And the AJ133 specifies a minimum of 91 octane, and there are reasons.
The NA engine has a higher compression ratio than the SC engine, but the SC increases the compression - that's the whole purpose of it.
Between the electronic fuel (mixture) control and the variable ignition timing & variable valve train, it will adjust as best it can to avoid preignition/detonation in the combustion chamber, but it does that by retarding ignition, adjusting fuel mixture & altering the valve timing to reduce compression as needed from inputs from the exhaust, anti-knock sensors, etc.... Over simplified for the sake of time & space.
But yeah, you can run 89 octane if that's what you want because it's your car. It's not good for the car, but it'll deal with it as best it can.

You'll just have reduced performance, lower MPGs, more engine troubles/gremlins & various faults including mis-fires and other random problems over time.
It probably won't damage the engine as much as other things will, like not changing the oil frequently enough and using oil that's not the right spec, not taking care of the cooling system and many other things that are not as critical with other and newer vehicles.

 

Mark,

You're spot on with the 91 min. octane requirement for the AJ133 5.0L NA. Thanks for the feedback, it is much appreciated!
It is easy to forget sometimes about the compression ratio differences from vehicle to vehicle in the fleet.

Just as a note, I do remember years ago when I had an early '90s W140 Mercedes S Class 5.0L, the premium fuel reminder was printed on the instrument cluster below the fuel gauge. I guess I just got lazy driving German sedans over the years!

 

Of course we can each run any octane we like to in our cars - we each own our own. So you have no one to answer to if you run lower than 91 by choice. And with the systems I wrote about before it'll pretty much prevent any catastrophic damage to the engine, but to do that it has to do things that will cause the fault codes to come up & reduce performance & as a result gas mileage.
I wouldn't remember whether the min is 91 or 93 octane either except for the decal our XJ has on the inside of the fuel cap door.

 

Mark,

I just went to look at the XJ fuel door and voila! - the 91+ octane sticker is there! - no excuses for my oversight.
I have owned the Xj for 6 months and I have fueled it up maybe twice now, yet I simply failed to notice that label on the fuel door.
Thanks for calling that one out, very much appreciated!



XJ fuel door label

 

Glad it all worked out!

Still working on my issue, doing a smoke test today then if if still find nothing I will move to the harness and PCM. From what I see the trims are fine and if not better than bank 1 at idle so there should be no other reason for misfire other than vaccum air leak or electrical. So I'm into that now.







Also yesterday I upgraded the bulbs to DS3 Leds and man they're bright.

 

Update,

We completed the 2nd Vaccum test today and got some results.
The main intake housing to the throttle body has 2 leaks. It had three but I fixed the other one the first time (2nd air injection hose).

Where it meets the throttle body its broken and was tapped sealed. During the test some smoke still made it through the bottom. Upon removal shattered... So the mechanics did know why or why it could've been misfiring. We also used a device to test the ignition coils all were firing.

So now, I'm going to trash the old crappy plastic intake and replace it with the mina gallery aluminum version.

Another part of the intake also leaked but not sure what that angle part is called.

If after replacing it still doesn't solve it. We will move to wire harness and pcm

 

Great findings.
Even the smallest leak can cause big harm, so there are an chance that once you get these fixed you are good.

 

Update, so replaced the intake tube. Car had to adjust for a bit as the idle got higher as the car was used to the air leak from the other tube. Adjusted it's idle but still misfiring/vibrating.

I'll do a adaption reset when I get my laptop with SDD but it looks like the only suspect left is the PCM and wiring harness. Assuming during the rebuild they missed a ground straps it shorted the PCM after a 10min test drive

Anyone have all the ground locations for the PCM in the xj? I have the diagram but shows it for the ones on top. Sort of

 

press x to doubt

honestly check valve lash and triple verify timing. beyond that you’re going to need someone with with an in-cylinder pressure transducer

 

So basically you think I need to reset the valves. Even with the compression being within spec? Sounds like hours of work without much evidence it has too much or too little free play between the rocker and tappet on bank 2. I've already put in more than 50+ hours ruling out all other simple issues and advanced issues. Also it's not throwing timing codes or any of the sort. But considering the trim is showing bank to is in the negative while bank 1 is more on the positive that would be the only evidence to make such a drastic tear down. But that could still be from faulty ignition and unburnt fuel from either 2 of the reasons.

I spoke to a tech 2 weeks ago and his idea was that it was wiring and low voltage to specifically bank 2 which is what I was gonna test next after ruling out the leaking intake tube

 

Ocwolfy,

Measuring bank 2 coil amperages sounds like a reasonable idea. Compare bank 2 to bank 1 coil primary amperage to see if there is a difference.
Did you find the ground location for the bank 2 engine harness? I believe this one is below the power steering fluid reservoir.
Good luck!

I have and update on my 5.0L NA XJ - using 93 octane fuel doesn't seem to be the solution - bank 2 fuel trims are still in the negative range and there are still occasionally cylinder 2, 4, and 6 misfires. The idle speed is consistent and I still feel the engine vibrations in park, neutral, reverse and drive.

 

Thanks Britechie,

As per my last post, I've since driven the xj 100 more miles since the intake fix with all leaks fixed. I reset the adaptation and noticed that apparently this XJ has one mismatch part that's either corrupted or changed within SDD. And it happens to stem from the BCM. There's also some recorded communications errors that stay after clearing. Some of the things I wanted to do was update or see if the PCM responds to self tests which it fails. Due to recorded communications faults its unable to update which supports my theory this may be communications or electrical related. And I know my SDD because I coded two jags the day after without issues (2014 XF).

So after this post I will be checking grounds, updating and the BCM to fix the mismatch error as I once did a replacment AAM and then seeing if I can get through to the PCM. Pretty much testing for eletrical faults and or coding.

Weirdly enough after 60 more miles (again cleared codes with SDD and installed new Intake) the car has a significantly less noticeable misfire barely even felt and the trims for bank 2 are pretty much 0.0. With the outside temps being 77F.

 

Ocwolfy,

It's great to hear that you're getting closer to the misfire root cause(s) and I'm eager to hear the results of your upcoming electrical tests.
Keep up the good work!

 

The only time I've had a similar problem on one bank I found it was the head gasket beginning to fail.
Did you have the heads checked and straightened ?

 

Kudos for your extensive diagnosis and repair efforts 🙄
When presented with random misfire, On an older 13year old) car like yours, we *always* squirt the injector and coil contact areas with electronic contact cleaner,
they even have a newish one that restores conductivity as well as removing oxidation, #5 l think they call it

So there's a $12.00, half hour fix for you to try.....Amazing how often it has resolved the problem 🤔

Side note on fake parts: The fake Bosch parts are missing the "bobbin" logo in the text group; Fake NGK plugs have sharper corners on the hex and the typeface is not quite correct

Best of luck !

Matthew

 

The whole reason it was at the shop for 3 months was for the blown head gasket it was replaced and the heads was machined. I've did my own compression tests and the numbers are good, also test for emissions entering the coolant reservoir and the test yield no positive result for mixing.

Also sorry still no updates for the misfire issue, I've been moving and setting up my new room in my place and haven't gotten to the garage set up so all my tools are packed away. I've been using my xk8 and my motorcycle when I leave the house which has been rare as I've been getting stuff delivered to setup

 

Thanks, no knock off parts here, same for plugs and coils. I've also swapped plugs and coils to the opposite side so I doubt it's connectivity issue but the other person who has the same issue in the forum should try it for sure.

2 days ago I had a job fixing a guys 2014 xf where it wouldn't start due to a corrupted instrument cluster and bad bcm. And considering my car gives communication codes like his somewhat was i would give some updates a try and then testing grounds before moving to check for physical damage to the electrical systems.

 

So finally had time to check the modules through sdd for corruption data and come to find out the BCM did and needed the update to be reinstalled. After this the communication errors were gone, after which I tried to update the PCM, says PCM needs no update, meaning the data is stable. So it's either there's a internal fault or the wiring. Because like previously mentioned, the engine makes excellent power, has good compression and no coolant or oil leaks or burning.

Between testing the amperage coming from the PCM and ground, is where the next step comes into.

Part of me also has a hunch I should just get another PCM and program it to the car and see what happens (yes I'm aware of the vin number situation on used ones, it's simply to test logic of if the original has a short like my xk8 did when I got it from the previous owner and to get another example after chasing an 02 code for so long) after which I'll send it in for repairs so to keep the same vin.

I don't mind spending 90$ -130$ on a used good one if it proves my idea without have to test the whole wiring harness.

 

Any success? I have similar issue, 2015 Range Rover autobiography. Cold start blinking cel, misfire cylinder 5,6 sometimes 7 and/or 8. All of them bank 2(checked by unplugging injectors and watching misfire counts in sdd. After short warm runs better, still can feel little shaking. Injectors bank 2 replaced. Plugs, coils swapped from bank 1. Cat looks ok in scope. Leakdown test 4% leak on cold start. Head gasket test passed passed.

 

Did you buy the Rover like that or it happened one day?

Because if you bought it like that it would provide no insight as I got mine like this as well.

Since my last post I ride my bike and drive my Audi a8 09 more so I haven't really been touching my xj as much as I love it. But not because of the misfire because I cracked the rim on a dumb bump in my area. Thank you Baltimore Maryland(had a client who needed something programmed in sdd.

On the contrary, I tried replacing the ECM, I coded the new on that came from an xkr. Same exact misfire condition, so ECM is ruled out. I checked all grounds, that's ruled out, cats are fine, and it's passed a compression test.

The only thing left for me to do, is when I remove the supercharger to service the coupler I'm going to see if the valves are fully closing, though from compression test they should. Or see if replacing the gaskets for the intake will also solve the issue.

But at this point I've just accepted the fact that my xj will forever misfire unless my intake job magically fixes.