Not sure your STFTs are helping you. Until LTFTs have had time to learn your STFTs are basically saying learning is still happening.

Once LTFTs have had time to learn and are high at idle but not high at higher revs then an air leak is almost certain.

As you know, finding a leak can sometimes be tough

Wacky trims can be a sign of one or more blocked cats.

 

Thinking outside the (air)box; I was wondering about something electric for some time already.

I guess there are 2 possibilities:
1. Your trims are OK, but just the way it is relayed to you is wrong.
Think bad connector, bad sensor, broken wire, ...

2. Or the values u read are correct, and your mixture is indeed bugged every once a while.
Think the injection system, perhaps a a faulty injector, but also the wiring to it, something that gets disturbed every once a while.

Same thoughts for the spark plugs and its supply system ...

Same thought on the O2 sensors.

You got some patterns on when it occurs, perhaps we can find an other theory.

 

It may also be trying to achieve something that's not happening. E.g. kicking the rear O2 sensors. You may get a code but obviously cannot get one if you keep clearing the codes as most are 2-trip i.e. must occur under similar conditions on 2 drives.

Sometimes you can see pending codes that help.

 

JagV8,

The trims always get perfect with RPM, would that rule out a catalytic converter problem? I hope... It still really really just acts like a leak somewhere. It's always a little higher on bank 2.

I have driven the car 3 or 4 times since I last did anything to it. The all interstate driving (work) generates good fuel trims, very good in fact. I went through town to my mother-in-law's house last night, fuel trims much less satisfying in stop and go driving.

I have too much going on this week to do anything but drive the car, so we'll see what happens over the course of the week.

Eric,
I have suspected a fuel injector issue, and it's still a theory of mine. It seemed to start after I ran the first can of BG 44k. I thought maybe it dislodged something from somewhere else in the fuel system and clogged or partially clogged one of the fuel injectors. Whenever the trims got so bad it misfired (usually during carb cleaner testing) it's always cylinder 5 that set a pending code. At least that one is easier to get to! I checked the plug, which is new and snug. I swapped with a spare coil on that cylinder, no change. The wiring to the injectors looks pristine.

It could be a wiring issue to the MAF? The fact that one bank is higher than the other makes me less suspicious of a MAF problem. The upstream O2 sensors are both new Bosch units. Downstream sensors appear to be working properly.

Like I said before, I am suspicious of the throttle body and in particular the TPS. I need to study more regarding these parts so I can better understand the symptoms to expect when they fail.

One day this thread will have "resolved" on it, what a glorious day. haha

 

You don't look to have data on whether the cats are OK or not.

Just driving along doesn't need much air through the MAF so also doesn't create much exhaust does it...

I'd first stick to not clearing OBD and chase whether you have an air leak (or codes).

 

It's been a couple hundred miles since I did a reset. The fuel trims are still pretty good but bank 2 will leap to high positive during coasting sometimes, bringing bank 1 up with it a bit. I feel a "difference" in the throttle, hard to put my finger on. It doesn't drive exactly like it did but still no codes or pending codes.

I have used cruise control a couple of times since the p1121 code a couple of weeks ago with no problems and no restricted performance.

I should be getting my company car in about two weeks. I will have to make some hard decisions at that time since I will have at least one car too many when that happens. I don't want a beautiful XJR sitting in the grass beside the house being neglected. The new car comes with new hours and new responsibilities so I will have less time to drive, much less work on the XJR.

I guess these are good problems to have....haha

 

My $.02: life got a whole lot more relaxed after I traded my 01 XJ8L for the like new RRS! Only had a wrench, pliers, and screwdriver out of the tool box once this month, and that was to replace a broken toilet seat. I think I spent more on tools in the last three years to keep up the Jag than I did on car parts -- and I bought a lot of car parts! Getting weekly emails for Amazon, Welch, Motorcar, the dealer in Las Vegas, wondering where I have been the last four months!

Have you noticed Ross and Brian are hardly ever on this forum now that they have newer Jags as their dailies?

 

I hear you Jim.

My wife's 2004 XJ8 (X350) has required MUCH less attention and it drives more like a new car than either of the X308 cars I've owned. Hers has needed some transmission attention but otherwise it's been dead reliable.

I do have a special affection for my XJR, but I won't deny my life would involve much more free time without it. It would be a good deal for someone if I sell it because I have done SO much to it in the last four months.

The next few weeks will involve lots of changes so we'll see how it shakes out with the cars.

 

I've spent a little bit of my Saturday off diagnosing (attempting) my car. I took a known good coil and switched it out with each of the coils on the drivers (LHS) bank just to try and rule that out. Well, I didn't do the last one because I came in to eat, but while the engine seemed to smooth out when I replaced the third one from the front, it was not pronounced enough to rule out placebo. It also seemed my vacuum leak got worse when I removed the coil cover. Since the vacuum leak varies so much on it's own it's really hard to confirm from spraying carb cleaner if it's a real leak or a coincidence.

I "think" I can hear a hissing sound in the area of the coolant reservoir, but it could be anything. I have sprayed all over that area and all the way up to the attachment to the brake booster, can't make the trims react.

When driving if I am pressing the gas at all the trims are fairly good, always below +10 and generally much lower. If I am braking the trims are still pretty good, although they go up a bit. At idle they are dead on at +12 for bank 2 and +7 for bank 1. If I am coasting but NOT braking, they can shoot up to +25 or higher sometimes, but this is especially a problem on bank 2. Then, occasionally they will dive to -25 or even -50 a couple of times. Sometimes at idle it will just randomly dive down to -40 or so, both sides, and the engine stumbles a little....then it normalizes again. (All of those negative dives, by the way, are STFT. The LTFT never goes negative, it just goes to 0).

Ok, going to eat now. I'll go back out later and play around some more.