Jerking on acceleration

[redcub61]

I have a 2001 s type 3.0 v6, at last year's service the garage changed the 3 hidden spark plugs located beneath the manifold, they tend not to change these at yearly service because of the work involved , since then when I have driven for approx 1 hour, the engine jerks on acceleration i have had 2 jaguar technicians look at it ,and of course it doesn't happen when they carry out a road test, it seems to be worse when the air con is on, the car has a full history , I have owned it for 6 years ,and it has only 55 thousand miles, any advice would be very much appreciated .

[J44EAG]

Misfiring, old plugs will kill individual plug on coils. Typically a V6 might develop the misfires at about 1600 or 2300 rpm. The effect is more noticeable climbing a hill fast under moderate load.

Did you notice any misfiring previously at any time?

Mike K

[redcub61]

Only started misfiring after the last service , no previous problems ...thank you for your help.....

[J44EAG]

New plugs can sometimes find a weakened plug on coil. Worth having the car diagnosed. A diagnosis machine can often find the offending duff coil pack(s).

Mike

[rosemary]

Hello redcub^!
Ditto Mike's comments, I had this on a 99, V6 Petrol, not only was it a cracked insulator on a Ford marked Spark Plug which even I could see when I decided to have a look for myself, these were clearly the originals in since the engine was built by Ford, under the manifold despite full annual services being carried out which I paid for in good faith for 5 years, the "on spark plug" coils were defective also. My solution was new plugs and "on spark plug coils"under the manifold. All mis-firing eliminated.
Dave