Hi everyone,
I have to change spark plugs due to misfiring. Mileage on my car is 39k kms. My mechanic said its better to change both plugs n coils at the same time. Is it true?
I checked on eBay and few other online shops. I can get 6 spark plugs from berksjags for £25. They say its OEM product. Is there a better product out there that I should consider?
Britishparts.co.uk sell ignition coils £30 each. Berksjags sell Lucas coils for £36 each. Which product is better in quality? Any other place that you guys can recommend?
Appreciate your expert advice.

 

Don't change coils unless they test bad. Sounds like he is a parts changer, not a Technician.

 

This depends on whether your mechanic has positively identified a misfire on one or more cylinders attributed to a failed or failing coil. Changing Coils is not a regular maintenance item.

Here is the Jaguar Maintenance Schedule for your vehicle:



You will see Spark Plugs are listed for change at 70K/100K miles depending on engine type. Coils are not mentioned anywhere as they are only changed if they fail.

Graham

 

Hi thanks guys. Yes I saw the misfire fault codes when he plugged in. Then he said its better to change coils at the same time. His explanation was since the plugs are new coils mite not be able to deliver the required output for new plugs. I didn't understand anyway. So i thought asking it in the forum. How do I find out wether coils have failed or in a bad condition. So I should change plugs only right?

 

You saw misfire fault codes so that suggests there is a coil issue.

A DTC code indicates a specific cylinder and the usual way to proceed is to clear the codes, switch coils and see if the code changes to the cylinder where the suspect coil has been moved. This shows whether it was:

1. a transient 'out of range' fault which set the code
2. pinpoints a specific faulty coil or coils
3. indicates a harness fault

I'm not one to throw parts at an issue. Changing all the coils could well get rid of the fault but you may end up replacing some perfectly good coils.

Graham

 

New plugs are easier to spark than old ones that are worn and dirty. So I don't buy that unless he has a better reason. 37k is very early for a plug change on these cars. Was it miss-firing for a long time to foul the plugs badly?

 

So basically misfire fault codes caused by coils? Not the plug?

 

NO - it's not as simple as being one or the other. The possible causes of one or more of the misfire DTC's P0300 to P0308 are:

1. Cylinder compression low
2. Worn camshaft / broken valve spring(s)
3. Fuel delivery pressure (low / high)
4. Fuel injector(s) blocked / leaking
5. Fuel injector(s) continuously open
6. Fuel contamination
7. Fuel injector circuit fault(s) (Injector DTCs also flagged)
8. Spark plug failure / fouled / incorrect gap
9. ECM to ignition module primary circuit fault (Cylinder misfire detected DTC also flagged)
10. Ignition module ground circuit: open circuit, high resistance
11. Ignition module / coil failure

Graham