See my post about this which is exactly right I promise.

 

Could you possibly go into more detail on this? I am having this problem (occasional rough idle, occasional loss of power esp. around 40-50MPH). Spark plugs are fine, nothing else obvious.

 

Did you check your coils? Odds are you have one that has an intermitient misfire so slight the OBD is not flagging it - very hard to pick up but is very evident when driving between 45 - 55 MPH. There is a tool to check the kV coil output and arc indication for each coil - it is a COP (Coil On Plug) tester. I spent the $$$ and purchased one as I was having the same symptoms you described and my mechanic swore all the coils were "A OK" as OBD was not showing any mis firing. Well guess what? The coil on Cyl #4 was crapping out intermittently as soon as I placed the paddle on the coil. The main culprit of misfiring coils is oil seeping into the spark well. Keep in mind even new coils will quickly fail if there is any failure of the spark plug well gasket. Hope this points you in the right direction.

 

How much was the tool?

 

No, I haven't checked my coils. Should a decent mechanic be able to check that? If the problem is caused by oil seeping into the spark plug well, is there a gasket that will need to be replaced?

(I've been reading the other threads on the intake elbow, and IMT gaskets, etc. I hope it's not the intake elbow, because there is no way I can pull that procedure off.)

 

It's usually the elbow or coils on the V6 of your era car.

 

JagV8 - I paid $85 for the COP Tester - Matco Tools - Coil On Plug Quick Probe VS - Model: COP963 - Really good tool to have at your disposal.

 

nickthap - yes, a competient mechanic should be able to rule this out - but not if they are not very familar with Jaguar vehicles. If you should find oil in the spark wells, even one - then a new cam cover gasket, along with the spark plug seals is in order.

 

Thanks. I understand such testers can mislead so I think I'll stick to "buy one or more new coils and see which ones when swapped result in a fix". (Or mode 6 when available.)

 

This COP tester has a sensitivity wheel to eliminate the erratic readings. It picked up the faint misfiring on one coil that the OBD II didn't flag and recently I was getting an intermittent misfire on Cyl #8 that was coming and going.

Regardless, it was time to change the plugs anyway (approaching 200K miles on the clock) and switched out all my coils with new ones, except #4 which was recently installed. Thanks.