Background - 8 yrs ago had random misfire which was almost guaranteed to occur on a straight, flat stretch of road at 70 kmph. Eventually code for misfire cylinder 5 shows up, new plug coil and away we go. 3 yrs ago single misfire on exactly the same stretch of road give an immediate misfire cylinder 6 code. New plug coil and away we go. Took car for a spin today (daughter now drives it and I was checking it out), was impressed with how well it ran and how smooth (winding, hilly terrain). Came to that same stretch of road, flat, straight and 70 kmph - constant misfire, no code. Acceleration or deceleration gives temporary remedy but return to steady throttle and constant misfire? Any insights? Eventually I will get a code and replace the plug coil (which I have one waiting) but why always on level, straight road at 70 kph and steady throttle? Do not wish to change all 6 remaining coils or all 8 due to cost.

 

It's the combination of factors (air/fuel/vacuum/etc).

Often it's even worse going uphill about 50mph.

Try to avoid it as it also damages the catalytic converter. They cost more than coils...

 

How many kilometres are on the vehicle?

Did the MIL flash during the misfire(s)?

 

230000 km. No codes. What is MIL?

 

With that many kilometres, the spark plugs should be changed if they haven't been already.

Flashing Malfunction Indicator Light = Catalyst damage is occurring.

 

No lights, no warnings. Plugs changed about 20000 km ago. Other times changing plug coil fixed. Why always at straight, level 70kph no throttle condition? First time it took almost a yr of random misfires to trigger a code, second time instant, this time several months have elapsed. Just did another visual for leaks of any kind and all seems OK.

 

Still see post #2

 

Most expedient solution would be change all plugs and coils. There is a company on Amazon called Scitoo selling coils for $120 for 8. Local price is $90 ea. Are they worth the gamble?

 

Bit the bullet. Just ordered new plugs and plug coils. There is at least another 200k km left on this beauty. A few minor glitches but best car ever owned except for the 1962 MK II, which was much more simple to figure out!

 

Still believe it is plug coils but while I await delivery of new plugs and coils, what would I do to check vacuum. Cannot find anything in the JTIS. Believe fuel system not suspect. Thank you.

 

It's almost always the coil (unless the plugs are overdue changing).

But no harm checking fuel trims / air leak.
Follow the many posts about checking LTFTs when parked.

 

Let me give you my experiences and use it how you can.

The first time I had lean codes were caused purely by a filthy MAF. Once that got cleaned, I did not have problems until one day after I washed the engine. My initial thought was water in the plug sockets- nope that was not it. I had replaced all plugs and coils and still had the problem and destroyed one of my cats that way. Got an aftermarket and that was fine, but still had the problem. Had an indy guy that had been a JAG tech, find a crimped piece of wire that was way in the back (called it a "trigger" wire or circuit- had to use a probe camera to see it, that he repaired and all became good. It was a wire that was in one of those plastic wire protectors and probably came to the assembly factory in that conditions pre-done (and they never noticed on the assembly) and over a period of time and perhaps my engine washing was the final straw. Regardless that solved my problem and i had the condition initially only at certain times and some of it was at 70 miles an hour or so just cruising down the road.

Now recently and this perhaps taught me something about the coil codes, I was going home from a frends's house and out of nowhere, got a limp mode, flashing CEL (MIL) pulled over and got my scanner, which I always carry with me. Got a p300 for random misfire. About 2 minutes later I checked it and had a p300 (of course) and a p304 along with p1313, p1314,p1315- which all point to a bad coil.

Got home at about 10pm- it ran fine at a babied 60, but was having trouble at idle at stop lights.

Coil 4 is very simple and I had extra ones and it took about 8 minutes to replace and all became good quickly. (Had a friend xray the coil and the little circuit board in it showed a slight burn in one small location).

Point here is that when it was truly just the coil I popped 3 other codes that told me that. The other time,I was concentrated on the plugs and coils as a problem and it was actually a wire that was having an on/off connection and probably came from the factory in that crimped contrition.

Hope that provides some food or wires for thought.

Tom in Dallas/Plano
05 S-Type 3.0 101,500

 

Here you go:

JagRepair.com - Jaguar Repair Information Resource

 

To get into it to that degree will require toys I do not have. If I don't get this figured out, will take it to the pro's. The strange thing is that it only happens under zero throttle variance. Instant cure is to either accelerate or decelerate. Resume steady throttle and the vibration returns?

 

As I wrote in the PM, the dealership has areas of diagnostics that they can actually see the count on each cylinder on the number of misses, especially if you are not throwing codes.

Does this happen if you go 30 or 40 miles an hour on any other road and not have any throttle variance?

Tom in Plano

 

Just took apart the air intake and cleaned everything (not dirty) and checked all wires for secure connection and visual problem. Nothing. Went for a drive and ran smooth as silk, hard acceleration was impressive in how it kicked down and smoothly took of like a scared rabbit! Went to my mandatory test stretch, neutral throttle at 70 kph (42 mph), shook like hell! Instant cure is to either accelerate or decelerate and it stops? No codes, no leaks that I can determine. Have had the car for 15 yrs and use mainly for high speed highway cruising. Two similar incidents in the past eventually let to misfire cylinder... code so I am going to just replace all the plugs and coils as this is driving me mad.

 

Pulled plugs and coils today. Plugs looked very good except #8 which was not as clean and grey as the others. Broke one plug coil in the process and lost a 7mm socket. Plugs from Amazon are junk and sent them back. Ordered locally. Is there any way to test the coils? The right bank had different numbers than the left. The left had 2 which I put in (#5,6), broke #6. Ordered a full set of coils from Amazon but reviews of Scitoo coils are not giving me any confidence. Would like to test what I have if possible or replace with quality stuff locally at $50 ea. (vs $125 for 8 from China)

 

Bit the bullet and ordered 8 new plug coils locally. This is an amazing car and I intend to keep it running (very quickly) for a very long time. Long story but cannot sell it ever and must keep it in showroom condition.

 

I would have gone to Rock auto and got the Denso ones for $32 a piece. My original ones had Denso name and part number on the coil.

https://www.rockauto.com/en/catalog/...tion+coil,7060

https://www.rockauto.com/en/moreinfo...363306&jsn=380

Tom in Plano

 

Coil testers exist, yes.