I have a 2006 S-Type 4.2 as my family vehicle and the car has been to the dealer 5 times in the last 2 years for very rough running when the car is cold and the outside temperature is below 40F. This condition has existed since the first winter and has not been corrected.

When I crank the car when it's cold(after sitting about 24 hours), the RPM's go to about 1800 for a few seconds and then drops to about 1200 RPMs.
When I shift into Reverse, (withthe outside temperature below 40F) the engine runs very rough (feels like its running on about 3 cylinders) and shakes the car for anywhere from 10 to 30 seconds. The shaking feels like coming to a stop in a manual transmisson car and forgetting to depress the clutch. (When I told the Service Advisor about this, they didn't understand, said they had never driven a manual transmission.)

I live in Atlanta, so it doesn't get very cold here, but last week when the temperature was about 20F, the rough running was the worse ever as I had left the car outside over night.

The computer has been reflashed with the latest software 2 times and the injectors cleaned 2 times.
The car only has 22,000 miles.

The car is now at the dealer again and they cannot find a problem - they tell me that the rough idle is normal for all 4.2 S-Types.
I've told them that there is no way this can be considered normal, and if it was they would not be able to sell any cars in the northern US.

I have a meeting with them on Friday morning (forecast is for the mid 30's) and the car will be sitting outside at the dealer for 2 days until then. When I arrive at 8AM, I am going to crank the car with the Service Manager and a Technican observing to see what happens.

2 Questions
1. Has anyone experienced this problem?
2. Is this a common/normal problem for 4.2 Jags?

This is my first post - sorry for the length.

 

This sounds VERY much like bad coils? They can ONLY be properly tested with a COP stress tester. Only the dealer will have this and not all dealers pay the money for it. Also check that the valve/cam cover gaskets are not leaking oil into the spark plug wells. This is a sure sign of the problem. This should NOT happen on a 2006 model as this problem has supposedly be fixed. BUT it was and is a major issue with the earlier model cars.
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