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A consistent tacho does indeed exonerate the ignition electronics, Jochen - but not ignition as such. If other components, like rotor, high tension leads, including shorts, etc fails - the tacho will still function normally. But a fuel issue is more likely…
Frank
xj6 85 Sov Europe (UK/NZ)
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If the tach is electronic, it merely counts ignition pulses. and, aye an excellent indicator that the low tension side of ignition is working. does not verify the HT lead and spark plug condition…
And a strobe timing light. Great to check the HT leads…
Of course if the tach is cable driven, n ot much if any help…
Disclaimer. A non expert…
Carl
Patrick,
I’m using the regular Super fuel (95 ROZ) with 5 % ethanol content. While I never go to the cheaper 10 % ethanol content I neither cough up the extra Euros for “Super Plus” (98 ROZ), as I never had a pinging issue.
Best
Jochen
75 XJ6L 4.2 auto (UK spec)
Thanks Frank and Carl,
just did a simple test: the rear carb sounds weird and the breather pipe is not much of an issue. The car started well with the pipe pulled off entirely. There was a marked difference in sound, but no immediate smoothing out with the pipe attached.
So, I guess it’ll cost some more time - the good thing being that the intermittant failure mode now seems to be pretty constant. Unfortunately, temperatures have dropped again to close freezing point and working on the car is no big pleasure.
Will address this issue possibly next week.
Stay safe and thanks again
Jochen
75 XJ6L 4.2 auto (UK spec)
Hi Jochen,
I got the Shell V-Power smartdeal a few years ago. For about €10 a month you can buy up to 1800 liters of Shell V-Power Racing 100 RON for the price of the normal Shell 95 5%. With a Jaguar that means saving €18,- per complete filling of the two tanks. V-power is currently €0.21 more expensive than euro95.
V-Power does not contain any alcohol and various cleansing dopes.
Yes, dear mer’cans, we pay €1,79 for a liter (0.264 gallon) of V-Power over here at the moment 
Verzonden vanaf mijn Galaxy
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The xk was designed for 98 octane, Jochen - and the original ign timing specs reflects this.
However, the engine will run on lower octanes - but the ign timing must then be adjusted accordingly. Ie, pinking is mainly a function of ignition timing - and backing off ignition is an appropriate counter…
Frank
xj6 85 Sov Europe (UK/NZ)
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In New Zealand we get 98 octane with no ethanol which is what I use. Makes your eyes water when it comes time to pay. I think current prices are around NZD2.60 per litre
On the octane/RON issue I use a millers octane booster in my more highly tuned engines - and my engine builder insisted I only use BP ultimate in the car, he was very specific. It’s a decent additive in any case.
For the Jag, I’m lucky enough to have a low compression engine which is a lot less sensitive to RON
For the intermittent power loss I have no specific suggestions. Only to say that the natural human instinct is to believe it’s a complex, expensive solution when in fact it will hopefully turn out to be something simple and low cost (I know a man who had an expensive full engine rebuild because it turned out the exhaust was knocking on a crossmember).
And so start with the lowest cost, most logical things first and work through them. New ignition system components don’t cost a lot and are dead easy to fit (watch for the specific 123 compatible coil), same with fuel filtration. They would probably benefit the car anyway and could then be eliminated as possible sources of the issue.
Frank,
you’re right, of course: the original OM calls for “Standard compression ratio (S) engines - use fuel with minimum octane rating of 98. In the UK use “4 Star” fuel.” - US listers don’t worry: Euro 98 RON corresponds with US AON 88.
That being said I’ve been running the car for 14 years now on RON 95 without any issues. Last ride included a spirited stretch of autobahn, but not in excess of 4500 RPM and no issues - return trip through black forest byways with an entirely happy engine - if any damage occurred it must have occurred later, but I’m still optimistic …
In two weeks time I’ll have access to an oscilloscope able to visualize ignition on each cylinder and hope to exclude or identify one source of trouble. The mechanic I’m going to meet is also renowned for his skills on carbs. So for the moment I’ll limit myself to checking the plugs and peeking inside the cylinders.
Best
Jochen
75 XJ6L 4.2 auto (UK spec)
My take is that if the car runs no better than on better fuel, and the ethanol doesn’t eat anything, the cheapest fuel is more than good enough and it certainly doesn’t cause such problems as Jochen had/ has.
The engine sounds happy and I can tell it is maintained well!
Thanks Bertie,
exactly my approach - I have no reason to believe anything totally untowardly should have occurred: no nasty noise and an intermittent failure mode indicating that it should be possible to resume action. If the scope identifies ignition it won’t be a big deal to provide for new whatever.
If it is the carbs I’d guess that a mild overhaul is no mistake either after 45 years …
Best
Jochen
75 XJ6L 4.2 auto (UK spec)
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It is unlikely that the fuel itself is the cause of any trouble, Jochen - though petrol quality and its octane are not the same things…
That said, at high speed/rpms the xk may sound perfectly happy even if it is not entirely so. Remember that ignition timing is measured at a specific rpms - and we rely on the dist to do the right thing at other times…
Frank
xj6 85 Sov Europe (UK/NZ)
**
I didn’t ask because of fuel quality differences, but because we had a lot of oldtimers go up in flames over here in the Netherlands. Independant research suggested that these fires were due to dissolving fuel system parts because of the 10% ethanol in our regular fuel…
That is why I only buy BP Ultimate or Shell V-Power whenever possible, as they do not contain any ethanol.
Patrick,
just a few months ago there was quite some debate on this forum about the trouble caused by E10 - then someone believed everything was messed or gunked up due to E10. Hereabouts most people argue that hoses and gaskets are harmed by E10 - in fact, just today my son filled up another car with E10 and I informed him to avoid it for the future.
But really, I can’t see a track of devastation caused by E10 either. You might get unhappy because the injectors of your K jetronic get stuck by parts of cork gaskets or some old rubber tube may give and you have to replace it. “A lot of oldtimers go up in flames”? - Naah … or the news somehow got around me.
5 % ethanol has been around for ages with no ill effect to my knowledge and so I see no reason to purchase the super-expensive fuel. But to each his own and peace of mind is a value in and as of itself!
Good luck
Jochen
75 XJ6L 4.2 auto (UK spec)
The MTBE-laced fuel required by CARB caused a lot of flaming cars, and the entire automotive industry to begin using new materials in fuel systems. But CARB soon abolished the requirement for MTBE, refusing to admit liability but claiming the stuff was showing up in the environment.
AFAIK, the 5-10-15% ethanol stuff doesn’t hurt anything unless your car was reassembled using some of the earlier Permatex sealants, which it could dissolve. It’s worth noting, though, that “flex fuel” vehicles intended to run on higher percentages of ethanol have a bunch of special features starting with a SS fuel tank.
Hi Jochen,
I’m in an oldtimer FB group in the Netherlands and in this group alone we had about 15 catastrophic fires in 2020. All of these owners had been using E10 as a main fuel source.
E10 gasoline is basically a fuel just as any other, but should be used only if it is burned in the engine quickly. Alcohol / Methanol is hygroscopic, meaning it attracts water. That can’t be good for our old Jaguar fuel system parts. Furthermore, most oldtimers are not daily drivers and the E10 fuel remains in the tanks for prolonged periods of time, allowing to wreak possible havoc much faster.
It is also less energy efficient, so the higher price for premium fuels pays itself back somewhat by lower fuel consumption. Furthermore, Shell V-Power, BP Ultimate, Total Excellium, ARAL 100 all contain cleaning dopes that can solve problems due to fouled fuel systems when used over a longer period of time.
I did the tests with all my vehicles:
The modern, computer controlled tin cans will not benefit very much from a premium fuel, as the engine management can adapt to almost anything. For older cars, I would always recommend the best fuel there is.
Patrick,
as I wrote, I’m no friend of E10, but the alcoholic content in “regular” fuels (5 %) has been there for ages and not done any harm.
More rust in tanks due to water may be an issue for us XJ drivers if used over a veeery long time - you’d be light years away from making a deal out of your 1800 litres a month flat rate though 
Gaskets and rubbers are an issue, but still - sad to hear you had a series of fires -, even with many, many fuel leaks I encountered myself and witnessed happening to other classic car drivers during joint excursions etc. there was never anything close to fire or high risk of fire - most dangerous thing I know is the carbs on my Spitfire sitting right over the exhaust manifold. Again, I wouldn’t like to play with fire, literally, but cars in flames are about the last thing I’d worry when filling up with E5.
As to the advantages with Vmax etc., it’s the other way round, I reckon. “Modern tin boxes” with adaptive engine management can make the most of these fuels. Correct, they will adapt to lesser fuels, but they can make use of higher octane rating or higher energy contents as well. To the contrary, our cars only can do that if we tune them accordingly. But would you dare to tease the last bit of juice out of your engine before high-load pinging sets in with a RON 100 fuel, and risk that some time next year when your son fills up he doesn’t know and saves a tenner for a few beers?
So, my condolences to the owners of the lost cars! Yet, I’ll stick to E5 just as much as to my favourite beer:-)
Good luck
Jochen
75 XJ6L 4.2 auto (UK spec)
E5 will work fine, i think.
I use the Jag as a daily driver from march till december, so don’t worry…i have gained back my investment for this year allready
Verzonden vanaf mijn Galaxy
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My understanding was that ethanol in gas was beneficial here. Pure gasoline will leave little bubbles of water in the bottom of the tank forever, until they rust through. Ethanol will absorb that water and take it on through the fuel system and engine and get it out of there.
… one reason less to be concerned about ethanol.
Best
Jochen
75 XJ6L 4.2 auto (UK spec)