I have a 2000 Jaguar S-Type 4 litre V8 (50K) that has started to run rough at idle. There is nothing noticeable in the acceleration and at various highway speeds. I suspect ignition coils as the spark plugs were replaced. Can anyone suggest aftermarket coils ie Denso NGK? rather than OEM from Jaguar. I have not got any codes on the OBD unit. Additionally I do not notice the rough idle when the engine is cold but after driving a while the rough idle is noticed when you are stopped at lights or signs. Thank you

 

Hot engine. Parked. Fuel trims, especially LTFTs. At idle & again at 2500rpm or so.

 

To address the brand of coils question, from 2002.5 on, the OEM was Denso, I believe. Denso, NGK or any quality OEM should be fine. I do not recommend Lucas (the Prince of Darkness) as I had one of these fail within a year. I replaced it with Delphi unit with good results.

Do avoid bargain eBay or AliExpress parts manufactured by the Sum Ting Wong Electrical plant #27, unless you like replacing parts...

 

If no codes then no misfires? What scanner are you using?
With a 4.0L and the year of your car the first thing to ask is have the tensioners been done?
Are there any noises when starting from cold? Turn off the radio and keep the drivers door open when you start the car.
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thank you to all who have replied, I ordered Denso coils. The OBDII is a cheapie made by BAFX. The high end shop also read with their unit and had no codes. There are no noises on start up and the tensioners have never been replaced. I shall return to the shop and ask for a scan repeat with it idling rough plus we can discuss the tensioners.

 

Is it possible that maybe the previous owner might have had the tensioners done already?
Well go ahead and change the coils but all misfires will set a code so we need to figure what is actually happening inside the engine because the OBDII system is not detecting any misfires?
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It's not uncommon for failing or weak coil units to not set a DTC. Once they fail completely, the DTC is set for the cylinder(s) having the misfire. Unfortunately, this can also result in damage to the catalyst(s).

Prior to installing the new coil units, it would be advisable to remove one of the cam covers and see if the secondary tensioners have been replaced with the later metal-bodied ones. If not, the primary and secondary timing chains, guides and tensioners should be replaced given the age of the vehicle.

 

thanks again for the suggestion. I am the original owner (50K on car) and know that only plugs and oil changes plus a new AC compressor have been done on the car

 

Since the timing chains and associated tensioners have not been replaced, you are risking catastrophic damage to the engine given its age regardless of miles driven.

Below shows the secondary tensioner and chains after replacement with updated parts on an AJ28.


AJ28 Bank 1 valve train.

 

Coils are a good start but remember misfires can be caused by other factors also