Occasional Misfiring but No OBD Codes

2000 model 3.0 V6 with 226,500 miles.
I have recently noticed occasional misfiring, usually at acceleration or sustained speed, but it smooths out at idle. Acting like it is running on 5 or 4 cylinders, but inconsistently, like it is only losing those cylinders once every 10 revolutions or so.
It is not generating any OBD codes.
I thought maybe a bad tank of gas, but I always buy Shell or BP 93 octane.
Ran it down to 1/8 tank, filled it up, ran another mile home, still occasional misfire.
I’m thinking either plugs, coils, injectors, or variable cam timing valves.
But any of those should generate a code, right?
Any ideas how to diagnose what it could be?


During the time I have owned my 2000 V8 I have had to replace two coils following similar symptoms to yours. The misfire was mainly on acceleration and on hard acceleration my car would eventually go into limp mode until I lifted off. This is like driving on pave. On neither occasion were any fault codes generated. I read somewhere that with early vehicles a fault code was only generated following a misfire if the misfire was repeated a certain number of times in succession. The nature of a failing but not completely dead coil is, I suppose, one of random misfiring.

Eventually, it got bad enough that I could detect the misfire at idle but that was not the case when the faults initially showed themselves. I was only able to locate the faulty coil each time by successive substitution with a new one although I was able to identify which bank contained the faulty coil by inspecting the O2 sensor outputs in the live data. That reduced it to a one in four guess!

Shropshire, UK

Thanks, Eric. I am not familiar with the use of the word “pave” as a noun, since in the US we use it as a verb meaning to make a road smooth. If you mean the road is bumpy like it was paved with bricks or big round stones, then that is exactly what I feel on these occasions. I will resign myself to changing all the coils and plugs. The three on the left are easy and the three on the right are under the intake manifold so I will clean the right cam timing solenoid valve while I have everything off. I did the left a couple of years ago.


It should have had an acute accent on the ‘e’ but I was lazy. Anyway, you guessed right – it’s a surface made of bricks that is often used by the vehicle testers to check for rattles and squeaks when developing new cars.

I know the V6 has one bank that’s difficult to access. In that respect the V8 is more straightforward in that you can get at both banks with equal ease (or difficulty).


The lefts are out. The coils have Ford part numbers so are probably the originals. The plugs are NGK Iridium TR55IX and the hooked tips are about half eaten away.

Anyone got any advice or tips or warnings before I start removing the intake manifold?

The intake manifold is off. The bolts were very tight, and I broke one. So now I have a broken bolt to deal with. Right hand front. Why do they use such tiny little bolts and screwed in so far? There is a kind of sleeve over them so it is difficult to get penetrating oil on the threads before you start. Using an 8mm 6 point socket, on one head the hex points stripped so I had to use a 5/16" socket on it. One of the spark plugs was not tight at all, the others were very tight. Also broke off the plastic hex head of the radiator drain plug.

The new spark plugs are in. Gap specified by Jaguar Vehicle Specs book 09/2003 is .051-.056 inches, biggest plug gap I ever measured. Must be ok for iridium plugs. The NGKs came already gapped.

Question for today is sealant on the coils. There was silicone sealant on the right hand coil rubber boots but not on the lefts. Was that sealant factory work or someone later?

The JTIS does not mention sealant but says “Apply a light film of Dielectric compound on the ignition coils meeting Jaguar specification.” What have any of you used?


I used a smear of proprietary silicone grease on the coils. The plastic covers on the V8 have a built in seal and I think you’re meant to replace the covers each time. I didn’t replace them but used auto grade black silicone sealant on these and have not had a problem so far.


The US use of “proprietary” means “its a secret formula and we won’t tell you”, but I think you mean any off-the-shelf dielectric grease rather than the Jaguar approved same stuff they sell at 500 percent markup. I used Permatex 09980.

The plastic cam covers on my V6 have pressed-in sealing gaskets which I replaced with a Fel-Pro set from Rock Auto.

There are three, the outer oval, the inner oval around the spark plugs, and the round one on the Variable Valve Timing Solenoid. The outer and inner press into a groove. There are a couple of tabs to guide you so you don’t put them in upside down. The outer has six wide places where it seals over a joint between two aluminum pieces, and I put a dab of Permatex No. 2 on them because there was a dab there before on the old gasket. The Fel-Pro set also comes with seals for the cam cover bolts.

My old VVT seal was distorted like somebody put it on wrong and it was leaking. Examining the intake manifold gaskets, I see they are dated 4/05, so somebody had it off not long after that, probably because of the oil leak, so that’s when they put silicone sealer on the coil boots. Would have been smarter to fix the problem with the VVT seal.

My new coils from Rock Auto also came with new rubber boots which fit snugly so I did not put RTV sealer on them, just the dielectric grease on the plug ends.

I had a broken flange bolt (6mm x 1.0 x 50mm flange head) where the intake manifold attaches to the fuel injector manifold, which luckily I managed to extract with several days of penetrating oil and carefully working it with vice grips. I went to the Jag dealer, two Ford dealers and two auto parts stores trying to find another bolt. The Jag dealer wanted $3.50 and it was special order in 5 days anyway, the Ford price for the same bolt was $1.27 but they had none, nor did Advance and AutoZone, so I found one at Ace Hardware Store for 79 cents.

All done, and she started right up and runs smoother than ever.
I guess this means 226,500 miles is about all the life we can squeeze out of a set of spark plugs. :grinning:
While I had it off I cleaned out the exhaust gas recirculation port as best I could with a bent wire. It gets gunked up and can cause an emissions fault code. What a stupid design to put it there, when it would have been so much easier for everybody if it was on the top or the right side. :confounded:
Two notes on reassembly of the intake manifold for anyone in the future attempting it.
There are eight screws. Note down the locations for the long and short screws.
Also note down the routing of all the vacuum and crankcase breather hoses and how they overlap each other, there is only one way that works.


Seeing that the V6 has replaceable seals I rechecked the parts catalogue but for the V8 there is no separate seal on the coil cover. Only the cover is listed and there are different part numbers for 4.0l, 4.2l & supercharged. So it seems you are meant to replace the whole cover if the seal is damaged. I imagine most people will do what I did and run a bead of silicone around the cover.

The mileage that these cars will do is impressive. Most of them in the UK get scrapped for rust because we put so much salt on our roads in the winter. I still see plenty be driven but they are thinning out now and the ubiquitous XF has taken over here.


Enjoyed reading this thread. Have not been here for a while but my 2000 V8 has done the same. First coil failure took a year before the OBD code identified it, the second failure the code was instant. Now have intermittent misfire but waiting for the code to ID the culprit coil.

Was surprised at the amount of work to access the coils in the 6. In the 8, it is fairly simple but you need really small hands. When the time comes for the coil change, will do the valve cover gasket too as the VVT seals leak.