So I just finished all new valve cover gaskets, new plugs (went with NGK iridiums) and new coils along with secondary tensioners.

Everything looks fine on initial start up but she starts to miss and eventually throws a code.

I've confirmed from unplugging each coil one at a time while the engine is running that cylinder 6 is the only one with no effect whether it is plugged in or not. All others significantly increase the misfire symptom.

I've rechecked the plug, I've tried 3 coils now (all different manufacturers), and I tried the plug against a metal ground while connected to coil and while bumping the key to mimic starting, there is indeed spark.

All my vacuum lines appear good, tried some carb clean and no signs of increased revving. Whatever this problem is, it's cylinder 6. (second from the front on driver's side)

What am I missing here?

 

I'll also add in case anyone didn't see my early post when I first bought the car. It had a misfire and when I pulled the codes once home the plugs second from the front of each bank had a misfire.

I pulled the coils and that's when I found oil in the plug wells. The valve cover gaskets haven't been done in a while and I know oil had seeped down into the cylinder.

But if this was a problem, I would expect more misfires across other cylinders since every one of them had oil in the wells.

I appreciate any help, I haven't found much in the S-type archives but I'm now searching other sections that have jags with 4.0s....

 

New problem, right? All was good before the recent work?

This may be of some interest. Note this is for generic codes, so double check in case the Jag-specific version is different:

https://www.obd-codes.com/p0356

The likeliest culprit is a wiring issue introduced during that work. Take a close look at the coil connector. Make sure the contacts are not pushed back. Then follow the harness and make sure no wires got pinched or damaged.

After that, it's time for some basic wiring checks, as described at the link above. Wiring diagrams here:

JagRepair.com - Jaguar Repair Information Resource

One last thought: Have you tried swapping the #6 spark plug? Kind of a long shot, but I wonder if a shorted plug could affect the coil response such that the computer thinks the primary side is at fault. I'm just trying to think what has changed (new plugs, among other bits), and this would be quick to check.

 

Sounds reasonable, but not always true. Several years ago, I had oil collect in the plug wells on one side of my '02 V6 and set a misfire code.

Even though all plugs and coils on that side were oil-soaked, only one cylinder set a code. This was probably due to that coil being a little weaker than the others, and so was the only one to cross the misfire threshold. That's my story and I'm sticking to it...

 

I appreciate the quick response! However in my follow post to my first, I indicated this car had a misfire on that cylinder when I bought it - along with the second plug from the front on the passenger side too.

Tomorrow I'm going to start the car and plug a test light into the coil harness to see if it does anything weird. There's definitely spark on the plug when you try to start...

I've tried 3 coils on this cylinder and they all do the same thing on this cylinder - pretty sure it's not the coil.

Even with this misfire it's running way better than when I first got it, but I'm obviously not done. lol

Cheers!

 

Is that with the plug removed? It's a lot easier for a plug to fire in ambient air. You need a much stronger spark to fire against the compression inside the cylinder. So be careful assuming all is good if only testing in ambient air. I do not care to discuss how I know this.

 

Sounds like it's a weak spark. That makes all its electrics/electronics suspects.

Almost always it's the coil but in this case seems not. Odd.

 

I've always just held it against a ground in the engine bay away from any combustible fluids while someone tried starting the car. Been doing that test since...I can't remember lol

 

So when I tested the plug it had a nice blueish white spark. Because it was late and I wasn't thinking, didn't even occur to me to recheck the plug gap. But I guess I was still thinking potential coil issue or connection issue.

I'll recheck the gap today, I had them all gapped according to the manual around .039. Maybe I accidentally gapped that one too close.

 

If you used iridiums beware as they are VERY unhappy about the electrode tip being touched by any metal as it needs to stay as manufactured (i.e. sharp).

 

I gapped them all with the same tool - obviously the tipped was touched by the feeler gauge (like the 7 others)

I'll check the tip for any damage in case I was a bit negligent, cause you never know. However this would of been the second plug I did during the entire reassembly - I would understand if it was on plug 7 or 8 in the process lol

 

Have you swapped the #6 plug yet with a known-good plug from another cylinder? I doubt you damaged the plug during installation. I'm just trying to rule out a plug that was bad from stock.

One other thought: You mentioned trying 3 coils at #6. Have you tried any of those coils on another cylinder? Just thinking out loud, wondering if something could be damaging those coils. Perhaps a bad plug or external wiring fault is burning out those coils. Another long shot, I know, but easy to rule out.

 

The iridiums used for the STR (NGK IFR5N10) are pre-gapped. Which ones did you use?

 

Going to check and swap the plug today, I'll use one of the old ones I removed because some of them actually worked - at work now so it'll be this evening.

 

Any chance of a mistake over which one is cyl 6? (It would explain why nothing changes.)

 

I have a regular S-type 4.0, not an R. I used NGK Iridium BKR5EIX11, the gap was approx .045 and the manual indicated .035-.039. So I gapped them accordingly.

 

6 is the second one on the driver side from the front right?

Either way, I can plug and unplug that coil while the car is running and symptoms don't change. If I unplug any other coil, the misfire gets significantly worse.

 

If you can do that then I'd say it's that one and you're back to figuring out the actual cause. (BTW it's not apparent to me which side your drivers sit, but #6 is bank 2 (as per international standards) and ought to be the third one from the front. You seem to be saying cyl #4.)

 

Forgive me since I totally forgot many of you have vehicles that are right hand drive. This is a left hand drive vehicle.

Hrmm - is this not the proper order?

edit: I had used several of the links on this board for the 4.0. Pretty sure I'm looking at the right cylinder - or I need a bucket of coffee haha

https://www.jaguarforums.com/forum/s...misfire-58838/

 

Sorry yes should be. It's before they use the standard. (Hopefully they didn't change the software yet leave the diagram as it was! You could test by swapping a coil for #7 in that diagram.)