On my '94 AJ6 XJS I have a performance discrepancy between cold and hot engine. When I drive it from a cold start (which is typically in the cooler morning hours) the car feels quick and responsive, but once it reaches fully warmed temperature it does not pick up in the same lively manner. I do feel that, during a cooler than norm day, immaterial whether earlier or later in the day, the car does perform better, but it's also true that within any given ''temp'' day performance degrades once it goes fully warm, especially after like a good stop and go segment.

Is it the intake air temperature, or is it the actual coolant temp doing this?

Thank you!

 

From the symptoms, it may be running a little rich when warm. I say this because a cold engine requires a richer mixture to run properly, which explains the cold start performance, and cooler air is denser, which explains the cool weather performance. I would start with the air cleaner, then the fuel pressure regulator. Either low or high fuel pressure can result in a rich condition. Low pressure due to poor atomization and high pressure due to larger fuel volume. After that it would be oxygen sense ra or injector cleaning.

I am sure others will have model-specific advice. The above was general mechanical theory.

 

Thanks for the info. I'll be doing some relevant inspection and testing. I have the means to test fuel pressure and air/fuel ratio, but I need to make sure that I have the correct spec to compare my readings to, both cold and hot. I'm also fairly sure that these specs would be rather industry standard than Jaguar XJS specific.