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[Malcolm_D]

Once warm the car idles with an occasional lump. If you press the pedal gently the revs rise without any problem, no sign of a misfire; press the pedal quickly and the revs go up but there is a clear misfire to be heard and felt.
The car has had a bit of a tough life recently and has been sat on the drive for a couple of months awaiting attention, I have started it regularly and allowed it to run until nice and warm with a bit of reving so that it is not just on idle.
Any ideas?

[Broomies Mate]

1) Spark Plugs

Is there Oil in the wells, if so, Rocker Cover Gaskets need replacing.

Water in the wells?  Mop it up and take it for a long drive.

2) DIS-Pack

Not a nice job, but remove it and visually check it for degredation, any sign of this and it's FUBAR.  May be knackered even without physical deformation though.]

3) HT Leads

Is one offering high resistance?  If so, will give you the symptoms.

[Malcolm_D]

Replaced dis pack, rocker cover gaskets plugs and leads in the past 6 months, no evidence of water but I will have a good look at the weekend. Thanks

[SteveS]

Malcolm,
Those symptoms seem to be very similar to the misfire I have on my 2.5 V6. It started a few weeks ago and only happens when the accelerator is pressed quickly with the engine revs fairly low. It is worse when the engine is hot especially a restart from hot and when hot the idle seems a bit unsteady compared to normal. Because it only happens under certain conditions it's not as easy to fault find as a continuous misfire.

I have had rocker cover gasket leaks a while ago and the plugs did get covered in oil, but there was no misfire. I also have had DIS pack failure, but that was a regular misfire on one cylinder all the time, so relatively easy to diagnose. From the feel of it it seems to be more of a fuel/air mixture issue than an electrical one, but I could be wrong. I thought maybe air leak bypassing the MAF sensor (might be worse at low revs/air flows) or a fault with the MAF sensor itself. I had a quick look around and found no obvious leaks and the Fault code warning light isn't on so the ECU must be getting a sensible signal from the MAF sensor.
Overall fuel economy seems normal as far as I can tell, but as the misfire isn't there much of the time I guess I wouldn't expect it to have much affect.

Sorry for hijacking your thread Malcolm, but the symptoms seemed so similar that I didn't think it warranted separate one.

Regards,

Steve

[Malcolm_D]

No worries about the 'hijack'.
I found that the underneath of the scuttle was wet, its a foam type material and drips appeared when it was squeezed. I am thinking that the scuttle is not watertight and that the water may have been dripping onto the coil pack. Removed the scuttle, dried it all out, checked the coil pack as best as I was able to but the slight misfire is still there, worse when driven.
Looking to remove the pack and leads, ayone able to advise me about how to test the plug leads and coil pack? I am ok with the mechanical stuff but useless with electrics! 

[markfree]

If you want to check your plug leads are firing ok - wait until it's dark, open the bonnet, fire up the engine, look for any blue sparks, if there is then 1 or more plug leads are earthing and will have to be replaced, if not look elsewhere for fault

[SteveS]

Malcolm,
As I mentioned in the earlier reply I did have a DIS pack problem, maybe a year ago, and I also found it difficult to determine whether it was an HT lead or  a coil pack problem. I had a regular misfire on just one cylinder and found no spark at the spark plug. I thought about switching the HT lead with another to see if the problem moved with it, but I opted to go for replacing the HT leads initially and found no difference, so then changed the DIS pack, which fixed the problem. Neither changing the leads nor the DIS pack is a quick job on the V6 due to the location of the DIS pack.
Maybe someone with more experience of DIS pack failures could comment as to whether they can fail intermittently i.e. firing on all 6 for one running condition, but then missing under another. Maybe to get this effect it is still sparking but only weakly. When mine failed it went from running fine to a regular misfire in the middle of a journey under al running conditions. I also saw a comment that they fail in such a way that 2 cylinders are affected. I am sure that can be the case as I believe there are 3 coils each feeding a pair of cylinders. If that is the case mine must have failed in the connection between the coil and the HT lead connection as the spark to all the other 5 was healthy.