same, just the first couple of minutes… Good to know it’s not only me…!
Either it’s just the way it is, either we have the same problrem.
I have tried, but to date I haven’t found a solution for this, so I’ve learned to live with it.

I have a slight bog as well. Runs fine when cold and runs fine when fully warmed-up. But for a few minutes in between in bogs down a bit.

TPS and CTS are OK. And I have a non-feedback ECU so oxygen sensors are not in the mix.

One of these days I’ll have to dig in and investigate more thoroughly… although it runs so well when warm that I’m tempted to leave well-enough alone :slight_smile:


My previous V12 never had this condition so I’m tempted to say it’s a correctable fault.

Finding the fault is another matter, as we know :slight_smile:

Perhaps one can hope that, whatever the fault is, it becomes much worse…and thus easier to isolate.


Hope the Jaguar God is not listening to what you are saying Doug…
Better go and do an offering of some screws and washers !!

My theory is a lot of it has to do with not having a MAF sensor as in later cars. Without a MAF, the ECU really has no idea how much air is coming in. The TPS plays a big part, but IMO the ambient air sensor plays a big part too in these special cases because the temp of the air is equally important.

I am going to spend some time experimenting with the ambient air sensor…i’m thinking of relocating it outside of the air cleaner. I seem to get better idling in the morning when the outside temperatures are much cooler, plus no bog when flooring it before the engine has fully warmed up. I know the ambient air sensor is a minor ECU trim adjustment, but it’s enough where if I pull it when the engine is warm, idle goes up quite a bit (too rich), and if I try to start it with it off once warmed up, it won’t start (too rich). With it pulled, there is infinite resistance, so the ECU thinks it’s pulling air from the north pole.

I’ve opened up my air intakes with the 2" PVC mod, but haven’t fitted cold air intake yet. My theory is that the air cleaner compartment (where I’ve relocated the ambient sensor because of my mod) eventually bakes under the hood, and sucks down very hot engine compartment air while idling, and can’t be cooled quickly with cool morning air while cruising. So I believe the ambient sensor is reporting temps higher than the air really is, thus leaning the fuel a tiny bit?

My infrared showed the air cleaner interior to be 150F. Ambient air was 75F. Engine compartment air was about 90F. Sender was reporting 780ohms (about 125-130F?)

One day I really need to fit a cold air intake so cooler air is always entering the air intake housing to help cool it. But for now I’m going to move it out of the air cleaner so it doesn’t get influenced by the air cleaner temps, AND I’m thinking of putting in a 100 ohm or even 200 ohm resistor so that the ambient sensor reports temps back about 20F cooler than it’s sensing, just to push things a tiny bit rich.

We shall see…

Greg, has your ECU been modified by AJ6 Engineering with the enhanced or super-enhanced mod? If so, it depends on the throttle pot for fuelling during accels rather than the MAP sensor. It’s presumably mapped for a fully-warm engine and might lean out a bit when it’s not warmed up.

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Yes, it has. Good to know, thanks. So my messing around with ambient sensor will only test idle.

If by later cars you’re referring to the 6.0L V12, I am afraid you’re not correct. There is NO MAF

No, I meant cars in general, by late 90s, did most cars have an MAF?