Potential cause of a rich fuel mixture problem

I shared earlier that my '92 XJS V12 is finally back on the road after a 7 year lay-up. However, it’s NOT been smooth sailing because she’s suffering from an EXTREMELY RICH FUEL MIXTURE which has resulted in both crappy idling & noisey backfires on the highway. The suspected culprit? My mechanic cleaned-out the solid residue build-up in both cats, however he FAILED TO REPLACE the oxygen sensors (also called lamda sensors) on both catalytic convertors! These guys sense the oxygen level in the exhaust & then instruct the computer to either lean-out or enrich the fuel mixture!!! Currently my exhaust is BLACK & it would seem that all 12 new spark plugs are already FOULED-OUT after only driven 90 miles! Am I UNHAPPY with this guy? You betcha!!! Turns-out that Jaguar recommends that these sensors be replaced every 30,000 miles & mine are PROBABLY ORIGINAL (a 1992.model car). So next I’ll replace both the oxygen sensors & the 12 “new” spark plugs & let you know the outcome.

so why haven’t you gone back to the mechanic with all these problems or did you pick a shonky mechanic/ & i’m even more baffled at why you’d want to fit another new set of plugs when the recent set are serviceable or you have so much money you can afford to be illogical…is the mechanic jaguar savvy?

Could be many reasons Glenn.
One is of course the O² sensors. As a test you could disconnect them and the ECU will go automatically to open loop (no Lamda feedback) and fuel the engine according to its base fuel map. If the ECU is properly adjusted the engine should run fine.

But it could also be bad/leaking FPR, leaking/sticking injectors or even a bad ECU.

And as Tom said, no need to change the plugs again.

FYI I have closed the thread you started with the heading about paint and then went out to discuss the over fuelling in the actual body of the post.

Also check the vacuum hose to the ECU. If it is disconnected a very rich mixture will result. Black smoke and engine chugging type of rich mixture.

Oxygen sensors? I have my doubts that they’re the root cause in this case.


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Thank you, my new friend, for your most knowledgeable feedback! I had just discovered the Lambda sensor possibility on the Jagbits website last night & according to them there are two different types of sensors: a single wire or a three wire which supposedly screws into the catalytic converter like a spark plug. So I next need to determine which one I’m dealing with; I normally would simply call the mechanic who removed both cats just last week to clean-out the buiĺt-up solids but he’s off on an Atlantic cruise!!! The timing for that scenerio really stinks from my standpoint, sort of like “Murphy’s Law” (whatever CAN go wrong WILL)! So looks like I’ll have to get out my floor jack & have a “look see” this morning. But I’m probably going to simply disconnect the leads as you suggested in your post

A '92 will have the 3-wire sensors. Jaguar begin moving away from the 1-wire sensors circa 1986-87


Sorry, Aristides, I accidently hit the “reply” button before finishing my post; “Murphy’s Law” yet again! HA! But to wrap-up: simply disconnecting the lead @ the sensor will be my course of action & then trusting the ECU to do it’s job. As to changing the potentially fouled-out brand new spark plugs, I only mentioned that one earlier as a sort of tongue-in-cheek sarcastic humorous style reference to Murphy’s Law again… “whatever CAN go wrong WILL”. Often that’s EXACTLY how stuff seems to work for me. Sincere thanks once again for your valuable feed-back, Aristides!

If the O2 sensors were that bad, it would idle fine and not run super rich while in P or N. The O2 sensors are only used while in D or R.

My 1988 V12 had an over rich problem all the time, in P or D, and it turned out to be a bad ECU.

I might be wrong, but I think that by 92 and with heated O² sensors Jaguar discarded the inhibit switch and open loop in P and N.

I would second that, very important and very probable.
Just yesterday I discovered that the hose was loose and my ECU would not shut the injectors on overrun.
The hose connects under the balance pipe and goes all the way back to the ECU, check the hose and both connections, also check if the diaphragm inside the ECU is not leaking, it should hold vacuum.

Yes, me too.

Sincere thanks, Aristides & Doug, for your informed input as your knowledge here is obviously far greater than mine! I confess to knowing NEXT TO NOTHING about the electronic fuel system on my XJS. Sounds like I need to locate the ECU & check those vacuum lines first as well as the diaphram. Can anyone guide me in locating the damned thing (ECU)? Many thanks in advance!!!
NOTE: I must confess to still being MADLY IN LOVE with my beautiful Jag, but I’m beginning to suspect that she’s even more tempermental than any woman I’ve ever known!!! HA!!!

They can be indeed!
But you will sort it out eventually.

Not a XJS specialist, but it must be in the trunk, RHS.

Has the idea occurred to you to just sell the car and be done with it?

IN REPLY TO RONALD STEPHENSON: I’ve been around long enough to have literally owned a probable “fleet” of various vehicles (am 74 years old) during my lifetime & definitely anticipate that a vehicle that has been in storage for over 7 years is going to have “issues”. I became a Jag Lovers member around 20113 when I acquired my first Jaguar, a older XJ6; that’s when I learned that one of our primary satisfactions as fellow Jaguar members is to assist others when they encounter problematic situations. This is why I’ve chosen to share my current challenging dilemma: to get knowledgeable feedback to better enable me to address the issues myself. So I fail to understand, Ronald, how you think your sarcastic comment could.possibly benefit anyone?!!


I’d suggest to check the fuel pressure regulator (or regulators , not sure there’s one or 2 on your car)

if it has failed, it could over pressurize the injection system, and give massive over consumption

Thanks so much for your suggestion; I"ll add it to my list!

I have a codicil to Murphy! “and once havig doe so, will cntinue to be wrong”.

why not just clean the plugs?


Thanks for the suggestion of changing the spark plugs but 12 brand new plugs were installed only 2 weeks ago & have logged maybe 100 miles (mostly interstate in order to “blow our the carbon”) total; plus have also added Lucas brand fuel additive to two fill-ups in order to help clean-up the ignition/ fuel system. Car was in storage for over 7 years, so some of the previous suggestions such as checking the vacuum line to the fuel system ECU along with the ECU itself are some of the boxes I’m currently ticking. I’ve developed a check list based upon earlier suggestions that I’m currently working on. Thanks again!

From memory of my 1980,s XJS,s the main sensor for fuel mixture is the 2wire coolant temp sensor on the left hand front side of the engine.
I apologise if this is not applicable to your model.