This be on my 2006 XK8; has almost 116k miles on it, plugs were changed in Feb of this year as this was the first time had the codes for multiple misfires along with P1316. After changing plugs, clearing codes, drove for several months with no codes set.
Then about three months ago, Check Engine lite comes on...once again multiple misfires, P1316 codes. I think the cylinders are different this time, but not 100% sure. Check hoses, wires, etc...find nothing, clear codes and all is good.
Two days ago, here we go again.
Since this is wife's car, I seldom drive it, so do not know how the engine is running prior to CEL coming on.

Asking for suggestions as to what to look for. I did do a compression check earlier this year on seven of the eight cylinders, could not get to one cylinder with the compression gauge I had; of these only one cylinder, #2 was a bit lower than the others, but not by much.

Thanks.

 

DTC P1316
FAULT DESCRIPTION Misfire excess emission NOTE: This DTC will flag only when accompanied by an individual cylinder misfire DTC: P0300 – P0308
POSSIBLE CAUSES ECM to ignition coil primary circuit fault (Cylinder misfire detected DTC also flagged)
Fuel injector circuit fault(s) (Injector DTCs also flagged)
Ignition coil failure
Spark plug failure / fouled / incorrect gap
Cylinder compression low
Fuel delivery pressure (low / high)
Fuel injector(s) restricted / leaking
Fuel injector(s) continuously open
Fuel contamination
Worn camshaft / broken valve spring(s)

If there's a vibration, for example, from tire way out of balance, out of round or very low pressure, damaged rim, cyclic dragging rotor, etc.... that oscillation can set off a false misfire code. Even possible driving on wet, snowy, muddy or dirt roads, especially with potholes. My wife has a lead foot. Does your wife do burnouts, donuts, drag racing, drifting??? Start with checking tire pressures if nothing else.

 

That's funny😂

 

Possibly oil in the spark plug wells?

 

Did a 80 mile roundtrip last night, with me driving, combination of highway and surface streets with no problems at all.
Wife takes XK8 out this morning, CEL on before she leaves the garage.
Quick solution here...do not let wife drive the XK8!....

Anyways, read codes this morning, same as before, 2 & 6. I remove all the coils from the right side of the engine, all look exactly the same, no oil whatsoever, swap coils 2&4, 6&8 clear codes.
I expect it will be a week or so to get enough engine runtime to see if the misfires follow the swap. If it does, then will get a complete set of coils on order unless someone has a better idea.

 

My wife started driving our '99 XK8 and it would frequently go into limp mode on her. She now reuses to drive it and declares it hates her.

P1316 takes 2 TRIPS – "indicates that the CHECK ENGINE MIL is activated by a fault occurring during TWO CONSECUTIVE “TRIPS”." In your test drives, read OBDII to look for pending faults that are on reserve before the CEL displays.

You mentioned the CEL lit at startup. Could it be that it is starting a bit reluctantly and registers misfires in that period of time. Does it have a rough idle cold? You might check out the post "TPS and PPS adjustments via ODB" for smoother idle. You might discover some missing horsepower at the same time.

Did you check the plug gaps? Originally, XK8s were specified 1.0 -1.1mm gap, 0.039"-0.043", then that was upped to 0.051" for 2001 on up. I never trust plugs being "pregapped."

 

I will check the pending codes when out driving.
Knew that P1316 took a few trips to set.
Could be that the misfires are all caught at cold start-up. Start-up is fine, no hesitation, but I do hear a slight miss, now that you asked.
I reviewed my service record, did not record the spark plug gap, but the change-out was in 2020 right after passing 100K, so not recent.
I looked at the TPS and PPS Adjustment via OBD posting, but since that was on the 4.0L engine with a completely different throttle body, I do not know if applicable.

Also in reviewing my service record, I read that in the past the misfires were all on the left side, now they are on the right side, although I did make a mistake in cylinder numbering sometime in the past as I used the incorrect diagram for the 4.2L engine, so my statement may also be incorrect. Pretty sure I did swap coils around on the left side in the past when the misfire codes appeared some months ago, but that is from memory, so ????

 

I'm pretty sure the only difference between the 4.0 and 4.2 adjustments is just the method of adjusting the cable, square plastic nut vs spring clip. All the principles should apply.to adjusting pedal opening and throttle plate zeroing, although I haven't gotten to our 2006 XKR yet. Won't get around to that until next spring. I'd hope the later throttle motor would allow pressing the butterfly closed manually for OBD zero reading.

 

No real cable adjustments to be made as the throttle motor moves the butterfly as commanded by the PCM.
will check to see what is reported for throttle position next time.

 

The cable pulls the pots in the Pedal Position Sensor. Make sure no floor mats or junk are under the gas pedal, fire up OBD scanner and note Throttle position with foot off and then depress gas pedal to the floor and verify it reads all the way up to 100%. If it falls short, you can tweak outer cable adjustment until it just reaches Wide Open Throttle. Pop the plastic cover off on driver's/rear location of engine compartment and see if there's some cable slack and adjust it out in small increments to make 100% on scanner with pedal buried. Then go back to zero throttle.to make sure reading shows it wasn't pulled it too far.

 

It has been a long time since I was last into the area where the Pedal pots are at; do not remember at all what the cable attachment even look like, I will check it out. I did add throttle position to be displayed, so I am ready to go.
Will report what I find before and after adjustments.
Thanks

 

More data:
XK8 sat for 48 hrs, started engine for first time after swapping coils around. Normal start. Drove it about 20 miles, then off for 2.5 hrs.
Prior to next start-up, had OBDII reader on. Throttle position reported as 2.7%, started engine, position went to up, then after short period at fast idle, position reported at 1.4%.
Drove about 15 miles, stopped, position now reporting at 0.4% at idle.
Checked for codes; had P0306, P1316 both pending.
Cleared codes, drove only about 2.5 miles, re-checked...no pending codes. Shut engine off.

Since I swapped 6&8 coils and still reporting misfire on 6, seems it is not the coil. Cylinder 6 is the one I did not get a compression check done on....looks like I need to find a way.

 

You might doublecheck the plug gap when you get the #6 plug out. The coil signals the ECM of a misfire by monitoring the primary current achieving a minimum current, even if there's a spark. A gap too far off spec might not reach that threshold consistently enough to make the ECM happy.

It seems that if the misfire is very consistent, the injector will be shut off at that plug and it should run noticeably rougher. Your case might not be that bad, but on the other hand, the CEL should extinguish if the engine has run problem free for a long spell.

You did mention the new plugs have been in two years and ran 15,000 miles. If you saved the old plugs, you might measure the gaps for information and slip them back in as they didn't throw the codes. I'd try that before springing for new coils.

 

Have to report a "duh" event.....I had the cylinder numbering backwards (again). My excuse is that I am used to working on Chevrolet engines where #1 is on the left side; the XK8 is opposite, so I swapped the odd coils around last week.
So starting over: I will swap #6 and probably #8 today, see if the misfire goes to #8.

Oh and I did not save the old plugs; tossed them after a couple of trips with the new plugs installed as all was good.

I did the 6 to 8 swap. Then out for errands with multiple stops/starts, all surface streets. Returned home with no CEL, checked for pending codes, nothing.
I did leave the coil cover and engine cover off as I thought I would be back in there real soon.
Will continue to drive around, see what happens.

 

Continued to drive around, 45 mile trip then short drives with multiple stops/starts, then the return trip home. No CEL, but have not checked for pending yet. Still have the left side cover and large engine cover off; so problem could be heat related. Going to put the covers back on...see if misfires return.

Later

 

hey Lanny - something else to check should it return is the wiring harness. on my 2000, I found several leads where the insulation was cracked/peeling so that in some spots, the exposed wires were contacting each other causing intermittent misfires. fixed by wrapping with electrical tape for now but will need to redo the harness at some point

 

Seems to me that there must be a, or multiple breaks in the wire insulation as the cause for these intermittent misfires. I have looked at each one when I did the coil swap, but did not use a magnifier...which I will do, as a crack does not need to be very large for those electrons to escape....