I can’t say I’ve encountered a pump in these cars where it’s working but not pushing volume. Maybe a high mileage fuel filter, but not an inadequate pump. I’ve heard of it in other makes/models, just not these era AJ-V8’s. I hope for your sake that it’s the problem.

 

Update - very glad to say the new fuel pump seems to have done the trick. I've yet to do a long journey but no errors or warning lights after a few drives, and it's starting straight away in the morning for the first time since I've owned it, which is nice and has to be a good sign. Apparently, the valve on the pump was visibly damaged when they took it out...

 

NBCat was the winner of this round 👏.
Good job! 🎉

Glad to hear your situation is at least improved. Now the road trip will be the final (hopefully) test.

 

Yes indeed! Thank you everyone for the help and advice, but special kudos to @NBCat !!

 

Turns out I spoke too soon! Literally in the last mile of a 2-hour roundtrip this weekend the dreaded restricted performance and then misfire errors re-appeared...:-(

Next step is to replace the fuel filter I think - my logic being that if the fuel pump was faulty that may have damaged/put extra strain on the filter? Or perhaps the other way around...Any suggestions for other parts that the faulty fuel pump could have damaged that would be worth checking/replacing before looking elsewhere?

 

So I'm pretty much at the stage of giving up until the smog test is due next year, although I may try and find a different Jag shop to get some fresh eyes on it, if I can face it later this month.

I am noticing there seems to be a pattern for when the misfire errors are showing up - it seems to be at regular intervals of between 1-2 hours driving. if I clear the pending codes, they don't appear until after another hour or so of driving. I'm going to measure the mileage between errors and see if it's consistent - if it is perhaps that could be helpful to diagnose (i.e. is there something that is checked/run as part of the emissions readiness checks at the same regular intervals?) I also find it strange that I can never feel or hear any misfires, the only indication they have occurred is the restricted performance for a few minutes, and the error codes - does that suggest a dodgy sensor perhaps?

Outside of that, having ruled out fuel pressure/filter, compression, spark plugs and coils I wonder if it could be a timing issue (given that it started after the timing chain and tensioners were changed) - is it possible for timing to be slightly out and cause this kind of error infrequently, or would it always cause errors more often?

I didn't end up changing the battery but will likely do that as well just to rule it out completely.

 

Timing wouldn’t phase in predictably, cam and crank sensors would post every time the engine runs.
It seems the least important but if you don’t know the age of the battery get it tested prior to shelling out on a new one, good battery condition is essential to diagnosis.

The codes/misfires came straight after the timing chain tensioner update work.

My guess an unmetered air leak on the induction, runs fine cold?

 

what kind of spark plug they use ?
I had multiple misfiring with Denso TT iridium which supposed to be high end plugs check the spark plugs itself mine fixed with autolite copper spark plug
also check the vacuum lines
finally disconnect the maf sensor and run the car for a day or 2 i highly believe its the maf and the spark plugs itself
also don't be afraid of ebay ignition coil set i run them on my both x308 they run great just get it from good reputation seller I wasn't lucky with used Denso coils

edit just saw you answered me . my suggestion get autolite 3924 or ngk copper plugs they're dirt cheap in rock auto replace them all the 8 plugs . not all ebay parts are that bad actually christopher himself which supposed to sell original tsubaki tensioners repack them and sell it for much higher price I run ebay secondary tensioner 50k miles and thankfully runs great . 2 years ebay ignition coil also fine

 

Thanks, @Sean B - good to know re timing and sensors. Yes runs fine cold other than the intermittent rough idle. The battery is only 3 years old but anything I can change myself and rule out from contributing to the issue is appealing!

The shop checked for air leaks as that was their first guess too - they replaced some parts and worn pipes but it didn't fix the issue.

Thanks, @mobile1 yes plugs and coils did seem likely but the shop is adamant it's not either. A replacement MAF sensor seems cheap and easy to swap though, so seems worth a try. If it's OK to run it disconnected for a few days I'll try that first.

The technician has suggested replacing the engine control module with a used unit - could that be the issue? Anyone have experience with them going bad? It just seems a strange coincidence that a sealed part like that would fail at exactly the same time that the plugs, coils and tensioners were worked on but I guess it's worth a try...

 

I'm not guru but I wouldn't touch the ecm since it was working fine before that tensioner replacement
the maf . I would diy the spark plug and new coil set I tried used denso but it was weak . for now try run it without maf and hopefully working fine

 

I am still leaning toward the plugs and coils as the real issue. If the car ran fine BEFORE they were
installed and ran bad AFTER they were installed that would be my first place to look.

 

@RandyS the codes show misfires on all 8 cylinders though, does that make plugs and coils less likely to be the culprits, or is that common?

 

When you replace plugs and coils, you normally replace all eight, and you normally replace
with all the same brand and type of plug and coils. In most cases only one or two would
prove faulty if any, but I have seen bad batches of both plugs and coils come out of the
factory (depending on the brand, of course) Denso, while being the most expensive is
also the most trustworthy and personally, I will use nothing less. We are kind of shooting
in the dark here, because hands on manipulation of the plugs and coils could show that
they are doing their job, however it seems to be the most expedient course in sorting the
issue. The other thing which was changed at the start of your issues was the timing chain.
so that might be the next line of reasoning.

 

Thanks @RandyS , that makes sense, and appreciate it's tricky without being hands-on.

Really appreciate everyone's help and suggestions - I'm not trying to second guess the shop, who have been amazing and definitely know what they're doing with these jags. It was more just in case anyone recognized the specific symptoms as at this point it does seem like it's something other than the likely suspects, which the shop will certainly have thoroughly checked by now (I've lost count but I think I'm at visit number 7 :-)).

This thread from 2012 does seem to match my symptoms exactly but unfortunately, the op didn't post an update or resolution.

Anyway, I will keep at it and update here if there's any progress!

 

These are the things that can cause the misfires you are experiencing. You seem to report the issue
will surface after a couple of hours driving, which is strange, especially with no effects while driving.
Faulty Spark plugs and/or coils would usually manifest errors long before an hour.

• Spark Plugs
• Spark Plug Wires and/or Coils
• Clogged EGR Valves
• Vacuum Leaks
• Damaged Head Gaskets
• Camshaft Sensor
• Crankshaft Sensor
• Oxygen Sensor
• Throttle Position Sensor
• Fuel Injector
• Catalytic Converter

The ECM would be the last thing I would look at. It is a lengthy list however it reflects
the order of the things I would check.

 

That's a super helpful list, thanks so much @RandyS. Checking/changing all the sensors you list would definitely seem like a good approach before the ECM.

 

Usually bad coil(s).

Also, you might look at the fuel trims in case they provide useful clues.

Before that, make sure ALL the OBD monitors are happy (have set, i.e. you have pending P1111 not P1000). To get there, do not clear codes!!

 

I'm curious what happened. did you solve the issue ?

 

Stil ongoing...

The fuel pressure issue definitely seems to have improved things as I'm getting the errors less frequently now - 110 miles since the last error and still clear, and it was 98 miles the time before that. Also, the pending misfire error codes have cleared themselves the last two times and have not caused the check engine light to come on - previously they would always develop into stored faults.

All the monitor checks except the catalyst have just passed so I'm going to keep driving and see if that passes as well... If it does, and nothing else changes, I can live with a very occasional Restricted Performance - when I next need to go into the shop for something else to be done I'll likely try the replacement ECM as the technician suggested. Not sure if the ECM itself is at fault or perhaps just a loose solder somewhere, similar to the brake control module ABS light issue.

I will update if the situation changes!

EDIT: I should also add that the rough idle has significantly improved since the fuel pump was changed. It is still present when starting from cold but less severe than it was, and it seems to clear up after a while.

 

@moohead, did the Technician replace the fuel filter along with the fuel pump? The filter should always be replaced with the fuel pump to avoid the possibility of low pressure to the fuel rail at the engine.

Have you checked the LTFTs and STFTs when the misfire(s) occur? Since they appear to be on several cylinders, most likely there may also be a vacuum leak. The LTFTs and STFTs can indicate if there is a lean or rich situation during the misfire(s).