Facelift - Chasing LTFT Trims

I have a 1996 XJS with the AJ16 engine.

I enjoyed a long period of time when the LTFT was at zero. Then it crept all the way up to 10.2 before I did something about it.

First I replaced the camcover gasket without any change.

Then I tightened the intake manifold gasket bolts. I found one, maybe two that seemed to be less torqued than the others. I got the LTFT down to 7.6

I just ran both TPS Reset and Oxygen Sensor Reorientation and now I’m down to 4.8

Besides the fact that I didn’t think the oxygen sensor reorientation did anything other than tell the ECU which of the four oxygen sensors is attached to which plug, I found a hairline crack at my rear exhaust manifold. (Bolts were all tight)

The odd thing here is that both my LTFTs match, however as I watch the STFT constantly change, I have noticed that Bank 1 STFT never gets as low as Bank 2. At best they are equal for a moment, but I never see any points in time where Bank 1 readings are lower than Bank 2. I have been watching this for days now. How could the LTFTs continue to be equal?

Not familiar with the AJ16 engine Veekay but logic says:

It’s not a V8/V12 so Bank 1 and Bank 2 is just the split exhaust.
When ECU corrects to enrich, and as you essentially have one Bank and not two, both exhausts will read the same thing.

A crack on the exhaust will raise the LTFT and you should address this first I think.

I’ve been chasing high LTFT’s on my Range Rover V8 and it’s a huge pain to find the cause as it could be more than one reason.
An unmetered air leak, a dirty/faulty Air Flow Meter, EGR valve and PCV valve are the usual suspects.
If at constant higher revs the LTFTs get lower it’s usually a sign of an unmetered air leak (the leak becomes proportionally insignificant at open throttle, but does matter on idle)

In my case I changed the PCV valve, cleaned the AFM, cleaned/checked the EGR valve but no joy.
It turned out that the AFM was permanently lazy, plus it had a leak from the Intake Manifold Gasket that did not show with the smoke test.


Thank you for taking the time to read and respond to my post.

The AJ16 has two separate exhaust manifolds for each bank. The manifold runs into it’s own pair of oxygen sensors with one of the two catalytic converters for the car in between. Doesn’t that mean I do indeed have two banks? One bank would be the rear Cylinders 1, 2 & 3 and the other bank being the front Cylinders 4, 5 & 6?

Since I have a crack in only one of two exhaust manifolds, that could affect the exhaust readings for just one pair of oxygen sensors. (My car has four total sensors, two for each bank) The ECU would trim out the fueling at only the affected bank of injectors, no?

By the way, I previously also had a leak at the intake manifold that was not detected by the smoke test, nor were two of three garages able to even find it! Not sure why the intake manifolds so difficult to spot a leak. That’s why I wanted to check the torque on the intake manifold bolts.

AJ16 engines are prone to crack manifolds if the fuel filter is blocked causing the engine to lean and overheat.

Worth checking.



Yes, I was able to replace the manifolds when I acquired the car. That was way more work than I anticipated. (Documents in posts in the forum if you want to read through it)

I’ve replaced my fuel filters two other times in the 75,000 miles I’ve put on her. Was getting around to do it again now that the tank was recoated.

Will make an attempt to seal the crack with Thermosteel. An off the shelf $8 product that may or may not work. I’m willing to gamble $8.

If I remember correctly, the saloon XJ6 X300 is the same.

Regards Jim



That depends Veekay, if the ECU has two separate control circuits for each batch of cylinders.
Even if it has, if both “banks” have high fuel trims means something “global”, most probably a leak at the intake, or PCV, EGR, AFM fault.

“Even if it has, if both “banks” have high fuel trims means something “global”, most probably a leak at the intake, or PCV, EGR, AFM fault.”

I agree with that part.

The part that has me scratching my head is that I see the STFTs averaging higher on one bank, but it’s not reflected in a LTFT difference.

Each ECU has a different modus apparatus… but usually LTFT take some time to register/change.
What you should look is the sum of the STFT plus the LTFT. This would reflect the total mixture correction the ECU has to do.
Is it the side with the cracked manifold with the higher STFT?

Before anything else you should fix your manifold as this would by default mess with your fuel trims.
Then you will have a better picture.

VeeKay, you’re probably aware of what I’m about the say, but I’ll say it anyway…

Jaguar’s early efforts at OBDII were a bit notorious for irrational data stream readings, phantom DTCs, reluctance to set DTCs, and the like. It’s not just me saying that; Jaguar dealer techs will say the same.

I drove an XJR/6 for years as a DD and battled fuel trim problems the entire time. Replacing cracked manifolds, changing O2 sensors, cleaning ECU pins, and all the other usual fixes never changed a thing. The fuel trims were always wrong…or at least they were always reported wrong in the data stream. At the time (this was a number of years ago) there was some speculation about non-Jaguar ODBII scan tools not interfacing correctly with the cars. I don’t think any conclusion was ever drawn.

Actually, I shouldn’t say I battled these problems the entire time. That’s not true! I eventually gave up the fuel trim fight. And the P0430 fight. And the P0411 fight :slight_smile:

I’m not suggesting that you should give up the fight. But be prepared for some frustration.


I am aware that the 1995 OBD2 programs are way off. LTFTs head off to 99 land and don’t come back. I think the 1996 was improved. I have positively corrected trims in the past, back to zero! (the TPS did it!!!)

Anyways, my fuel economy has gotten better since I went from an LTFT of 10, to 4. I am now just under 18mpg, city and highway mix.

Right now, I’m going to address that exhaust manifold. I know when it broke. I started the car and I heard a bang! It was then that I knew.

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I would not worry too much if they are only 4%…
You can’t expect them to be zero.
I was pretty happy when I got them from 20% down to 3% on my Range Rover.

BTW, on the Range, it will trigger a fault only if they are above 10-12%.

Thanks. I do still have a scent of fuel when I drive….I have to fix that manifold….

How old are your Oxygen sensors?

The downstream sensors have 8,000 miles on them. (Bosch 13789)

The upstreams may have 60,000 miles on them now. The upstreams are only there as checks, they don’t affect the fueling, they are there to make sure the downstreams are working.

Well…after replacing my fuel filter and the camcover and spark plug gaskets, my LTFTs have been dwindling. I’m at 1.8% now, and while I’m happy with that, I’m sure I’ll be at 0% soon.

I’m not sure which one fixed the problem, but the STFT ranges still differ between banks, and I haven’t sealed the exhaust manifold crack, but here we are.

I do want to add that I believe used too small diameter section of the orings at the fuel filter. Maybe that played into this? I used an A011 size last time, which is 9.25x1.78. This time I used the A110 size which is 9.19x2.62. (Viton)


Went in to apply the Thermosteel patch…and wouldn’t you know it…the glue inside was dry. It was old. I couldn’t use it. No one near me sold it, so I ordered one and was ready to put the car back together again…

While there, I did manage to find that one of the two exhaust manifolds…the one that had cracked, had gotten loose. The bolts were not tight, so I tightened them. I’m sure the STFTs will probably be more consistent now. Will report back.

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I’m delighted that somebody has met with success in the fuel trim battle ! I doff my cap :slight_smile:


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Thanks. This is the second time I’ve won this fight. The first time required a brand new TPS.

Does anyone know if a leaking camcover gasket can cause trim issues?

I understand if a spark plug gasket has failed, it can lead to trim issues, since a spark plug may be drowned in leaking oil? How about the camcover itself?