Ecu running leaner over time

I’m posting this because I’m just curious as to what’s going on inside the engine.

I’ve had my 88 XJS V12 for about two years now. I bought it barely running, and it had sat 7 years. PO didn’t know what they were doing, and the car ran super rich failing several smog checks before it was put into storage.

I got it running decently within 6 months, and have continued to restore it over time.

I switched to synthetic oil about 8 months ago, and try to drive it at least three times a week. I’ve put about 5000 miles on it now in two years.

I have noticed over the last year, not only does the engine run better, but i have slowly had to adjust the ecu leaner and leaner. A year ago, i had to set the ecu at halfway full rich to idle smooth. Now it runs smoothly with proper voltage numbers at only one click rich.

My theory:
The carbon build-up from the past super rich running is slowly cleaning up so and you’re getting better compression and higher vacuum.
The ECU sees higher vacuum and leans the mixture.

You’re probably giving that heap an Italian Tune-Up every now and again, too, and that helps break the piston rings loose so they seal properly as well as cleans off some carbon deposits.

Yeah, i assume carbon build up too. Although it wouldn’t be winter time with colder air intake? Although that makes no sense manually adjusting for different times of year, as ambient air sensor should take care of that.

I am now just 1 click rich, yet the ecu numbers are 2.25V and 2.45V, a bit on the rich side but it runs so well, i don’t want to mess with it . A year ago to get numbers like that, i had to go about 5 clicks rich.

I guess synthetic oil (it barely leaks now too), hammering it a few times like Kirby says, and i have run BG44K fuel additive a couple times, have all made a difference.

Its been a long strange trip since end of 2018! Thanks guys!

It does seem to be running better, no?

Yes, runs better. I only recently adjusted ecu more lean because it smelled a bit rich idling, yet it still idles smooth after adjustment. So may as well improve mpg.

That adjustment should only affect the idle condition. Any effect on mileage will be minimal.

Really? I thought that the ecu mixture adjustment was for all times closed loop is operational, which is most of the time.

Open loop is only during P, N, not fully warmed up, or full throttle.

Think about it. Closed loop means the O2 sensors are directly affecting mixture. If it’s too lean, they send info to the computer to richen it up, and vice versa. They have to keep the mixture in a VERY narrow band to allow the 3 way catalysts to work. They will fight to keep it there no matter what. The only times they are not doing that is cold start, full throttle acceleration, and when it’s in neutral or park.

If you look at page 39 of the Engine Performance Manual, S-58, it says to use the mixture adjustment on the ECU to set idle emissions. That’s all it’s for. The manual also discusses all aspects of the fuel injection system.

Ahhh, thanks for clearing that up. I’ve misunderstood all this time.

Question though, if the ecu is adjusting fuel mixture with o2 sensors in closed loop, shouldn’t that be good enough at idle? Why do we need a fuel mixture adjustment at idle?

You adjust it when it’s open loop, i.e., in park. I don’t know why Jaguar decided to put it in open loop in park and neutral, unless they didn’t have enough flow to keep the O2 sensors working right. But they put it in closed loop at idle when in gear. Why, I don’t know. I don’t know of any modern car that is in open loop except on cold start.

Here’s the S-58 manual if you don’t have it.S58_XJS Engine Performance-752.pdf (1.5 MB)

The Oxygen sensors are far away from the head and unheated, so on idle they have hard time keeping hot enough to work.
Plus, Jaguar thought that it’s not very suitable for the idle to be going up and down as the ECU cycles from lean to rich ab vice versa, and a tad rich idles smoother.
If you have heated sensors you could very well disable the open loop at idle by disconnecting the feedback monitor plug at the ECU, but as i said, it will give you a slightly hunting idle.

Well, I decided to adjust it 2 more clicks lean. It’s now at neutral setting (exactly halfway between full rich and full lean). So it’s gone a total of 4 clicks lean since I last set it correctly in the summer.

I now get readings at idle (850RPM) in park with the thingie removed (cap that forces P into closed loop) of 2.05V (bounces between 1.95 and 2.15) and 2.25V (bounces between 2.15 and 2.35)

Car still idles nice and smooth, same as before. But I do notice the throttle is a bit smoother going from idle to moving. So the ECU adjustment mixture kicks in at idle, and then turns off as you give it throttle? So this is why you want it not too far off so there aren’t any huge rich or lean adjustments as you open throttle causing a hesitation? This makes sense to me now. Roger Bywater was explaining something like this to me a year ago, and jal5678 now confirms how. Thanks!

I guess after a year of fixing things and running clean oil, the engine is just running more efficiently, not needing the richer settings for a smooth idle which was disguising issues.

One last question if anybody knows, at what engine temp does this ECU fuel mixture at idle kick in? Is it something like 160F/70C? Or much lower?

Actually, sometimes on idle the O² can’t heat up enough to work, the ECU senses there is no signal and fuels the engine according to the base fuel map setting.
When you give it throttle the O² heats up, starts to work again and the ECU goes back to closed loop.
If the base map is correct the transition from open loop to closed loop will be unnoticeable, if not there will be some hesitation.

My 88 has three wire O2 sensors, so they are already heated up.

I thought the closed loop doesnt start until the engine warms up? Or does it start when cold?

I do know that the air injection dilutes the mixture that the oxygen sensors see, thereby enriching the fuel trim when the air injection is operational. The air injection is coolant temperature controlled, so this occurs until the engine starts to warm up. I think the system is in closed loop all the time (except when in park or neutral) by design.
The idle trim is to smooth the transition from open to closed loop operation (shifting into drive); the system stays in closed loop at idle as long as you don’t shift to park or neutral- that is why you will see a cyclic 50 rpm fluctuation in idle hot.
That’s why I didn’t remove the air pump on my ‘88. I figured all this out because the prior owner had the diverter valve vacuum lines such that the air injection was on all the time, and the oxygen sensors were non functional even though I had replaced them, and I was getting about 8mpg.

Hmm, interesting. I have removed my air injection system. It still idles smooth enough though when cold. Good to know closed loop is always used in D or R regardless of engine temp.

If I recall, you have a Roger Bywater enhanced ECU; perhaps he changes the fuel map to accommodate. Did he ask if you retained the air injection?

That’s incorrect. It’s in open loop whenever the air injection is called for. Has to be, it certainly couldn’t be measuring oxygen in the exhaust while you’re pumping air into the exhaust.

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