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03 XJR timing slip? Rough idle and Pending Misifre on all cylinders
Hello friends, I thank you in advance for all of the kind help you've given over the years.
We're talking about my 2003 XJR-R1 with 191,000 miles. I was driving ~75 mph on the highway, smooth as can be, and suddenly noticed that I had more deceleration "pull back" than usual. When pressing on the gas at all, the car felt fine but when letting off, it wouldn't feel like the usual effortless glide.
When I got off the highway, and sat at a light, the idle was extremely rough, shaking the car, but not stalling. I would put it in Neutral at each light because it felt like it wanted to stall... Air conditioning was off. I was thinking, "Bad gas with water in it?" This is southern California so it's more likely to get bad gas with crap in it -- Not like up north where condensation in the tank is likely as weather cools at this time of year. Tank had a little over 1/8 remaining.
Pulled it into a gas station and put some injector cleaner in before filling it up, but this didn't improve anything. I could start it but it would stall after 2 seconds. I could start it and gas it a bit and it would roar, but RPMs would come back down to stall or limp. I connected my code reader and had nothing other than P0171 / P0174 "System too lean" in both banks. At this point I was two minutes from my destination so I completed the drive and parked it.
After sitting for a few hours, I returned, opened the hood for inspection - all air intake hoses etc. are attached and intact. It still would only start and then stall - BUT - if I give it gas, throw it in Drive, and start to drive, it will stay running. Meaning, it only stalls from starting after being Off - whether cool or hot - but when it has moved a little, it will stay running and idle roughly when stopped at lights. Still no new codes at this point. Drove it home and favored highways since it drove nearly-normally on the highway. I "needed" the A/C on during the drive home and the added load of it also didn't stall the car at lights - but it definitely wants to stall.
By the time I got it home, I had Pending codes P030X - misfire detected - on all cylinders, along with P1316 "Injector Circuit / IDM Codes Detected". This is in addition to the P0171 and P0174 "System too lean" on both banks.
This feels mechanical to me. Like when a fuel pump goes and there are no real codes because the engine doesn't have a computer/sensor-related problem to report. It's a 2003 so I know it has the upgraded cam tensioners... But I guess a timing chain can still slip at this age & mileage. Also reminds me of when a head gasket would go and cause two of four cylinders to have no compression on my old four-bangers. I have no white smoke at the tail and the car detected misfires across both heads, so I have doubts that it's head gasket at this point (8 cylinders misfiring would seem more like 2 head gaskets failed on all four cylinders at once and no compression would exist anywhere).
I'm not sure how to read timing marks on these engines, and I certainly don't wanna go pulling radiator & the front of the engine off if there's something else I can look into. When I start the car, should I be listening for something? Some audible indication of timing issues?
I have read about trying a hard reset so I'll go do that shortly and report back. In those threads, it seems owners had codes for misfire on all 8 but there was no noticeable drivability problem.
The fact that it happened at high-ish highway speeds makes me think that something mechanical rather than electrical occurred. Remembering that acceleration is "slightly affected" but it's still feels like it's running smoothly on the highway - aside from the over-deceleration.
I'm also aware that I should collect some live stats from the code reader, so I'll return to post those as well.
Thank you so much for adding peace to my life through your collective wisdom and understanding!
Garrison
Whenever I see the sudden onset of misfires in multiple cylinders, my first question is, how recently had you purchased fuel? I'm wondering if you could have gotten some fuel contaminated with water.
As the fuel is pumped into your tank, the gasoline and water are swirled together, but as you drive, or the car sits stationary, the denser water separates and settles to the bottom of the tank where it is inhaled by the fuel pump.
An easy test is to add two bottles of Heet fuel system drier to your fuel tank, sit on the rear bumper and bounce the car up and down to agitate the fuel and help the Heet to intermix with the water, then start the car and drive it to see if things improve. If they do, it would be prudent to change your fuel filter, since it will have accumulated water.
Another possibility might be a failing fuel pump.
It seems unlikely that both primary or secondary timing chains could skip simultaneously and cause misfires in both cylinder banks.
Whenever I see the sudden onset of misfires in multiple cylinders, my first question is, how recently had you purchased fuel before the onset of these symptoms? I'm wondering if you could have gotten some fuel contaminated with water.
Funny, that's one detail i thought about but left out of my description. The night before, I had a yellow warning for low fuel, and stopped at this mid-city Shell station that advertised $5.39 on the sign but was actually $5.55 at the pump, and thought "Ick, I hate shady den of thieves places like this..." So I only put $20 in the tank to get enough for today's trip.
I'm going right for the Heet and will let you know tomorrow!
Also super encouraging to hear you say that both chains would have to slip to cause misfire errors in every cylinder. I was mentally preparing for the worst.
see if things improve. If they do, it would be prudent to change your fuel filter, since it will have accumulated water.
Another possibility might be a failing fuel pump.
Also, I replaced both fuel pumps 2 months ago (and the filter) and I know how to pop out one of the relays in the boot to see if the primary pump is failing and force the secondary to take over. I'll give that a try if the Heet doesn't fix it.
Gosh, I hate changing that fuel filter! hahaha such a mess. But I will at your advice if water turns out to be the situation. Thank you for that, I never would have realized to do it.
Your kidding!! Your paying $5.50 a GALLON for gas??
I get 93 premium where I live for $3.39 a gallon.
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ya but i live in a city so we don't use much gas like i used to on the east coast.
the one thing i detest about the west coast is that after you get to texas, they don't sell anything better than 91! i buy octane additive from time to time just to relive the glory days of east coast gas lol
I remember a gas station (I think Sunoco) where at the pump you could dial in the octane rating you wanted and pump would mix. Don’t remember max but it was more than available anywhere else.
okay today I have 2 bottles of Heet in, shaken not stirred, let it sit 2 hours, still stalls when starting from cold but if i put my foot on the gas a little and wait 30 seconds it evens out and stops stalling. It shakes but after driving 2 minutes shakes much less than before. Still accelerates and drives the highway as if nothing is wrong aside from the heavier deceleration.
I took live readings to share while cold idle, hot highway driving, and hot idle. I only understand a few of these statistics, like i can see that the alternator is working. I can see the timing advance seems normal which means no slipped teeth? I don't know what fuel trims should be and I understand that would be a piece of this puzzle.
and here are the codes at the moment.
again - seems to be improving slowly and i don't know how quickly to expect $20 of bad gas followed by a fill-up of good gas, injector cleaner, and Heet to work its way through.
Your fuel trims look very high, I've seen within 10% of zero stated as the safe range to show nothing is wrong.
Considering they are showing 25% & higher extra fuel, yet you've got lean codes suggests some sort of issue. I'm not sure where I'd go with that, I'm sure others will have ideas. I suspect a fuel pressure test might be an idea at the rail, as a blocked fuel filter would make the car lean even when it's massively increased the injector timing (as per the trims) to try & compensate.
FYI, injectors flow at a set rate, so for more fuel they fire for longer. This of course relies on the correct fuel pressure so they deliver the amount the ECU expects.
Considering they are showing 25% & higher extra fuel, yet you've got lean codes suggests some sort of issue. I'm not sure where I'd go with that, I'm sure others will have ideas. I suspect a fuel pressure test might be an idea at the rail, as a blocked fuel filter would make the car lean even when it's massively increased the injector timing (as per the trims) to try & compensate.
Thank you for thinking it through! And for teaching me what fuel trims "actually mean".
I installed a new fuel filter when I changed my pumps two months ago. I never buy cheap parts but... could be that the material or structure collapsed inside and is blocked??
I'll pop out each fuel pump relay to check both pumps, and report back later today.
High positive fuel trims are consistent with water in the fuel, which causes the oxygen sensors to see a "lean" condition because less actual fuel is making it through the injectors due to the presence of incombustible water. The ECM is extending fuel injector pulse times to add more fuel, but because of the water, the oxygen sensors are not reporting the expected change in air-fuel ratio (AFR), so the ECM keeps adding more and more fuel until it reaches the limit of its adaptation capability, typically +25%, at which time lean running Diagnostic Trouble Codes are triggered.
The Heet is denser than fuel and settles to the bottom of the tank, where it intermixes with the water and forms a partially-combustible mixture. While the water/fuel mix is working its way through the system, the engine will continue to run roughly until all of the water has passed through the engine. Depending on how much water was contained in the bad fuel, it can take 100 miles or more for it to work through the engine. A good extended drive at highway speeds usually helps accelerate the process.
If you don't notice a difference after 100 miles or so, further diagnosis will be required.
After reading through this thread, the problem appears to be electronics rather than mechanical. And, my first guess would be the MAFS. The next thing I would do is check the voltage supply using the attached. This is a 10 to 15 minute job just to make sure you have the voltage to run all of the electronics. When you're satisfied that you have the voltage, pull the harness off the MAFS, then turn the key to the "ON" position, I think that's position II, just don't hit the start position, yet. Check for any new codes on the scanner; you might/should see a P1104 or P1107 code. I've read somewhere here on the Forum that with the MAFS disconnected the ECU will substitute a base map so the car will run. Try starting the engine and see what happens. I haven't tried this, so if it works, please let us know.
Like I said, this is just a guess, but the symptoms are consistent with what I've experienced during my 21+ years of owning my XJR. If it is the MAFS, I've got a couple of good ones in the garage that I could send you for less than you'd spend on ebay or for a new one. I also attached a paper on fuel trims that's pretty good reading.
Last edited by hispeed42; Nov 10, 2025 at 11:43 AM.
High positive fuel trims are consistent with water in the fuel, which causes the oxygen sensors to see a "lean" condition because less actual fuel is making it through the injectors due to the presence of incombustible water. The ECM is extending fuel injector pulse times to add more fuel, but because of the water, the oxygen sensors are not reporting the expected change in air-fuel ratio (AFR), so the ECM keeps adding more and more fuel until it reaches the limit of its adaptation capability, typically +25%, at which time lean running Diagnostic Trouble Codes are triggered.
The Heet is heavier than fuel and settles to the bottom of the tank, where it intermixes with the water and forms a partially-combustible mixture. While the water/fuel mix is working its way through the system, the engine will continue to run roughly until all of the water has passed through the engine. Depending on how much water was contained in the bad fuel, it can take 100 miles or more for it to work through the engine. A good extended drive at highway speeds usually helps accelerate the process.
If you don't notice a difference after 100 miles or so, further diagnosis will be required.
Cheers,
Don
Brilliant. Also explains why the situation is slowly improving as I drove it (for only 15 minutes) yesterday. Thank you so much, my anxiety has fully disappeared thanks to your guidance.
03 XJR timing slip? Rough idle and Pending Misifre on all cylinders
