Since joining this forum in 2009, you're first person I've seen making this claim. I've run mine on 87 octane probably half the time I've owned it and never noticed one bit of difference.

 

In my experience when i purchased my jag the car would " FAILSAFE " 98% of the time .... i didnt know why untill i stopped using REGULAR UNLEADED GASOLINE and started using PREMIUM UNLEADED OCTANE AKA 93.. you may have been using regular unleaded in yours with no problem but mine didnt straighten out untill i changed fuel grades.. within 2 days of starting to use 93 premium unleaded fuel all of my warning lights went out and the car hasnt "fail safe" ever since... If your car calls for premium unleaded why use regular unleaded or 87 in it anyway ? smh

 

ummm sir, you also have a 2003 4.2... 3 years newer than mine wich may explain why you are able to use regular unleaded fuel... mine is a 2000 4.0 and im telling YOU that untill i started using premium 93 grade fuel in MY car that it " FAIL SAFED" ALMOST THE WHOLE TIME I DROVE IT. when i switched to premium the problem stopped and ALL of the warning lights went out... this was MY EXPERIENCE... WE ARE ALL HERE TO HELP PEOPLE WITH OUR EXPERIENCE.... SMH

 

Worth knowing!

The 4.0 has a simpler PCM from a different maker (than the 4.2), and cruder O2 sensors, either or both of which may be the reason. More data would be good but we should bear it in mind. The simpler one is also burdened with doing all the control of the autobox unlike the 4.2 which has a dedicated TCM.

It could instead be something else, but I don't see an easy way to find out.

 

No need to shout or shake your head.

No other owner of any S-type has mentioned experience remotely similar to yours. I cannot imagine any technical reason for insufficient octane setting the CEL and inducing failsafe.

All engine variants used in these cars over the years appear to be very tolerant to lower than optimum octane levels. Specifically, many 4.0 owners have used 87 octane by accident or purpose and report no loss of performance or mileage (the expected side effects) never mind not setting a CEL. I'm wondering why yours is different.

 

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In reading all of the post from this thread no one has mentioned what I've experienced with regard to using 87 octane fuel and switching to 93 octane and my car completely not fail safe engine mode any more but if you read allbthe post and I capitalize for emphasis NO BODY HAS EVEN ASKED THE ORIGINAL POSTER WHAT FUEL HE WAS USING . I came on the post to tell my experience with hope that it would help him/her and appears that a couple of you are minimizing my experience ... When manufactures make cars they recommend what fuel to use... Jag clearly recommends premium fuel . Most cars that are recommended to be on premium run horribly on regular 87 octane. They knock and don't get good gas mileage. Now with that being said I stand by my experience. What would I gain by telling the original poster and other jag owners to try using premium ( wich we all should do anyhow being we are drive $50,000 plus cars ) and see if that helps the problem ? *WE are ALL here to help each other big or small when we have issues... Whats the worse that could happen from using 93 and seeing if the fail safe mode disappears ? 50 more cents a gallon ? I think it's worth the try... Some seem to wanna be right so bad that it feels better to emasculate everybody else that may have a different fix than you... To the original poster and others that may be having a similar problem, it won't hurt to try what I've suggested for a few days and see if your situation won't get better... If anything you have only lost a few more cents a gallon. Good luck

Sent from my iPad

 

Funny, that's what crossed my mind too, so we agree on that point.

From the dozens/hundreds of posts over the years from owners of the 3.0 and 4.2 engines, fuel octane levels do not visibly affect engine performance, mileage, set fail safe, CEL or restricted performance.

I'm curious (as possibly others are) as to why your 4.0 is different. Note also that none of the posters in this thread with problems have this engine.

I'd be curious to know if there's a latent fault with the knock sensors. Possibly by running the car on 93 octane (two full points higher than the 91 recommended by Jag) there is no detonation present under any condition and a 'dead' sensor would go unnoticed and inhibit the fail safe warning. JagV8 is the expert on control logic and may be able to comment.

 

I have just fixed my 2000 4.0 that was going into engine failsafe mode for a while now.
Whilst in engine failsafe mode it felt like it was running on 6 cylinders and was horrible to drive. I tried cleaning the throttle body. I was getting all sorts of codes, C codes too.
I would pull about 5 codes after this had happened.

Anyway, to cut a long story short, I bought a second hand accelerator pedal, fitted that and now it's as good as gold.

Thanks
Mike

 

No need for the shouting match, fellas.

Just because a change of fuel has solved or created a problem for you and your car, DOES NOT preclude that from happening to someone. What most people don't realize, or forget, is that fuels are a very, very regional thing. Fuels vary greatly depending on your location, country, climate AND SEASON. The engine system itself can make a big difference as well. That makes for loads of possible running problems or issues. Current fuel always has to be considered when diagnosing drivability issues. Sometimes a change of fuel is all it takes to get things back to normal. By the same token, one guy in some area can use lower grade fuel and get away with it just fine; somewhere else the story is way different.

Don't discount one experience just because you've never heard of it; and also don't assume what worked for you will work for someone somewhere else.

How's that for muddying the waters??

 

i cant comment on any one elses experience but my own... In my car and in the fuel fill port it is clearly labeled '' PREMIUM UNLEADED FUEL ONLY " . i was using regular for like 2 weeks and the whole time it failed safe and the check engine light was on.... one day when i pulled up to the gas pump something inside of me said use premium .. i filled up on premium and within one day it didnt fail safe but a couple times... within a couple days it completely stopped and then the check engine light went off... ive used premium every since and it has not fail safed again... this is my experience... may be what one of the people here need to try.. it wont hurt... even cars that are made to run on regular run even better with improved performance and mileage on premium fuel... lets let the posters try it.. it wouldnt hurt

 

Fancy having to ask you poor guys in northern climes to COOL IT!!!

+1 to xjrguy; it's Christmas ... who needs the agro?

Or is the US/Canadian border the latest in a long line of international borders where guns at 30 paces will solve everything? This latest is by no means the first instance - a trend is developing and I trust that Jim and the mods take steps to stamp it out once and for all.

These forums are full of great advice; often laced with wit and good humour; and replete with pics and stories of delicious cars - the best of the best. I came here to share; to be informed and yes, entertained; so I feel a great pity and sadness for those who must flex the muscles of their superiority by putting others down.

Please put it away ...

Ken

 

from what ive seen in the many post i read i do not recall ever reading one of you asking or some one posting anything about what fuel type they have been using. all ive read is other suggestions or questions about the issue at hand wich is fail safe problems.. we all know that the gasoline used now is nothing like what used to be used . its has ethanol and burns faster.. regardless i believe that i have a valid and viable suggestion for us to try

 

No shouting match here. I only suggested that people try it. i have no idea if it will work for some or not.. It worked for me and my car runs great now.. that is what's important to me. However if my experience can help one other person then that was my aim... some minimized my experience because they never heard of it before. And u r right.. i live in central florida so i can imagine that the fuel could be different here

 

Well done beema,

guns back in holsters ... of course your experience is valid and for the very reasons Steve (xjrguy) has indicated.

I for one, am sorry you were trodden on, from such a height - but I am sure our Northern friend is big enough to acknowledge we are all here to share and learn from each other - not to engage in a points scoring contest.

Again, Merry Christmas to all.
.

 

Beema-

I agree with the others about not starting a pi**ng match, especially at this time of year- but please don't drag out and rehash old tired myths that have been proven to be false time and time again.

-Cars built to run on regular will not run better on premium.

-Ethanol blended fuels do not 'burn faster' to any practical degree. Flame front speed has no bearing on detonation which is the sole purpose of using high octane fuel.

If the OP(s) are not already using 95RON/91AKI fuel then obviously it will do no harm to try this. It would be useful to hear their feedback one way or the other.

My point simply and solely is that your experience flies in the face of the wisdom accumulated here across virtually all models of Jag and engine variants with the sole exception of the X-type. For reasons that no one has yet explained, a small minority of owners of those cars have been able to link a failsafe/CEL to useage of low octane fuel. Some of this same group have also noted audible pinging (detonation) , reduced power and increased fuel consumption.

Being that yours is the first and ONLY data point that is similar to the X-type, I'm curious and a little skeptical. I have no doubt that what you relate is true- the light turned off after filling with high octane fuel, but is that a direct result or merely a coincidence? If it's a direct result, I would want to dig deeper into the technical explanation.

 

OK 'Uncle!'

I meant no disrepect to our new member, just making an obviously clumsy effort to avoid another round of octane or fuel wars.

Much like oil and filter discussions, there's tons of opinions, claims and assumptions on the subject. Filtering out the facts from the unmentionable stuff is sometimes difficult.

I do appreciate his data point as mentioned above and hope it leads to us all learning something new about these cars.

Cheers.

 

Hi Mikey,

I guess that whilst you are focused on the fuel issue, xjrguy could have been writing about Australia when he points out that regional, refinery and a heap of other issues worldwide should caution against assumptions and blanket statements. Your zeal for real life, scientific data support is commended.

I will not even start on the vagaries of diesel fuel here - and rapid emergence of "premium distillate" demanded by modern high performance diesel engines - irrelevant here because you chaps never had the luxury of seeing the upgraded re-mapped later 2007/8 Twin Turbo Intercooled Diesel 2.7L V6 S-Type and its startling performance offset by 60mpg highway fuel economy.

My point is much more that we view our posts with courtesy and care for fellow members - as Australians, we learnt long ago that our laid back humour often does not migrate well to other nationalities. We tend to mock any airs and graces - and in particular, the self appointed genius.

This aint a lecture, but let me give an example quoted from your post; " OK 'Uncle!' "

I have no idea what that means - so my mind says:

"Is he placating me? - friendly smile?
"Is he ridiculing me? - veiled sarcasm?
"Is he inferring I'm a smarta**e - rude, but I'm so old, that cuts no ice...

So now I assume the former - but what if he had called me "Auntie" - then he's probably calling me a "fairy". Now that makes me angry, so I"m comin' after him with my handbag!

See what I mean?
.

 

'Uncle' is a phrase commonly used in North America to signify that the person concedes defeat in a contest, or possibly declares 'no contest' and wishes a truce.

 

Thanks Mikey,

I am none the wiser but feel much more comfortable - truce is good - winners all round - even tho' my handbag is an old one, it's a favourite!

Back to our Jags - coz I am negotiating a new purchase. More with pics to follow, if all goes well. Oh Santa ... please, please, please.
.

 

Since my first language is not English, I do enjoy trying to 'translate' some of the comments, which can be confusing. Overall, I find it amusing and entertaining...