It does and I totally forgot to take a snapshot of it before I cleared the codes. I wasn't thinking straight because I suddenly had coworkers calling and texting me that the sky was falling down at the office. I've been kicking myself for the past hour since I remembered about the freeze frame data.

 

After researching yesterday, I began to suspect the MAF sensors. So I sprayed them down with MAF cleaner, reinstalled, and the car started normally but was still idling rough. I checked my fuel trims and they read STFT: 25 LTFT: 99.2 on both banks! I was able to drive it around the neighbourhood but the idle was not clearing up. The fuel trims on one bank dropped down to more normal levels while the other bank dropped but still was reading high, with LTFT around 26. I shut off the engine and scanned for codes and got P0505 (Idle Air Control System), P0507 (Idle Air Control System RPM Higher Than Expected), and P115D (Mass Airflow Circuit Offset). I cleared the codes and let the engine run for a while. In park, the revs were up and down continuously in a controlled pattern. In drive, the idle evened out but throttle response was all over the place, unsafe to drive. I shut down the engine again and scanned for codes...this time I only got a "permanent" code of P115D. So I'm thinking my MAFs need to be replaced.

I don't know what's happened to this car. It used to be so smooth and comfortable and reliable until this God-forsaken year. Starting in January, I started getting a low speed suspension rattle from up front which still hasn't been resolved, followed by terrible steering wheel shaking which was resolved for the most part with new tires and balancing, but seems to be slowly returning. Then when the lockdown started, I took the opportunity to do a bunch of preventative maintenance, including new shocks all around. But the car drives like crap ever since. The ride quality is harsh and bouncy at the same time, the handling is wishy-washy, feeling unstable at high speeds, the steering wheel always feels loose, now the roof is knocking and popping; the only thing that still feels right is the acceleration power. I drove my girlfriend's 2011 Infiniti G37 and it makes my XJ feel like some cheap jalopy. I don't understand it. Is it my fault? Did I install all these new components incorrectly? Its not like installing suspension parts is rocket science.

The car used to be epic, the best car I ever had, but now I actually dread having to drive it. The poor driving quality coupled with these recent issues have really changed my perspective. This might be the last Jag I own.

 

Understandable sentiments given the amount of love and attention you’ve shown your car.

I haven’t had a chance to look over all your codes yet, but for MAFs you should be able to look at each MAF sensor separately and see if they are registering the same readings. If not then swap them and see if it’s the circuit or the sensor.

EDIt: Also have you tried disconnecting the MAFs and see how it does open loop?

 

Great suggestions, thank you. I'll try them and report back.

 

For driving dynamics and ride quality: Did you have 4-wheel alligment after touching suspension parts? XJ are known to be very sensible for it. (based what you wrote, i think you have some toe-out)

 

Yes, this car is extremely sensitive to alignment. I did have two separate alignments done at Firestone and just had another alignment (I think just front) done this past weekend at an independent shop. The steering wheel was crooked after the engine mount replacement, so they sent me to the alignment shop (free of charge). The high speed stability improved a good deal with the new engine mounts but the steering wheel still feels too loose. The servotronic system in the steering rack is supposed to tighten up the steering at high speeds and keep it loose at low speeds (for parking ease) but I feel no difference anymore. The steering feels too light all the time.

I was planning on finding a performance shop to do a more precise custom alignment but now I've got to figure out this MAF problem.

 

Ok. Hmm... thinking the way how variable steering servotronic works; Does it have full assistant or lowest as default, in case that there are connector loose from steering rack? Anybody got info?

EDIT: If there are no powerfeed or connector loose, you get low assistant.
Light steering on speed can be caused by kinked low pressure/return pipe or blocked resevoir filter or speed sensing error, but then you shoud have DTC.
Found these from X351 service & repair manual.

DIY trick to find leaks from intake system: Ask a friend who smoke (if you dont) to blow smoke in to the system and watch where it comes out. (moderate dark place with help of flashlight for spotting)

 

If there is a problem with the servotronic, it will default to lowest assist. Makes me want to try with the connector unplugged like a lot of BMW and Audi guys do. The manual says doing so will store a code but not the CEL.

 

Yes, I found that pretty much same time you wrote. Check my edit:

 

Alright, I just went out intending to try these suggestions out. The car started normally and idled normally for a minute or two while I was trying to get my OBD app to connect. But before I could check the MAF readings, the engine started to misfire. The CEL started blinking, so I shut it down and scanned for codes. I got pending misfire codes for Cylinders 1, 2, and 4, P0300 (Random/Multiple Cylinder Misfire Detected), and P1315 (Persistent Misfire). The P115D MAF code was also still "permanent."

 

OK, so I just started the engine again and pretty quickly started to misfire again. The MAF readings were both at 0. I unplugged them and the misfires stopped but the engine started to rev up and down in the same consistent controlled way it was doing at one point last night. I didn't pay attention to the exact numbers, but the revs stay solid at something like 1500 and then go to 1700 for a moment. That happens consistently every few seconds. Never goes below 1500. The revs go down to normal for the most part when you put it in Drive but throttle response is weird.

So yesterday when it was exhibiting this odd rev behavior, the MAFs were plugged in. Now it only behaves that way if they're unplugged.

 

Both MAFs measuring zero is strange as it’s unlikely both MAFs would fail at the same time. Even more weird as you are presumably getting extra air in the engine, yet no reading on the MAFs. Perhaps you have a massive vacuum leak, but even then the MAFs should register above zero when you open the throttle. Maybe try one MAF plugged in at a time and see if readings can be had from either. The loop should still be open, so presumably your car will still run.

I assume the alterations in throttle is the car adjusting the fuel ratio based on emissions only? What do your fuel trims say with the MAFs disconnected? Do they normalize somewhat or is mapping still way off. If mapping is still very lean, probably suggests a leak.

 

I took a chance and ordered new MAF sensors since I'm pretty sure I cooked the old ones by over-oiling the filters. So I put the new ones in and clear the codes and start the car. The long term fuel trims both fluctuate together around 45-50 as the rpms settle down. But as soon as the engine begins to idle normally, my OBD reader loses communication and won't reconnect. Anyway, I sit idling for a few minutes and everything is fine. I drive around the neighborhood for about 15 minutes thinking the issue has been solved when suddenly it starts bucking again just as it did last Wednesday morning. I park and run a scan and the P115D code is still reading "Permanent" but I can't really trust my OBD since it loses communication. I'm going to have to get a real scanner since I'm flying blind now.

 

Are your MAFs still reading no voltage/ airflow? If so you may have an electrical issue on that circuit.

I believe your idle normalizing is normal, even if something is wrong because your car will adjust injector pulse width timing based on your fuel trims. It is easier for your car to set ideal timing based off of fuel trims under static running conditions (ie idle), but significantly more difficult under dynamic conditions (ie driving around the block).

 

I've been too ill the past few days to dig into this any further but I did get a brand new iCarsoft LR V2.0 scanner delivered. All its features are a little overwhelming, so I need to dig through it all and see what my options are. I'll of course start with checking the fuel trims and MAF rates. But I did decide to check the battery voltage and it was very low, 12.2 after sitting a few days. I started the car very briefly just to monitor the voltage. On start up it dropped to 8.4, then ran pretty steady around 14.5. So I took the battery to Autozone to have it tested and charged. They said it was about 60% charged but the battery was still good. When I picked it up later in the evening, my multimeter read a voltage of 12.75. I'll check it again in the morning before putting it back in the car. I know low batteries are the root of all evil in modern cars, so I'm hoping this has something to do with my problems. And I'll be able to reset the pesky BMS module with the new scanner, to boot.

 

Well, after another week of being too ill to work on this problem, I put in the fully charged battery and reset the BMS. Low battery warning gone (thankful for small victories). It takes two tries to get the engine running. I guess I still haven't figured out how to use the iCarsoft scanner properly because I can't see what the fuel trims are, so I plug in my bluetooth OBD reader and open the app. The engine was idling a little roughly in park at 793rpm, both STFTs were 25%, LTFTs 0% (probably because the battery had been disconnected), Intake Manifold ABS Pressure: 6.67 psi, MAF air flow rate: 9.77g/s, Throttle Position: 14.51%, Fuel Pressure: 1115 psi, Upstream O2 sensors were not reading, but the others were steady at 1.27v (I'm guessing that has something to do with being in open loop). That's all the info I could glean due to the limitations of the app and because as soon as I put it into drive, communication is lost.

Anyway, I start driving around the block and its driving wonderfully. Very smooth. Well, at the third stop sign (after maybe 5 minutes), without any kind of warning, suddenly the engine stalls. I start it back up but now its running rough again, and I have to keep the rpms up to keep it alive. I pull over to the curb and it stalls again. I did notice that as the engine begins to stall, the OBD readings come alive again, and I was able to see that the STFTs had dropped to somewhere in the mid teens. Unfortunately, its hard to pause and really pay attention to the readings when the car is dying in the middle of a semi-busy street.

I try to restart the engine several times. It will crank and fire up fine but the rpms almost immediately drop and the engine dies. If I leave it alone for 10-15 minutes, the engine will start and run but roughly. No codes at all.

I have got to figure out this darn scanner and get some realtime readings, if my upset stomach will allow it.

 

Installed VAP tuning on the Green Beast. It really makes an improvement in the power of the engine and it runs smoother and a little quieter. Have to be careful driving it as you can get to "go to jail" speeds really quick. I wonder if it the tune removes the 121mph limiter. Not going to find out around here with the cell phone blabbing idiots in their powder blue Priuses driving 5mph below the speed limit sitting in the left lane.

 

Yes VAP can and does up the speed limiter. Different models get different limits. He changed my XJR from stock (155 mph??) to 200 mph.
Have not been able to hit the limit yet but when I got it on the dyno the guy did get it to 200 mph in 6th gear! My speedo stops at 190 mph but he was hooked up to his computer for the dyno runs.
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I have a 2011,and haven't done anything with the brains of my XJ-L, and I don't have a limiter on mine, at least not one of 122. Why?

 

What engine do you have? It was different depending on what tires were installed too.
We were told the 121 mph was just a tire limitation but we really don't know.
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