New car not running quite right at the minute...

[e34.535i.sport]

Ok so I'm guilty - I sold the e34 a while back and bought a 1994 Jaguar XJR. Brilliant car by the way, and I will never go back to a manual after driving this!

Recently however my mpg around town has dropped from an average of 16mpg to below 11 :( On the motorway I can barely get 20mpg being very careful, which is worrying! Other symptoms include a very very slightly rough idle (was absolutely perfect a few weeks ago - you couldn't tell it was running from inside) and sometimes a bit of slight hesitation at low revs.

My thoughts initially was a vacuum leak but having searched my faithful bimmernut I'm now wondering could it be that the o2 sensor has turned gangster on me?! Would this have a similar effect? The drop in mpg seems too drastic to be caused by a very minor vacuum leak to me... Any ideas/thoughts very much appreciated. :)

(Incase you're wondering I'm posting on here as the Jag forums don't seem tohave the same level of interest/knowledge/motivation as the bimmernut!)

[Rick L]

94 Jag should have OBD II...? Have you check the codes with a OBD reader? If your car has sluggish acceleration, slight rough idle, and bad gas mileage, check MAF. There are cleaning spray for MAF. Bad gas mileage can also be O2 sensor. And neither of the two might not throw any codes unless they completely fail. But try cleaning MAF first. Remember to not touch the sensor end as they can be damaged easily.


Ok so I'm guilty - I sold the e34 a while back and bought a 1994 Jaguar XJR. Brilliant car by the way, and I will never go back to a manual after driving this!

Recently however my mpg around town has dropped from an average of 16mpg to below 11 :( On the motorway I can barely get 20mpg being very careful, which is worrying! Other symptoms include a very very slightly rough idle (was absolutely perfect a few weeks ago - you couldn't tell it was running from inside) and sometimes a bit of slight hesitation at low revs.

My thoughts initially was a vacuum leak but having searched my faithful bimmernut I'm now wondering could it be that the o2 sensor has turned gangster on me?! Would this have a similar effect? The drop in mpg seems too drastic to be caused by a very minor vacuum leak to me... Any ideas/thoughts very much appreciated. :)

(Incase you're wondering I'm posting on here as the Jag forums don't seem tohave the same level of interest/knowledge/motivation as the bimmernut!)

[sal_park]

Any ideas/thoughts very much appreciated. :)


First thought - the rough idle and bad MPG may have nothing to do with each other. I'd suggest trying to fix the idle and the keep an eye on MPG.

Have you tried disconnecting the MAF sensor (or whatever the XJR has) while the car is idling ? I believe the idle should get worse if the engine's not running in limp home mode.

Finally, what about pushing some gas (butane or similar ?) around the inlet manifold and vacuum hoses when the engine is idling ? If you've got a leak the idle should speed up. I've not done this myself, so could have described the procedure incorrectly, please be careful with this !


hth

sal

[BMW4LIFE]

+1 on the O2 sensor....its not a cheap fix so you have to make sure....

This might sound weird but sometimes the simplest thoughts can confirm things like this...

I would say...take out the O2 sensor and drive with the car for like half the day or so and see what happens to the car...if you see a drastic change and the MPG gets worst i woul say that maybe its something else...but if you see no change at all then your O2 must be the problem (or at least a big part of it)

Good luck with the Jag =/

[e34.535i.sport]

Hey guys. First of all thanks for the responses - very useful. Just to elaborate it has a slight hesitation on low speed take off at low revs and only slight throttle - if i hammer it it will pick up quickly like it usually does! And the fuel economy is ridiculous still!

I recently did a fairly big job on the car and had to remove intake/maf/fuel lines/supercharger (and more!) ... Got it back together and sorted out a vacuum leak on the intake with a new gasket.

The vacuum leak was my first thought so i tried spraying some carb cleaner around the intake gasket and no change in idle speed so i guess that's still ok.

To be honest i'm not totally sure if the reader is OBDII or not - checking that on the jag forum just awaiting a response! I plan to get a reader for it as i suppose it will pay for itself eventually! For the time being i want to get on top of this fuel problem because i don't want to be running rich and killing the cats (I'm not into animal cruelty! :p )

Now I have had to unplug the maf and the coolant resevoir sensor a few times recently (taking them on and off to get at things so i'll make sure the maf has a good connection and try to clean it so thanks!
*I forgot to mention that the 'low coolant' light is on and the sensor is broken, it's hanging loose below the resevoir (not leaking though and level is fine). I was assuming this would be totally unrerated to my symptoms as it only monitors the level of coolant - am i right?

Removing the o2 sensor... Hmmm i've heard this is a bit of a job sometimes do i need any special tools to do this? I have no experience with this so am i right in thinking it is on the early sections of the exhaust somewhere?

[whiskychaser]

AFIK Jaguar didnt share their diagnostic info too readily. Even on ebay nobody jumps out and claims 'this will work on your XJR'. Maybe worth having it done at dealer if it doesnt cost a limb or two?
While the low coolant light might bug you, I doubt it has anything to do with it. Of course a duff temp sensor would though. I bet a workshop manual is as scarce as the proverbial rocking horse s... stuff too?
Have you seen this item about testing O2 sensor(s)?:
http://www.lambdapower.co.uk/diagnosis/diagnostic_index.asp#q3
Hope you havent scrubbed the new tyres too much:D

[BMW4LIFE]

Removing the o2 sensor... Hmmm i've heard this is a bit of a job sometimes do i need any special tools to do this? I have no experience with this so am i right in thinking it is on the early sections of the exhaust somewhere?

Granted every car is different in their difficulty but I wouldn't imagine removing the sensor will take you longer than an hour or two...and as far as special tool...I highly doubt that as well...

If you have some extra time...its worth throwing the car on some jacks and at least taking a look under the car...Its not going to stand out like a soar thumb or anything but just follow your exhaust lines and you'll sure see it...

Hope that helps

[repenttokyo]

jag adopted obd-ii in 94? in north america, it happened in 1996...

[whiskychaser]

jag adopted obd-ii in 94? in north america, it happened in 1996...

UK didnt implement it until 2001 for petrol and 2004 for diesel.

[e34.535i.sport]

Hey Colin thanks for that link! I read it (or tried to haha!) and thankfully I think I sorted the problem this morning! :p

So this morning I had a bit of time to have a look around - Ruled out the MAF by unplugging - the idle went crazy! Found the o2 sensor quickly (it's the four wire type... There's two types apparently!) and traced it back up top to where it 'plugs in'. The plug end was in a particularly awkward place near the firewall along with two other plugs (not sure what for!). I had unplugged and replugged these when I removed the injector harness a while back and it seems I must not have plugged it back in properly as I pulled it off and the car continued to run a little bit iffy... When I re-connected it the car seemed to idle better so I took it out for a test drive - Reset the MPG metre and instantly back up to 16-17 round town (with the same driving stye) which has made me very :D

I'm made up it was such an easy fix and didn't cost me anything (bar a lot of petrol for the last week or two!) even if i do feel a little bit foolish!

Many thanks to you all! :D

[whiskychaser]

Glad to hear you got it sorted! The big bonus is it didnt cost anything. Now all you have to think about is what would you do if it wasnt as simple as that :D Big of you to come clean about it:) Result

[e34.535i.sport]

Haha thanks Colin - I'm too honest to be a proper scouser I think :p

:( Looks like I spoke too soon however because I got 9.9mpg on the way to work this morning! If I was hammering it I wouldn't mind but i was driving 'normally' ... Seems to have gone back to using silly amounts of fuel almost overnight, which seems strange. I'm only basing this off the OBC but I know it's relatively accurate by how many miles I'm getting for 1/2 or 1/4 tank etc I have been getting. I wonder if the o2 sensor is working inconsistently rather than not at all? I've been searching around and found that coolant temp sensors can also cause fuelling issues?! I can get one of these for less than £10 so I might just do that, but not quite as easy with the o2 sensoras I'm looking at around £80 for that :( If I didn't think I was in danger of killing the cat by running rich I wouldn't be so worried but as they cost a small fortune I need to see if I can trace the source the problem!

[whiskychaser]

I thought you might have 'borrowed' one of the big 'scopes from a lab at work and checked the O2 sensor(s). Just imagine you driving around with one on the passenger seat and a belt round it. Imagine you explaining it to the bizzies (they still call them that?) - 'Yeah its my sat nav':D Its hard to say if the temp sensor is duff or not without reference to spec. Clearly if its not working then the ECU thinks the engine is still cold and you are effectively running on 'choke'. If thats the case the exhaust will probably stink of fuel and it will run lumpy. Maybe check the resistance of the sensor when cold, at 1/4, 1/2, 3/4 and hot on the gauge? You should see some logical change but it may not be linear.

[BMW4LIFE]

my bets still on the O2

Think of it this way...if you plan on keeping the car for a while that O2 will need to be replaced so whats another 10000 miles....

IDK if I were you I would bite the bullet and go for the O2

then if the problem remains unresolved then I would sell it and buy a Honda Accord! ;)

jk

I hope it works out for you bud!

[e34.535i.sport]

Haha ye the 'bizzies' (we also call them other things that I won't go into! :p haha) would not be impressed with that! :D

So do I check the resistance with a multimeter then yes?! I'm useless with electric stuff give me suspension anyday my primitive mind can just about handle nuts n bolts! :p

Cheers JK i'm tempted to just change both sensors and hopefully be done with it... If I don't procrastinate for too much long the decreased fuel consumption will offset the price of the sensors anyway I suppose! (Normally I'd just do it but got a mortgage on the horizon now and Christmas is looming!). Better get over to ebay I think...

[whiskychaser]

Yeah a multimeter is fine. You are only checking the change in resistance:
http://www.jaguarfuelinjectorservice.com/Testing%20Testing%20the%20Jaguar%20Coolant%20Tempe rature%20Sensor.htm
Dont know if this is bang on for the XJR but gives you a clue:D

[Grantus]

If there is a coolant sensor that reads temperature and is faulty, that can cause excessive fuel consumption because it tells the computer that the engine has not reached operating temperature and the mixture then becomes enriched.

[e34.535i.sport]

I 'bited' :p the bullet and bought both sensors. The o2 sensor is a PITA to get at (well at the connector end anyway!) but will be able to change them over myself which is a plus. :) Ended up searching around and getting both for <£80 from 'Britishparts' which I was quite pleased with :)

If that doesn't help it will give me an excuse to come over and visit anyway ay Colin?! ;)

[whiskychaser]

There isnt anything in the 'service bay' at the moment John:D

[e34.535i.sport]

There isnt anything in the 'service bay' at the moment John:D

Ha ha ha I like! :D