Just to add to this..coils can definitely have intermittent problems. As they heat up the internal resistance can increase and the windings breakdown causing them to misfire.

 

Perhaps a bit off topic, BUT - I am fascinated with the idea of dual fuel conversion to LPG in an XK!

I made several similiar conversions to some very large Detroit iron back in the 80's - Lincolns and Mercurys, when the price of fuel here went over $1.00/gal. But they had gigantic trunk spaces for the tanks!

Where in the world did you locate the alternate fuel tanks? These beautiful babies seem to have every cubic inch of on-board space utilized... DaleD

PS the Ford engines ran really well on LPG! D

 

well petrol here works out at $9.50 (£6) per gallon (UK imperial) and LPG is roughly half the price.....so dont complain about the price of petrol in the US!
the conversion is neat - fits in the spare wheel well see here

and i carry the skinny spare wheel or a can of tyre weld!....dont use the boot much anyway.

 

its back - the misfire that is!
cyl 8 again!
swapped cyl 8 plug and coil with another cyl but still showing as cyl 8 on the code reader - restricted performance etc
booked in to the jag specialist next week, but in the meantime.....

im going to do a compression test.
i take it i just pull all the coil leads off, connected the compression tester to cyl 8 and crank it over for a couple of secs?
anyone know what the compression reading should be?
then i'll do the test on each of the the other cyls to see what they are showing.

i still feel that it is harness/electrical related. going to get the specialist to try and check the harness (not sure how they will do that though)

running out of options......

 

are there any signs of oil/water in the plug (or plug well) present as soon as you remove it? perhaps fluid leaking from the valve cover gasket?

 

All the swapping and moving around and you still have a problem with #8. Did you pull and replace the plug? If you did I would focus on the wiring to the #8 coil and trace it back. I recently came across a connector problem and we were only checking the wires, check the wires and the wire plugs associated to that cylinder. This might help http://www.gusglikas.com/images/Auto.../jagxk1997.pdf I am short on time but will try to track it down for you later today. You did say you did a compression check? Good Luck!

 

Could an intermittent #8 fuel injector cause this problem?

 

Compression test are best done with all plugs out but you can get a good approximation on only one cylinder. You need to hold the throttle pedal all the way down hard. That will cut off the fuel supply and also open the throttle plate for full air availability. Look for 155 psi +-. If lower than 145 I would suggest that you might do a compression test on all cylinders.

 

Newboy - I promise this is my last off-topic question - but how much LPG can this donut hold? What range does that give you? Thanks, DaleD

 

little bit of oil in the bottom of the well, but not sure if this is because i oil the threads before i put the plug back (and then fumble around trying to get the thread in the hole!)

dont think it could be an intermittent fuel injector problem because it runs bad on both lpg and petrol (and the lpg system has its own, separate injectors)

swapped cyl 8 plug and coil for cyl 4 plug and coil. still missfiring on cyl 8. i agree it points to electrical, but cant fugure out where. had a look at the wires, but they look ok, not burnt/damaged near the coil, but then they go off into world of black tape and isulation, not sure whether to start pulling all this off.

it wasnt so bad on the way to work today, missifre was barely noticeable (25 miles) it was terrible yesterday - chugging like an old taxi, restricted performance etc. This, I hope (and pray) points toit not being a compression/gasket problem, as sure that would be consistently bad all the time?......anyone agree? Will do a compression test over the wekend i hope (time/weather dependent).

BTW - everyone thank you for all your help so far - it is greatly helping and i hope be keeing it all one one thread it will be a useful source of reference to others with similar problems in the future - assuming i get it resolved!!

(DaleD see my other post about LPG - saves off topic posting!)

 

I screwed up on my previous comment to your ongoing problem. What you're describing sounds almost exactly like what happened to me 2 summers ago. An intermittent misfire. You were talking about the coils and the problem was so similar, I was thinking you were talking injectors and not coils -- until I saw Gus's post. Anyway, my intermittent problem was with the injector. I changed the injector -- not the coil and that has solved the problem. The injector tested normal, but I was getting frustrated like you. Only way I knew is 1 time when it first happened it threw an injector code and misfire code. But Jag wouldn't change the injector at first because it never threw and injector code again -- only the misfire.

 

going to run a compression test tomorrow so this comment may be largely academic but came across this post from this thread
https://www.jaguarforums.com/forum/s...ht=compression

and as there is just a little oil in the spark plug well (boot side) could that be enough to cause the missfire (i know some of you have suggested that earlier). just curious as to how it causes the missfire. would be great if it was just a case of changing the cam cover gasket although i am sure its electrical.

PS - ive got no blue smoke - so am hoping its nothing worse.....

 

After reading this link it leads me to think the oil referenced is from the top end of the plug and not the cylinder end. If oil gets on the upper end of the plug and around or near the coil pack it could cause fouling / misfire of the plug. In this case you are talking about a valve cover set of gaskets. If the problem is in the cylinder you would in most cases see evidence of oil burning or oil in the coolant system or the other way around. You would also see evidence of oil on the electrode of the plug. Please tell us what you find.

 

time for a little update.

took out all the coils and plugs at once and tried to do a compression test, but even with the MAF still connected the car didnt turn over with any force - weird, so i got zero reading on the guage. i've done this right havnt i?

the guage may be faulty so i have ordered a preplacement but looking on you tube, other people cars turned over nice and quick - mine was really sluggish (fully charged battery)....dont know why?

looked at all the plugs - not oil fouled - just gently burnt as they should be. both banks had traces of oil in the plug wells, but nothing much....

really perplexed - when i put it back together (v frustrated admittedly as i couldnt get the compression readings....my fuse was running low by now as the compression guage had an adapter on the end to fit into the Jag spark plug hole - i unscrewed the guage....and the adapter stayed wedged in one of the plug holes - right business trying to get it out.....not impressed!) it seemed to run ok with just the faintest of missfire,

done a couple of journeys and the missfire comes and goes - surely this must be electrical?

oh well, i am at the end of my mechanical knowedge so it goes in to the garage next week - heres hoping they find the cause....

 

I am so sorry that we could not assist in locating the problem but I think you are close to it. It seems every time you work on #8 plug it seems to run a little different for a short period. This time you removed all plugs and it did the same thing. I think you are closer to the problem than you think, the question is where!