While looking for new spark plugs for my '02 3 litre I came across an interesting discovery...
2002 requires Autolite APP605 - Heat range 5
2003+ requires Autolite APP103 - Heat range 3

From what I've read there was no real change to the engine over it's life in the X

I also believe that a cooler plug (heat range 3) is more efficient at dispersing heat

If the above is correct, would there be any argument against going to the APP103

Any & all insight would be greatly appreciated
Gary

 

anybody have any thoughts on using the later spec plugs

Gary

 

Anybody willing to share some knowledge...

 

bump for info

 

Hi, Kota62!

I have been watching and waiting for some replies and discussion on your question since you posted, as my 2.5 liter is ready for a set of plugs and I wish to make the best choice, too.

There is a rather good treatise on spark plugs (sometimes called "Sparking Plugs" in British English, wrap your tyre around that one!) at Wikipedia Spark plug - Wikipedia, the free encyclopedia with a good discussion on the why and wherefores of the Heat Range Question, but a response from someone with real-world experience on how different heat ranges affect the Jaguar engine performance would be nice.

In the past, I always ran the cooler plugs in my vehicles, but the nuances of computer contolled engines might preclude this. Cooler plugs used to mean less likelyhood of pre-ignition, and that meant you could wrap the distributer up a degree or two more advanced, and the end result was more power and better mileage. I always knew I'd tuned an engine to it's maximum potential if when starting it, it would have timing so advanced it would stand on it's ear for a second before roaring to life.

But times, fuels, engines, theories and computers have changed all that, and it would be nice to hear the modern thinking on the Sparking Plug!

Also, with the Right-Hand Drive Jags, are the plugs Left-Hand threaded???

 

It is my understanding that with the introduction of the OBD-II computer systems (ie, any car made after 1996 has this system), they have knock sensors that not only retard the timing to prevent knocking, the computer also attempts to advance the timing till it starts to hear the knocking and then backs off the timing just a little bit. So, with this in mind, the computer is going to keep the engine right on the edge of knocking to get the most power/efficiency out of the engine.

As for the left hand threaded plugs, do the righty tighty, lefty loosey bit and tell me what you find. If you start seeing spots, keep going, the plug is about to loosen up.

 

So was there a real world reason why Jag dropped the heat range from 2002 to 2003

My preference is to run a 180° thermostat and cooler range plugs thereby helping the car run just a bit cooler

 

Kota, you can run a cooler thermostat in the car, but you are going to find that your mileage is going to hit the floor. I will guess that you will about 5 mpg if you drop in a 180 degree thermostat. I'm sure you are wondering why. This is why. The computer monitors the engine temp and will run the motor rich when the motor is not up at temp. Since the computer is looking for 195F, when the thermostat is maintaining the motor at 180F, the computer is going to think the car is not warmed up yet. I'm sure you noticed how your mileage fell off during the winter some. This is due to the very rich setup Jag has for when our cars are not at temp. Well, you are going to have this condition all the time. So, your car is going to run rich all the time, leading to poor mileage.

As for why they went from the APP605's to the APP103's, I am venturing to guess that the 103's are a little cheaper plug ($0.10 at my local Advanced Auto) and when you are talking about making as many cars as they did and saving a little here, a little there, it means more profit for the auto maker. That simple change didn't affect the output of the car, yet saved the manufacturer $0.60 a car. Add in other cost saving measures like that, they start to add up over time.