Adjusting ECU fuel mixture

1988 V12 w/ O2 sensors

Wanted to share my experience in trying to adjust my ECU fuel mixture.

Over the last couple months, I would weekly try to fine tune my ECU fuel mixture to help smooth my idle. I used the capacitor to try and get 1.9V on each bank while in closed loop mode (using O2 sensors)

But I would get slightly different readings week to week, and I noticed different readings if I did it with the plug off to force closed loop while in P, or if I did it while in D. Very frustrating.

I tried going a bit over 1.9V, and a bit under 1.9V. The idle smoothness in D did not change one bit. Roger Bywater had told me the ECU will always try to reach stochiometric so it runs correctly based off O2 sensors, no matter where I set it the ECU. He said this ECU setting is mainly for a smooth transition if you floor it which forces it into open loop.

Well, I did notice if I turned it too rich, to perhaps 2.1V or higher, when I floored it from a stop, the engine would bog and hesitate. The higher the richness (I tried 2.4V) the longer the hesitation, up to 2 seconds!

So what I’ve done, I adjusted it quite rich, test drove it, it bogged. So I’d turn it back (lean) 1-2 clicks and test drive again. It would still bog, but the time was less and less. I’d keep going 2 clicks until finally there was no hesitation when flooring it from a stop. (I did this at my normal operating temperature.)

My theory - if you set the ECU mixture too rich, like 2.2V, when you floor it and the ECU takes it to open loop, (which is 1.9V), for a second the ECU suddenly sees it must lean the mixture.
By running it a tiny bit on the lean side, when flooring it, going to open loop is a richer adjustment, which is what the engine wants.

Feel free to tell me i don’t know what I’m doing. :slight_smile:
But I really like the throttle response now when flooring it, I can spin the tires, and the car idles exactly the same.

This explanation of things sounds like the issue I am having with my 4.0L It will idle and I can increase revs slowly but if I goose it, it bogs out. I am going to do a bit of research.

It’s very hard to argue with success :slight_smile: but I always thought the primary function of the TPS was to add enrichment when the throttle is opened…akin to the accelerator pump in an old carburetor…to prevent bogging down.

If that’s the case then I wonder if your adjustments are masking a problem with the TPS?


I’ve replaced my TPS with the more reliable red one, and tested my TPS, it works great. I also have a new working enrichment vacuum switch, as well as the one on the throttle that comes on. Ive tested both, I am def getting enrichment when flooring it.

I still think there could be a pause in the ECU trying to figure out fuel mixture from open loop to closed loop, and closed to open, if those feedback values are way different. Bywater said it was important to get that feedback voltage at closed loop as close to open loop voltage, just for this reason, if you step on it.

I had tested giving it just over half throttle from a stop, and there was never any hesitation (because it was still in closed loop?). Only when flooring it (which is open loop) was there hesitation.

Our 80s technology ECUs are not the quickest tools in the shed…do you remember using an IBM 486 from the mid80s?


Win 3.1, riiiight?

Before windows!!! DOS.

Yea, I * sorta* remember that!

My introduction to the land of computers was a Trash -80…!!!

I think that if the engine runs already rich and then you further enrich it when you floor it, it gets overwhelmed, especially when in low revs, and thus the hesitation.

What you want is to adjust your mixture to the closest to the closed loop value as Baywater suggests and maybe one click to the rich side.
That’s what I did anyway and have no hesitation whatsoever.

I am not an engineer and all of this is way out of my league, and even after many months (if not years) of chasing “pur”-fection, I still don’t understand if the ECU knows that your are flooring it based on the TPS (electrical signal) or the MAP (vacuum).
If the latter, which I think is the case, then the choice of putting the Lucas ECU in the boot and connecting it to the manifold with the tiny and very long tubing, one can argue that this is the likely culprit of the “delay” in pedal response.

Electric signal will reach the ECU in a microsecond (actually less than that), – even a slow computer will have sufficient time to “react”. However, if the MAP sensor takes a millisecond (or more) to sense a change in pressure, and then the ECU tries to compensate for the sudden influx of LOTS of air in the manifold – well, there we have it.

I’ve tried running a 2x larger ID tubing directly from the manifold to the boot, a very unscientific way of doing the experiment, and did not see a change. Granted, my a little bit more modern 36CU does not have idle adjustment screw and it does self-adaptations (meaning that my findings may not correlate 100% with yours).

FWIW, the computers from the 70s and 80s are slow and primitive from today’s standpoint, but let’s not forget that with the same technology, we still managed to put a man on he Moon and to make the space shuttles fly:

Which brings to mind the exact intended function of the TPS…which I’ve never been 100% certain about.

My understanding is that the primary function of the TPS is to signal the ECU that the throttle is being opened so fueling can be momentarily increased…to prevent bogging.

It’s also my understanding that the TPS doesn’t have much (if anything) to do with mixture control at steady throttle conditions.

Yes? No?

I’d be happy for clarification and enlightenment :slight_smile:


When you floor it from a stop, vacuum drops from 17 to 0, TPS voltage shoots from .32V to 5 (?)V, ECU changes to open loop mode, and the throttles are wide open. That’s a lot to deal with in a quarter second. Sure, computers were relatively powerful back then, but speed was not their strength.

Good discussion and I’d love to participate, but I don’t want to take this thread off-topic.

In the archives, one can find many old posts by “yours truly” – one can see that I have actually driven my car several miles with the tube to the ECU blocked, i.e., no MAP input. It was drivable and no black smoke that can be seen from miles. Based on the above, it appears that when the the 36CU senses fault in the MAP readings it does things off of a pre-programmed map. No sure how and not sure if this applies to the 16CU the OP has.

I have not driven with the TPS disconnected. Have tried to rev the engine stationary and it does look like throttle response is not as sharp as it is when the TPS in “online”.

But, I also have flywheel and crankshaft position sensors, which help the ECU decide what to do, so, no clear answer from my unscientific experiments.
Have not tried running it w/o both MAP and TPS signal…

The usual fix for this symptom is an Italian Tune-Up.

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Just for the record, I’m talking about flooring it from a stop where you push the pedal down in a blink of an eye.

Whenever I take a good half second to floor it, everything was and is always smooth.


Well, just don’t do that. Problem solved. :wink:


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my wife rarely rides in the XJS, but i was showing another person how performance is!

i abused the car badly 6500+ rpm many times ,OK thats all said and done!
next day took the wife to a BIG local car show driving NORMAL,wife said “the car seems so nice and smooth and sounds better”.
from the lips of a darling!

about that antique ECU stuff(crap).
my ECU does all i need and MORE.
it has a Low and high accell fuel pump adjustment in 1% increments!
and i can test if settings are correct, by the RICH /LEAN knob, it shows readout how much a % change is needed, and i adjust accordingly, 1% at a time!
plus many other options! pic of programmer i can also turn on or turn off open/closed loop any time!


Follow up… I think Doug was right yet again. :slight_smile:

Me messing with ECU fuel mixture was masking the issue, not TPS, but throttle.

After removing my throttle bodies, I gave them a very good clean, dialed in on making sure throttle gap was 0.002", discovered a bad throttle bushing and so replaced both with brass bushings, and did the whole readjustment of throttle linkage and making sure TPS voltage was still 0.32v at idle.

My throttle response is great now, and I’ve adjusted ECU back to be what it should be. Idle is a tad bit smoother too. All in all, it really is important, I guess!

Wish I could remember what it is that I might’ve been right about ! :slight_smile:

As to throttles and adjustments…

On my own car I’ve somehow managed to get everything tweaked ‘just so’ and the engine behaves very well. I’m 100% happy.

But it has taken a lot of experimenting. In my case, at least, there is a lot of wear/slop in the entire linkage system, beyond just the usual/common throttle bushing thing, which confounded my efforts at times.


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Doug ; i agree one needs some feel for what the engine needs to make it acceptable!
some times we have to diviate from the book !