bit of a long post, but will set the scene:

1996 XK8 120k miles with dual fuel LPG conversion

1 month ago it developed a missfire (rough idle and poor acceleration under load) bad on petrol but much worse on lpg. (understandable as lpg needs a more perfect spark than petrol to work well) it threw up the restricted performance message occasionally too.

hooked up my basic fault code reader and it showed missfire on cyl 8.

disconnected the relevant coil and no difference to engine running so deduced it was the coil.

put a replacement (albeit second hand) coil, reset and cleared the codes and it ran like a dream....for the first 10 - 20 miles....now the missfire is back.....but not all the time!

I can be driving and stop, start and accellerate and it runs like a dream (petrol or lpg) and then it will missfire and run like a dog for a while.....and then back again!

done about 250 miles now since coil change like this. It shows no fault codes nor restriced performance.


any suggestions? my routes/deductions so far as follows:

• cant be injectors (it missfires on petrol and lpg and the lpg doesnt use the petrol injectors)
• throttle body all clean and tight
• no obvious leaks in the vacuum hoses - but not sure how best to check
• no restricted performance warning on dash - and no codes on code reader!
• could it be the maf - should i clean it? (expensive to repace if it doesnt turn out to be faulty)
• could it be electrical - where should i check
• would a more specailist/advanced code reader show the faults?
• should i change all the plugs (they were only done 10,000 miles ago) and they look ok to me?

'tis a bit of a mistery.......

 

It may be time to do a compression check my friend...

 

Correct plugs? Gap?

Easy to clean the MAFS, probably won't hurt but I would be surprised if it didn't throw a code running on gas. Us colonials may understand the concept but aren't familiar with LPG.

Compression test couldn't hurt as mentioned. My '00 with 106k miles had new car compression but you have have had a lot more time to be exposed to the high sulfur gas.

 

ok, i'll give the compression test a go - hadn't thought of that.

i just "feel" it's electrical as woundn't the engine run rough all the time if the compression was an issue - sometimes it runs fine, then lumpy....i thought it would be lumpy all the time if compression was a problem?

couple of things:

seen a few compression tester kits for sale (on ebay) but knowing how deep the xk8 plugs are and how thin wall casting the spark plug removal tool has to be to get them out, will a standard cheapo kit do the job, or should i be looking at one with special adapter?

secondly, can anyone point me to the charts which show what compression the cylinders should be at?

 

pull the plug too, make sure its not oily, recheck to make sure the misfire hasn't 'moved' to another cylinder. Sometimes more than one goes at once.

Also, double check to make sure you got cylinder 8 correct ID, that can be confusing to some.

 

The spark plugs are just normal size from a spark plug and a compression tester standpoint.

One comment I would make that applies to any deep well spark plug location; remove the plug with a special plug socket with a foam gripper insert, replace it with a standard deep well socket. Tightening with the spark plug socket, especially a new one, may leave the socket in the hole due to the insert being tighter on the plug than the ratchet extension connection.

I have no knowledge of the affects of a LPG conversion but here are the specs for spark plugs and gap compliments of our missing friend Harley D.

 

When you throw the Restricted Performance are you getting a code? In the interim I would suggest checking the plug connection at the TB See my link http://www.gusglikas.com/AutoRepTB.htm This problem has thrown Restricted Performance and Fail Safe but it is a simple check. The next few check are simple, check for a vacuum leak I had a leak that did not throw a code and the car was sluggish and I did not know by how much until I made the repairs. Another link http://www.gusglikas.com/AutoRepairA...HoseRepair.htm The best repair for this is to replace the hose. What I had found that works in locating leaks is Throttle Body spray cleaner and just spray it around and when the engine begins to shake you found a spot. I would also check your MAFS, make it is clean and working properly along with that check the plug connections. This is assuming you compression test is good.

 

My guess is that either your new/old coil is bad or the harness is damaged on #8. It's not compression or it would not have run well at any time. When you changed the coil you disturbed the harness and it started working again but it may now be settling back to its old ways. Have you tried pulling codes again? When there is a misfire, after a certain number of occurences, it will set another code.

 

thanks people - lots of useful input.

i'll try the following and report back (drives me mad when people ask forum questions and never update with the results so will try and do so!)

buy a new spark plug and do a straight swap
i'll check the TB plugs again (had a quick look previously)
try another replacement coil (i have 7 other second hand ones that i bought at the same time....they cant all be duds!)
will clean out the plug well - as its no8 and the cover isnt bolted down tightly - it could be water/oil in there
will try looking for a leak in the vacuum hoses with a quick start spray
am hoping its not compression as the problem did initially go away when i changed the coil (dont think that would have happened if it was compression problem)
its still not throwing up any fault codes despite 10+ trips totalling 300+ miles since i cleared the codes and changed the coil.
harness could well be damaged - i guess an auto electrician or jag specialist is the only way to diagnose this one

 

ok - update
still not throwing up any fault codes. booked into a jag specialist tomorrow for a fault reading to see if they can find anything.

i have cleaned the TB connectors, and the MAF. also checked for vacuum leaks.
replaced the second hand coil on cyl8 with another secondhand coil - still no improvement.
it is pointing towards electrical/coil i think as car is bearable for the first 5 - 10 mins driving (i can still detect a slight missfire) but gets much worse as you drive it and after 20 mins is chugging like a taxi and wont pull under load - does this point to a coil failing?
tried pulling the coil connectors one at a time on one bank (the easier side to get to) and all cause the engine to run worse which should indicate the coils are ok.

i'm really hoping the jag specialist can find something when he puts it on his code reader, as i'm running out of options!

 

What is installed to enable the duel fuel and is the TB functioning in both applications? I am thinking your TB or attachments for duel fuel are having a problem and that is my best guess.

 

Also, check the part load breather, could be clogged...there is a 'cleaning' procedure in the FAQ thread at the very top of this xk8/r subforum. Very common on the 4.0 engines.

 

Good call Matt!

 

well - update time......

the jag specialist hooked to up to their code reader and it came up missfire cyl 8. nothing else showed.

so went home and tried a third secondhand coil and it did run better.
so i'm guessing my replacement coils were duds.
but its still not perfect....i can still feel a slight missfire - and its worse on lpg.
well i guess at least i have eliminated the probable problem for now - i may just try one more coil for luck (i've got 8 to get through!).

i hope this post/thread helps someone in the future, but i have to say i am not totally happy that i have found the solution so i'll keep an eye on things!

 

Good deal!

Have you checked the wiring to the coils and for a good connection at each one?

 

I'm a little late with my $.02. I'm with oldmots thinking coil or harness. I had a bad coil and it was intermittent also. Usually under load, but sometimes it would just pop up. Read misfire cyl #8. Dealer refused to change coil at first (under select edition warranty) said it tested fine and sent me home. Problem continued and I made them change coil and it has been good for over a year. Now that a new coil did some good with yours, it still seems electrical and I'd really look at the harness.

 

Newboy,

At this point I would try substitutions to see if you can get the problem to move to another cylinder. First step might be to swap the coil AND plug from cylinder #8 with, say, #7. If the problem does move to #7, life gets a whole lot easier. If the problem does not move, at least you've eliminated those components.

I agree with those who think it's electrical, so if the problem stays on #8, then probing the harness might be the best next step.

Good luck,

 

...and make sure #8 IS #8 cylinder IDs can be confusing on the earlier cars.

 

def cyl 8 - double checked with the numbering

moved bad coil to front of bank (radiator end) and put a known good coil in cyl 8 for easier access of the bad coil.....cyl 8 is hard to get to.

cyl 8 now running fine (disconnect coil lead when engine running and it virtually stalls)
disconnected "bad" coil from front cyl and engine hardly stumbled, so repalced with secondhand coil and all seems ok.

when i say there is still a slight missifire....its prob me being ultra picky - i'm sure most people wouldnt even notice it, car now drives fine and pulls well, but i notice every little thing on my cars, hence i may try another coil...just to see

(ps - cleaned the part load breather at the same time as the TB - it was fine)

Finally does anybody know any glue/bond/resin that would hold the brass fixings back into the block. The bolts from the plastic coil cover screw into these and all the fixings have now come out of the block! i need some super strong, heat resistant glue to get them to stay back in there. i'm sure not having the plastic coil cover firmly screwed down didnt help as moisture can get in to coils/plus area?

 

JB Weld.

Make sure that all the surfaces are clean of oil with something like brake cleaner spray.