Recently acquired a 1987 XJS 5.3L, don't know the history but the car looked good. But when I start it in the morning, its idle always shoots up to 1800-2000 rpm right away and will stay there for about 5 minutes till the engine warms up, then gradually drops down to around 1100-1200rpm. When the coolant temperature gets to around 95 degrees, it will back to the normal idle around 750rpm. The car runs OK but feels not enough power when accelerating.


Any idea? Am wondering if anything wrong related to cold starting? or vacuum advance module?

 

This is an AAV (Auxiliary Air Valve) that is opening too far, and only closing when the car is properly hot. This is the cold start device on the V12, Do a search on it on this forum and you will find lots of advice on how to fix and adjust it.
Greg

 

Here ya go

AAV rebuild procedure.doc