Background: Its a 2004 XJ8 check engine light came on got code p0300.

2 Questions

1. For ignition coils do i replace all 8 or just the bad one(s)?

2. can you tell/link me to the ones you recommend?

 

Trinity, because you are getting a random misfire code (the P0300 code), I would tell you to go out and do a long, hard acceleration. This should cause a second (possibly 3rd) code to appear which should be something along the lines of P030X (where X is going to be a number between 1 and 8). This will tell you the cylinder(s) that are the problem child. NOrmally, you only need to change 1 coil at a time. Now, with this being said, I would also be looking at a few other coils and inspecting the rubber boots between the coils and the spark plugs. If those are starting to crumble when rubbed or are showing signs of damage, then I would replace the boots on all 8.

As for brands to get, I will let other XJ8 owners chime in on that. In short, you are going to get what you pay for. You buy cheap, you are going to be replacing the coils again shortly most likely. Granted, not saying to pay dealership prices either. There is a happy medium in the center.

 

Welcome to the forums Trinity Degraffenreid,

I've moved your question from General Tech Help to X350 forum. This is the place to post technical questions about your model.

Graham

 

So I often try to determine quality if anyone is actually able by the manufacturer and photos. In 2021 I purchased 8 coils from A-Premium on Amazon because I had a #8 coil failure that actually melted.. Of that batch of new, the first one failed in 13 months, all 8 failed within 18 months. The coils in the engine were also aftermarket when I bought the car in 2020, seems like they were same quality

When I installed the entire set, great performance for about 10 -11 months, then I thought I needed to replace the Iridium plugs due to some occasional and not logged misfires which I had checked when I replaced the coils, after all I was approaching 100K miles, but what I removed were almost perfect condition. After chasing bogies for a while even with new plugs, the first coil failed at 13 months with a pinhole right at the top of the boot and obvious arc burn.
I again pulled the plugs to recheck the gaps, all perfect. then over the next 5 months each one failed at a different location on the engine. I had used the old coils as replacement.

So beware of A-Premium if you are planning on keeping your old X350 for a while. Noting the newer Ford MoToCraft designs, the entire barrel is silicone rubber top to bottom, but I wonder how well they hold up to moving heat out of the coil. Anyone have the $80-$150 a set of 8 coils last longer than 18 months of actual driving use?

 

Some of you might ask why that coil arced thru at that spot? Answer?
Cheap spring metal used in the connection to the sparkplug.
That spring on every one of the failed coils corroded creating a high resistance connection at the spring to plug top, spark needs to go somewhere, cylinder head is close there and zaps right through at the cheap non-high dialectric plastic thin spot.

These were A-Premium coils for $89 per set of 8, Made in CHINA. Caveat Emptor ya'll

Seems like a very poor cost saving manufacturing measure, those springs maybe cost $0.10 per thousand for non corrosive metal. All parts are built to a specification, specifications can change in the same manufacturing facility based on different clients.
When the quality client production ends, the manufacturing goes cheap. NOS parts in OEM packaging cost 10-20 times more each,

.

Temporary fix is, use a very high quality conductive lubricant, extremely thin coating on the spring. Too much and the conductive lube will travel down the boot next to the spark plug and short it out that way.

 

Thanks for that tip. I was always hesitant do the hard accel with misfires, but you get the code locked every time exceeding 3k RPM and do not need to wait for the computer to finally get around to setting the DTCs on its own time.

 

Believe the coils can be a luck of the draw.
First set set off several check engine lights.
Informed them & got a prompt refund.
Tried two more sets, been over 5 years now.
First set has about 20,000 miles, the other set 17,000.
Lifetime warrantry with the old stockers in the trunk.
With a Jaguar reader in the glovebox in case of a misfire.

 

So I found a set of coils that appear to be constructed similar to the OEM versions, under a "MAS" brand, operative word is of course "Appear".




The MAS coil set includes MAS branded Iridium Spark Plugs, so I pulled all of my used coils which slowly replaced my last set of cheap junk coils when they failed, and pulled my new NGK Iridium plugs so I could clean the cheap spring corrosion off the end, gapped the MAS Iridium for ~1mm ( 0.39 inch), took a good look at MAS plug and coil quality, and completed the swap.

Twist of the key and as expected, smooth running ...codes cleared at least for now. Fuel mileage now recovering closer to 25 MPG around town, back to full smooth throttle response.

Time will tell if these MAS brand will last or are simply garbage, but at least they are not trying to fool anyone they are anything other than some off brand.
At least the California based seller states they own the MAS brand and stand behind the quality for the one year warranty period, and are very responsive to questions within a couple of hours..

That is only slightly better than unbranded, unidentifiable, or "A-Premium" BS quality that began failing at 10 months due to corrosion on the spring.

 

Well the first set of coils, had four take a dump
So i bought two sets of Duralast coils from Auto Zone.
Lifetime warranty, & the car runs great again.
Plan to do the other set tomorrow, after they get here.
Other set still runs fine, but better coils seems like a good idea

 

buy used ones from anything like 07 up they will get you farther than whitebox stuff