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p773990: Ah, in your photos it certainly looks like the Sensors are on a short piece of pipe so 'excuse me' if they are not !
'Ally' is aluminium (or aluminum if you prefer !) Although Stainless Steel would last longer: Just Google 'Jubilee Clip' ?

Serves me right for trying to help I guess.....

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Ally = Aluminium, Jubilee Clip = screw clamp.

It looks like yours have already been modified, hence the need to weld the O2 sensor bosses in. Parts list shows the sensor bosses in the inlet pipe to the cats not as a separate part.

enter image description here

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Sorry, that is for the SC Sport, it is a short stub pipe on the FFRR. See https://parts.jaguarlandroverclassic.com/lr011092-pipe-exhaust.html

Ridiculous price but can be got cheaper https://www.johncraddockltd.co.uk/lr011092-exhaust-stub-pipe-turbos-to-downpipes-5-0l-v8-petrol.html. Still bloody expensive for a bit of tube with a flange on each end and a boss in the middle.....

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Holes in the exhaust do have potential to throw readings from O2 sensors, usually by holes acting as a venturi which causes air to be sucked in. But I'd only expect that to happen in an exhaust that has very little back pressure. Of course, a normal exhaust running at part engine load would have very little back pressure. Put all that together and it means that holes in exhausts effect mixture readings more at part engine loads, while your fuel trims are consistently high across the full range of engine loads.

Interpretation of post cat readings can offer insight into whether pre cat probes are working properly, but it's easier to detect front probes that are reading too rich (so leaning mixture off) than reading too lean (so richening mixture) using the post cat probe reading method.

Since front probe bosses (screw in points for probes) seem to have been tampered with, perhaps the 2 front probes have also been replaced with incorrect spec probes for the vehicle. Wide band lambda sensors are available in a wider range of specs than narrow band probes and are not nearly as intercompatible as narrow band probes.

But I'd be changing the Maf before any probe changing or boss welding.

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So why the Maf? Due to age? Or simply to hard to determine if it is reading out of spec. (sort of a preventative measure).

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So the exhaust for the 4.2L and the 5.0L are the same? I could probably find some decent used ones here if they are the same. The older 4.2L are getting scarce around here but 5.0L are easier to find.

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Has my exhaust been changed to the 5.0L? On the JLR site it does not show that for the 4.2L exhaust

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p73990 wrote:

So why the Maf? Due to age? Or simply to hard to determine if it is reading out of spec. (sort of a preventative measure).

Because you've disproven a lot of things already, Mafs do fail in time / use, you'd probably spend as much time and money replacing lambdas as you'd spend on a maf, a maf can be changed with very little effort compared to welding (or maybe even bodging) pinholes in lambda sensor bosses. Vehicle work doesn't get much easier than changing a maf. If we're wrong and a new maf doesn't fix it, keep it as a spare (in case you intend to keep the vehicle as they do tend to fail in time anyway) or sell it on as a part bought in error. I don't often advise swapping a part that hasn't been proven faulty but it seems a sensible next step in this case. I've spoken to owners of SC RR's who have had the same problems as you fixed when they changed the maf.

I'm not saying the maf might have completely failed, it definitely hasn't completely failed... but it could be reading out of spec (outputting a signal that's around as linear as spec but under-reading across the entire air flow range). They can do that with age, use, or if they get dirty. I used to see a lot of under-reading Maf's on Lexus vehicles, some of the air filters owners were fitting imparted a yellow dust onto the maf sensor which insulated the innards from flowing air and caused the mafs to go out of spec. Often when people think they're cleaning the maf sensor they're actually just cleaning the IAT intake air temp sensor that is built into the same unit as the maf. The maf itself is far more delicate than the IAT and it can be difficult to access so attempts to clean it can cause problems in themselves.

Most mafs output a voltage signal that represents airflow, some mafs output a pwm or digital signal. Of course there's a wide range of mafs of each of those types and you need the specific correct maf for the vehicle. Without re-reading the thread I don't know if you've said how long you've owned it, maybe if you've only owned it a short while it's possible a previous owner fitted an incorrect model maf. I'd only but a genuine maf for a RR SC, or in your situation I might check to make sure the maf is the voltage output type and if it is then make a project circuit that reads maf voltage and outputs a different voltage (in your case a higher voltage). There's a guy/thread on this forum where someone (I forget who or I'd link to it but I expect someone else reading this will link to it) made such circuit/Arduino project that does just that.. He fitted a completely different spec maf designed for a different model vehicle to his P38 which usually wouldn't work on a P38 but the Arduino project he made allows (within reason) any Maf to be fitted to any vehicle.

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p73990 wrote:

Has my exhaust been changed to the 5.0L? On the JLR site it does not show that for the 4.2L exhaust

Looks like it might have been. The 4.2 SC L322 appears to be the same as that on the Sport. https://new.lrcat.com/#!/12119/94841/99132/7893

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Fair enough. I like to determine and validate things first. we have eliminated almost all of other possible causes. I was coming to that conclusion as well when I seen the date on it. And when I repair the exhaust I may have to replace the O2 sensors anyway if they dont survive. At least Im not throwing parts at because Im just guessing.

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Davew
You do have it right. They are short pcs of pipe but the humidity and salt they use on roads here makes exhaust work difficult. Any weak parts are usually broken as soon as removal is attempted which can snowball into a lot more work. As you can see from the pics access is difficult. I didnt mean to offend, I just needed some clarification on terminology.
Environmental conditions here are different to yours. We have constant humidity except in winter and then they use salt on the roads. Because of this ally and steel here, do not play well together unless very well isolated. Any hole in the exhaust system will continue to grow at an increased rate. This area of Canada is known as the rust belt. Most vehicles wont make it past 15 years. This is not something anyone would know unless you have spent time here.

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Not cutting you off though, we could keep going with testing etc if you like. But it does seem there's not much more to learn from testing and it's time to start thinking about which components the results of those tests could point to. Components/things those results could point to include the maf, fuel pressure sensor, lambda sensors (whether or not they're correct spec), fuel spec, false (unmetered) air leak between maf and throttle body. I probably forgot something!

But it's occurred to me there's one more thing I'd try before replacing any parts. At the rear of each fuel rail (near the firewall on the engine) there's a fuel injector harness connector... Disconnect one of those connectors and start it up, so it only runs on 4 cylinders because 4 cylinders have fuel injectors disconnected. Let it run until the engine warning light comes on (and probably flashes). Use your OBD tool to confirm that it's logged faults for at least one disconnected fuel injector. Then turn the engine off, reconnect the loom, turn ignition on, clear the engine error codes. Turn ignition off, restart the engine and immediately do some mixed style driving (like I advised before, don't just stick to a steady cruise or steady engine load). This doesn't seem intuitive I know, but there are good reasons for this advice. Back in the days when 4.2 SC RR's were relatively new, lots of people in my business (LPG converting vehicles) reckoned these vehicles had 2 fuel maps that the ECU's switched between, which they blamed for poor LPG conversion results. My take on it is that there's only one fuel map but there's a kind of underlying/initial adjustment/map and that adjustment/map is learned during early engine running from new and after and severe error codes are cleared with the idea being to have the 'usual' map have fuel Ltft's of around +7% when learned. But if that initial/base adjustment/map is learned wrong then even if all mechanicals and sensors are in good order and well within spec the Ltft's may be well outside of that average +7% when learned. You can't read that base/initial/underlying map on any scan tool but the concept isn't really that unusual, for example a lot of vehicles that run wide band probes kind of do much the same but in reverse, on those vehicles the fuel trims for idle are usually not anything like what would usually be expected (for an old skool vehicle with narrow band probes) until the fuel trims for under-load driving are learned, and then the base/underlying map for idle conditions is based on the under-load trims and the idle trims start to read as would normally be expected.

Not really relevant but I did have a look at the last couple of scan tool PDF's you posted, thing is in those data sets you'd selected S2 (post cat sensor) Stft's which are less relevant than S1 Stft's so the overall data sets were not as useful as the earlier data sets I based some of my advise on. Still they do confirm consistency with earlier data sets so they were useful in that respect.

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Holes in the exhaust would cause the O2 sensors to report a lean condition, thus directing the ecu to add fuel. If the right amount of fuel is being added, that would show up on the spark plugs as a rich condition provided no extra air is being drawn in. But if the Maf is under reporting the amount of air. It lowers the the amount of fuel added and also lowers the actual amount of the fuel the long term trims have available. Which then creates the high readings. Because the reality is not enough fuel is being added since the sensor is under reporting. And this would then actually show up on the plugs as a lean condition similar to a vacuum leak. The Ecu then uses this along with barometric pressure, air temp, engine temp to then determine how much fuel to add and also how much fuel pressure it needs.
If I got this correct, it has not been a worthless exercise. I have actually learned the basics of the system system operates.

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You are not cutting me off. We have eliminated every thing other than some possible very tiny vacuum leak.
I will try unplugging the injectors which in simple terms sounds like it will cause the ecu to relearn parameters based on the actual wear and tear of the engine. Effectively relearning the base fuel trims for the current condition of the engine.
As per my previous post I think I understand where we are on the trouble shooting and the Maf sensor makes sense. Also since the holes in the exhaust are downstream of the upstream sensor. I dont think they would really affect the upstream sensor readings, Which I think you mentioned are the primary drivers of the fuel trims.
Other than a decent Fluke multimeter I have no other tools available to actually check each sensor out. And without a base line it wont mean much. Im learning stuff here so Im happy. Let me know if Im getting it correct.
What? I have fuel trims for all 4 O2 sensors? How did I not see that! Getting old sucks.

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Should i do this before or after changing the Maf?

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And I assume this should be done with engine at operating temp,

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Before because if it works it could fix it, then you wouldn't need to buy any parts.

I'd do it with a cold engine, that way you're doing the same as I did when I first found out it could reset the underlying trim. When I'm cutting petrol injector wires to wire in an LPG system, sometimes the easiest way of finding which colour wire leads to which petrol injector is just to cut a wire and see which cylinder the petrol ECU reports has the misfire and open circuit petrol injector.

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Some would call that cheating. LOL (instead of trying to find everything the electrical drawing) I assume you have to cut them all regardless. Easier, faster, why not?
I dont mind buying parts once im sure they are the right ones. And this is now starting to make sense.
Ok will do
I assume you want me to log data as per your list earlier?

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Hehe yes cheating in a way but I have to cut all injector negative pulsing wires anyway. Even if I'd looked up wiring colours on a diagram there'd still be the need to confirm which colours went to which injectors, with my method there's no need to confirm.

Might as well have the data logger connected. In fact you could use it to make sure Stft's don't hang at extremes for too long, if they're hanging at an extreme change the throttle position and the Stft's will momentarily return to zero which resets the timer for Stft hanging at extremes and prevents throwing a MIL code before Ltft's are learned.

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Ill see what happens. might take a few days though Its cold and crappy here still for the next few days.
Im thinking I should change the Maf anyway since it is most likely polluted with oil from the previous air filter. I need to get some exhaust gaskets anyway