Series 3 Air Fuel Ratios

So I needed an air-fuel gauge for my V12 project anyways so I got an AEM gauge with a Bosch probe. I was curious so I welded on an extra O2 bung to see what the EFI was doing on my XJ6. To my surprise, it stays at almost the same AFR’s no matter the driving conditions apart from off-throttle coasting. It likes to stick around 14.5:1 ± 0.5. from what I know about performance engines this is a rather lean mixture especially when under high load. Does this sound about right for this engine or is something amiss making it run lean? I notice that when you floor it from a stop it stumble for a moment and almost sounds like it lean pops for a second and then is all fine. I have also been fighting a rough idle that I cant seem to track down, I have all new plugs, wires, and a Petronix distributor. I am at a loss and the lean mixture seems to be the only thing that seems amiss to me.


It does sound lean to me at motorway speeds I would be a little afraid of holeing a piston, happy to be told wrong.

The only reason i’m not sure is the fact that the engine runs fairly low compression ratio’s right? like 8.?:1 ? So detonation or preignition would be unlikely even when lean. I happen to have a second ECU from a later 1987 model, and another airflow meter. I am tempted to try swapping them out one at a time to see if its either, but and ECU failing like that or at all is really rare right? In any event I have been putting 91 octane (highest we have in Cali) to try and avoid detonation.

I think its the increased heat from running lean thats the problem, burnt pistons result.

Does the XJ6 ECU has the same Base Fuel Map adjustment like the V12 or is it done only from the MAF sensor ?
If so, have you checked that is set properly?
From my experience on the V12, if the Base Fuel Map is way out the Ecu can’t enrich/lean the mixture enough to bring it to the correct ratio.

This is very close to Lambda = 1, William - which is indeed as it should be, and the basic target for engine management systems…

The more common mixture adjustments relates to CO levels - nominally set to 0,5 - 1,5%. The main reason for setting the engines fat, particularly the xk, relates to difficulties, with the rather primitive engine management system, of avoiding lean running under all circumstances.

Apart from other drawbacks; lean running means loss of power - while running fat, up to a point, has no negative effects on performance. Adding, as Robin implies, that excess fuelling contributes to engine cooling.

Be aware, that detonation is residual fuel being spontaneously ignited by combustion temps, due to too early ignition for the fuel octane used, and the compression of the engine. The xk engine was designed to use 98 octane, and the original timing specs reflects this. When ‘pinking’ is experienced the prescribed remedy is to back off the ignition timing. This basically detunes the engine from optimum power - but so be it…

Running lean may lead to inlet backfiring due to inconsistent ignition of a lean mixture - the remedial action, when experienced is to fatten the mixture. The two symptoms are different, and have different causes - and different solutions…

All that said; the mixture is adjustable; on the xk this is done by the AFM bypass - you do not mention if this has been tried? As the range of this adjustment is limited; if the desired CO level cannot be acquired by bypass adjustment - the AFM can be opened and the flap spring reset…

The ECU bases the mixture computations on AFM flap movements - the program is otherwise fixed; there are no internal adjustments.

However, the Lambda system, when fitted, provides feedback to the ECU to alter mixture based on O2 readings - basically striving to maintain Lamba = 1. The CO adjustment must be done with the O2 sensor disconnected - properly set up; the engine runs perfectly without the Lambda…

An O2 sensor has two readings; oxygen present (lean mixture) or oxygen absent (fat mixture) - it can not read actual mixture. The Lambda system simply forces the ECU to alternate between fat and lean running - to average Lambda = 1. But the system is basically disabled during pedal movement and high loads - to prevent lean running…

xj6 85 Sov Europe (UK/NZ)

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So are you saying that this AFR is fine even under load? because I find that a little lean. but maybe i’m too used to Turbo engine AFR’s which should be down in the low 13’s or lower depending on how hard you are pushing the engine.

I failed to mention that was the first thing I am going to try but since it has no effect after basically idle or very low throttle inputs I figured it wouldn’t help a lot.

Forgive me but what is CO in this context? Do you mean Carbon Dioxide? I dont see how I wouldn’t be able to adjust the mixture with AFR gauge which does in fact tell you how rich or lean the mixture IS… just not if that is what is being commanded.

Sorry just struggling to follow along on some things. Thanks for the Info though!

That looks OK for warm cruise, but I would expect to see 12-13’s at WOT. Maybe your ECU is not seeing the load signal to trigger open-loop operation?

Your lean misfire under acceleration may point to a bad TPS signal, but airflow or MAP sensing could be an issue as well.

You know I had problems with the car draining power even when off a while ago which I have since fixed but it managed to leak a lot of battery acid and while cleaning it i noticed that the full throttle switch was covered I wounder if I managed to kill my full throttle switch.

EFI systems of that era have a system that enrichens the mixture under full load. For example, on the V12 there is a throttle switch and a vacuum switch telling the ECU that it’s under load, and the ECU responds by kicking it out of closed loop and into open loop, running on the base fuel map with no correction. The base fuel map is deliberately a bit rich.

So, if I were you I’d be looking for a similar system on your SIII and then checking to ensure it is working as intended.

This looks like a great place to start - and an easy test?

Probably it looks to be a switch on the electrical diagram for the EFI I have so I should be able to test with a multimeter.

CO is carbon monoxide, William - a measure of incomplete combustion. The fatter the mixture the more CO; with ideal mixture (Lambda = 1) only CO2 (carbon dioxide) is produced - complete combustion…

With lean mixture, the combustion temps increase, producing NOx (nitrogen oxides) - which is a also unwanted.

In earlier days only the simple CO meter was readily available and adequate for mixture tuning. Now complex gas analysers are used to enforce ever stricter emission regulations - but also useful for better engine tuning…

I am not familiar your ‘AEM gauge’. It may certainly be based on gas analysing, but without showing the precise gases produced. Which in this case is likely insufficient, or simply too crude - neither xk nor the V12 was meant to run at Lambda = 1 at all times…

Two points; the xk and the V12 engines’ management system are very different - and the two ECUs are incompatible. And the V12 is more advanced than the xk…:slight_smile:

As an aside, you mentioned uneven idle on your xk - this is a much rehashed issue. It is a matter of degree before it should be regarded as a solvable ‘problem’ - to make the xk idle silky smooth requires efforts that is barely worth it. So how uneven is it…:slight_smile:

As an aside; at higher revs the xk is indeed silky smooth - even with serious faults. The lower the idle the worse it tends to be…

xj6 85 Sov Europe (UK/NZ)

Sorry AEM is the company that makes it. its a Wideband O2 sensor used mostly for aftermarket tuning with aftermarket ECU’s. It has a gauge and an input for ECU’s

The idle is frustratingly inconsistent. sometimes it idles as smooth as I could ever expect it too but most of the time it idles and most definitely feels like it has a very slight miss. if you put your hand by the exhaust pipe you can feel it very faintly “miss a beat” every few seconds.

You can always look at cam timing whether it cures the symptoms or is the issue (in my case, both!) after you went through air leaks and everything seems good, but isn’t. I had smooth days, now it is smooth days only but the top end power is gone with less valve overlap. The exhaust felt the same way.
Back on topic, it should enrich at full throttle via the throttle switch (vacuum full load switch on non-federal cars); but with the lambda connected it should be a good sign if it tries to hold lambda=1?

I have observed this on much younger cars, and have developed a suspicion (assuming everything else is right - valve clearances etc) that in some cases it may be caused by the closed loop operation of the ECU. The mixture is frequently varied to cause the EGO sensors to switch, as the injector parameters required to achieve lambda=1 are tested. If this is right, maybe smoother idle is observed while the system is in open loop, such as cold/warm-up idle.

This is a bit speculative on my part, but would be interested in whether the pattern fits your case…

Or one cylinder isn’t firing all the way all the time. I wouldn’t ever expect perfect consistency all the time and it will likely be uneven being an XK but if it can run smooth I doubt it is du to the mechanism of lambda feedback?

For the most part it seems to run better when warm so I am not sure that is it. but occasionally when fully warm it will run poorer than when cold which is baffling to me. What annoys me is that it seems to have a mind of its own so its super hard to diagnose whats wrong. I just recently fixed my hard/long starting issue, i figured out that my cold start injector would click but would not dispense fuel. once I changed it for a BMW equivalent it starts first crank most times.

The uneven idle on the 4.2L EFI XK engine has been discussed endlessly on this list for at least the last 18 years that I have been a member. Some members have posted lists of the many steps that they have performed in order to “fix” the uneven idle issue, but I don’t believe anyone has ever come up with a permanent or complete cure. I experienced it regularly in my two Series III XJ6s over the past 19 years.