Hi All,

As always a great wealth of information is available on this Forum and my compliments go to all the very knowledgeable members!

I've just spent a couple of years restoring my pride & joy 2000 XJR8 to almost concourse condition (that's a couple of years on & off BTW). One of the last tasks I carried out was to fit a new stainless steel semi-sports exhaust system, new CATs, new oxygen sensors all round, new double iridium spark plugs, new coil packs all round, cold air induction kit and to change the supercharger and engine oil / filter. All appeared to be running fine with the car driving well for a few days, but I noticed the fuel consumption had taken a nosedive from originally circa 22MPG to 11MPG (UK Gals).

I ruled out all the changes above as possible causes except for the new CATs. I had my doubts over the non-OEM replacements bought cheap on ebay from day one. They were poorly assembled and looked in my opinion to be very restricted when offered up against the original CATs that were at least 50% larger (Cell surface area / Canister Size). Having kept the original CATs I decided to re-install them with the new oxygen sensors to see if fuel consumption returned to normal.

Shock, horror... To my disappointment not only has the fuel consumption remained very poor at around 12MPG, I now have a car that idles like an old washing machine spinning a load of wet towels, it regularly drops into 'Reduced Performance Mode' and this evening has thrown the following OBD Codes: P1642, P1646 and P1647.

I acknowledge these codes relate to the Oxygen Sensors and Can-Bus Errors, so I've checked the plug connections, loom, wiring routes etc. but can't visually identify any potential issues. The sensors worked fine in the new / removed CATs but having fitted them to the old CAT's they appear to be misbehaving somewhat. Is there a way of testing these sensors using a mult-meter or similar? If so, what readings can one expect between what pins?

Also, can I assume that perhaps the ECU has some form of 'learning curve' memory whereby it collects and stores data from various engine management sensors and trims the engine to run accordingly? If so that might possibly explain why, having replaced many components in one session that the engine now runs like a bag of poo!

Can anyone please clarify my thoughts / queries, and if I'm not barking up the wrong tree is there a way of resetting the ECU to start the learning process from scratch? BTW, I've already tried disconnecting the battery and shorting the leads together. This had no effect and didn't clear any OBD codes, and after clearing them using the OBD scanner tool they reoccur within a few minutes of idling or engine restart.

Hopefully having got this far a trip to Jaguar isn't necessary.

Thank you for reading my post, and thank you in advance of any advice you guys can offer.

Regards

Chris

 

 

Number one is to check for unmetered air leaks, any joint on the induction side - somewhere is sucking in extra air, the ECU is dumping more fuel in attempting to even it out, hence the lumpy running/gas mileage.
The O2 sensors (upper) look like they're non functioning with those codes. This will only cause more fuel to be dumped as the ECU inhibits them.

A hard reset will put the ECU back to factory settings, but doesn't address hard faults like non functioning heater circuits in the O2 sensors - you need to either put the original uppers back in or double check connections are 100% on the new ones - which ones did you replace the originals with, and did you do all 4?

 

There is a possibility that one issue may solve the scenario?

After a reset, the ECU has to relearn your driving habits/patterns (all ODBII cars do this). Though you shouldn't be getting any performance issues during this learning curve. Did you purchase the correct O2 sensors....check this thread...https://www.jaguarforums.com/forum/x...cement-195018/

For vacuum leaks, review this thread.... https://www.jaguarforums.com/forum/x...7/#post1471002 ...yes, its another issue, but the areas to check are the same. Be sure to check the vacuum on the back of the TB to the small MAP almost directly behind it on the firewall, its a blind fit and easily missed. For better direction on that MAP... https://www.jaguarforums.com/forum/x...8/#post1452568

 

Dump the cold air induction kit; check the MAF (clean, bent pins); try again.

 

Minagallary induction kit. But plenum box didn't fit!!


Induction kit adaption using original air filter box
Guys... Thank you for your replies & most welcome advice. Here's what I've tried since my initial post:

1. Replaced old coils packs which resulted in an evident misfire during idle but didn't throw any fault codes. Engine went into Reduced Performance mode when test driven. I've since re-fitted the new coil packs.
2. I've checked for air leaks on the induction ducting, and the other tube at rear of TB with soapy water (acknowledging this duct is under negative pressure, but I would have noticed bubbles getting sucked in or even possibly a stutter from the engine), cleaned the MAF sensor again. But still no noticeable improvement in the last few days of driving.
3. I've checked all wiring and connectors etc. associated with the 4 new Oxygen Sensors and all appears to be in good order. I could reinstate the old sensors as a process of elimination, but would rather not because I had to cut the old plugs off to get a ring spanner on the sensors to remove. The new sensors are admittedly not OEM, but were purchased from a reputable UK motor factors. Not sure what brand they are but will find out. Probably still got the packaging kicking about.
4. Personally, I didn't like the cold air induction kit purchased from Minagallery in the States. The swept induction tube and MAF housing is fine but the plenum was a very poor fit. I doubt they'ed ever tried actually measuring the space! I suspect most of the combustion air was coming from the engine compartment which clearly ain't Cold! I've since *******ised the old original air filter housing to accommodate the induction duct and fitted a new K&N panel filter. Again I have checked for air leaks around the adaptions I've made and everything is air tight. I very much doubt the air filter housing itself is totally air tight, but then I can't see this having any effect on engine performance.

I think I've covered most of not all the advice provided but I can't notice any appreciable improvement in either quality of the idling engine (lumpy) or the MPG which is still hovering around 12mpg.

Any further thoughts would be appreciated.

 

 

Bit of a downer cutting the plugs off - Have you checked the fuel pressure? What spark plugs are fitted, hold old?

There is some information on the forum about the X Type O2 sensors fitting, a cheap option is to buy SH units and fit, I think the grey plug version, upstream?

As you're sure there are no air leaks, the O2 sensors are the fault....fix that and your idle/gas mileage should be correct.

 

Hi Sean,

Thanks for replying.

I wasn't anticipating re-fitting the original O2 sensors at the time so went a little gun-ho and cut the plugs/leads off to use a socket after not being able to budge a couple of the sensors with a normal spanner. I've read Highhorse's post re O2 sensors with much interest and note he refers to a marking '3000' etched into the hex head of each sensor. Having checked all for of the original sensors I can only vaguely identify the numbers 245 on sensor body of two. Unfortunately, I don't recall the new sensors having any ID before fitting them, so sounds like I need to pop one out to inspect any such markings.

Not sure I have any means at my disposal to check fuel pressure, unless there's an on-board sensor I can interrogate with the OBD scanner. I'll have a look see ASAP. If not I assume this would need to be measured close to the TB, and being an electric pump the engine need not be running, so connecting a pressure gauge to the fuel line and switching on the ignition should result in pressure reading?

Something interesting I noticed today when watching the trip computers Instant MPG. When coasting or decelerating say down a motorway slip road the MPG display is very erratic. Rarely exceeding 75mpg and often dropping into the low teens without touching the gas peddle. If I recall correctly when this was my every day car about ten years ago, when you took your foot off the gas the instant mpg would rise to 99.9 within a couple of seconds or so and remain there until the road speed was low enough to labour the engine. I wonder if anyone could please verify this for me in case my memory playing tricks? Either way, I would not expect such an erratic mpg reading while coasting downhill from 70ish to around 20mph.

BTW.. JTsmks, thanks for your reply. The plenum box in your photo is totally different to what Mina Gallery sent me after about two months of trying to get them to reply to my emails and full fill the order. I note also the transformation piece onto your TB looks a lot better than the 90deg hose fitting they supplied me with, which had to be carefully cut to the bone to enable the bonnet to close. Maybe I just had a bad experience with these guys, but I paid top $$ for a few bits of tube I could have picked up on ebay for less than Ł100.