Hello,
Tonight while driving home the VDP started running rough, like it was running on 3 or 4 cylinders.
Is there a way to test for a bad coil pack other than replacing them one by one with a known good coil?
I replaced all of the packs back in 2019, and I haven't put many miles on it since then.

There was a couple of Jaguar TSBs on changing the gap and ensuring you have the correct Champion plug

JagRepair.com - Jaguar Repair Information Resource

This gap has an effect on the service life of the coil ( larger gap ) as well as getting the coil to fire as it is weakening ( smaller gap )

This is a current / heat build up life shortening relationship as they have little if no cooling

O'Reily auto parts store has a tester in some stores in the back room

See the following Youtube video on how to operate because they may not

Bring heavy leather gloves for protection as a new X300 coil is in the 35,000 volt range

Remember the on / off switch is in the back of the tester

This test is not under real world conditions of warmed up and failing but is a easy start

Call the store first as all stores do not have the tester and my have been phased out for safety reasons

The adapter is not listed in the cross reference bokk with the machine but off the top of my hear it was # 32 or # 34 abouts

make sure the wiring is good

otherwise you have to throw parts at the problem. aftermarket coils are really hit or miss

If your present coils have the translucent potting material (as opposed to black) you can run the engine in a dark garage and see if any coils are arcing. Any that put on a light show are faulty. Note that lack of a light show doesn't prove the coil is OK.

The only official test in the manual is to check primary resistance which, from memory, is .75 ohm.

Bear in mind that if a coil is failing intermittently then any test might show it to be OK if the problem is not occurring at the time of the test.

Cheers
DD

Yeah, the aftermarket coils are most likely miss. There are some good posts about coils on this forum. Do a search and you can figure out quickly if your coils are crap. The only upside is that many of the crap coils can last anywhere between 6-12 months, and at the prices they charge, its not the end of the world to drop $50/year to replace them. The downside is that it doesn't appear that the good coils can be found any longer, at least not for less than $225/each!

As far as testing them, don't bother. There's no reliable way to test them. They'll bench test fine, but run poorly in your car. You'd need a fancy oscilloscope to properly test them.

So, what Doug said about using an ohm meter to test them is incorrect information?

Keep in mind the coils heat up in use and expand causing a broken wire inside to not make a connection

A meter test with a cold coil may show you something as a cold coil but a good indication ( or bad in this case ) is a cracked outer coil plastic

The walls of the valve cover coil wells that they sit into can become pitted as current arcs from a defective coil to the valve cover

Although the valve cover is electrically isolated from the rest of the engine this arcing can cause the " clean , pure " DC power the ECU is using To become induced with "dirty or noisy " power

This can drive your ECU nuts and cause problems defying logic to understand a other problem like spark timing

AC and DC power can exist at the same time in a wire or circuit but you want clean DC power

You can test your car for this by turning on the stereo to a AM station and hear it pulse with changing engine RPM

This AC and DC power in a circuit at the same time is how the old school vacuum tube amplifiers for your guitar works

There is a interesting video on Youtube on hoe this works in a guitar amplifier if you are interested Brenden


and part 2


Oil in the spark plug wells as well as oil in the coils attachment to the spark plug can be cleaned with some carburetor cleaner

Yor alternator has a separate noise suppressor that cleans up it's power output from AC to DC after it goes through the built in diode package

This suppressor can be easily tester with a meter and again a defective suppressor can be heard on a AM station

)Your stereo does not function like a vacuum tube amplifier ( solid state amplifier that can not be harmed as a tester ) but can be used as a tool to detect dirty DC power

Not all X300 models have this suppressor and can be seen under the car by the alternator attached under the the left chassis rail in the vertical line under the air filter

The suppressor is a upside down tall cylinder

Cleaning the large woven ground strap between the starter / engine block and the car frame helps with this dirty electricity and is best gotten to from under the car with a 13 mm socket and a 8 or 10 mm on the car frame connection with some light bulb grease located by the cash register in small packets at the auto parts store ( a dielectric DuPont MOLYKOTE® 4 or formerly Dow Corning DC -4 like I used at work )

Vee is correct. There is no way to test a coil properly with a multimeter, you'd need a proper set up with oscilloscope and a good sine wave to compare. Sadly cheapo coils don't last long and can fail in spectacular manner so easiest and best way to do it is to replace them.

To give you an idea how complicated testing coil packs is, have a look at this video:


That's probably best explanation you can find on youtube too.

Problem solved.
I bought a 6 pack of QYL coils from Amazon.
Replaced the first old coil with a new one.
Stumble still there.
Changed it back and replaced the second coil.
Problem solved. Running smoothly again.
Thank you for all of your answers.

When did you buy them? I was unaware they were still available for purchase.

Last week. On Amazon. Search QYL Jaguar 4.0.

I have been running the QYL’s for over 3 years and not had a problem.
These are not often available, so if you think you need coils or will do soon, get em now.

There is the possibility the 2 rear coil wire pairs were swapped

The AJ16 cylinder numbering system is reverse of the earlier AJ6

From front to back now

The connector latching on to the coil is not the most confidence boosting

I replaced them one at a time. Also, the wire connections only reach the coil they’re supposed to attach to.

I do check, but nothing ever comes up!

Would buy a set if I could

Here's the link above. It says there are 2 sets remaining.

They don’t claim QCL but made by ‎A-Premium.

Might be as good, just have to wait and see but at that price get the 2 remaining!

That is certainly not QYL. I bought this set already. I haven’t put it to use, but it doesn’t seem like it will last very long.

Please keep the forum posted to let us know how successful these coils are.

Thanks.