Hello fellow jag owners.
I have a 2004 vanden Plas. At idle in park, im getting about a 50rpm drop them it bounces back up to normal, then repeat.
Not really noticable when driving, no problems with acceleration or anything, that is smooth.
Plugs only a year or so old. No codes being thrown. Doesnt seem to be a misfire, just rpms drop and rise slightly over an over. And a misfire would throw a code.....

My first thought was a dirty throttle body, which was pretty dirty and I cleaned it up today after changing the oil. But the clean throttle body did not help the issue.
I dont notice any vacuum leaks, etc.
Looking for suggestions on where to look next. Thanks.

 

I'd watch the fuel trims. (Get engine hot first then park.)

 

Hi,
the fuel trims will show an adjustment to the changed situation in the intake area. Possibly you could see which bank is affected.

If the throttle is smooth and clean, I would continue to look for air leakage, if necessary with a smoke machine.

A malfunction of the petrol vapour recovery system is also possible, but would trigger a code.

Fritz

 

Misfiring coil(s)

 

Thanks for the suggestions. Unfortunately the only diagnostic I have is a code reader, I do not know how to look at the fuel trims.
Not sure how to do a smoke test either.
It's definitely not a misfire. That would throw a code, and I have had misfires before, its not missing a cylinder.
I'll keep searching for leaks I suppose, and maybe clean TB again. I recall years ago cleaning the tb seemed to resolve the same issue.

 

How would I tell which coil? no codes being thrown.

 

Obtain a COP tester. Amazon sells them. They will find the offending coil(s) very quickly. Well worth the purchase.