Took my 96 XJS 4.0 out for a drive yesterday and the throttle seemed to be sticking. I could take my foot off the accelerator pedal at 40MPH and the engine would maintain the rpm and speed. I then shifted into neutral and the rpm increased confirming the throttle was stuck. However, when I applied the brakes and slowly came to a stop, the engine rpms came back to normal idle. The throttle seemed to stay at the highest rpm after letting off the accelerator pedal.
Today, I removed the throttle body and cleaned it with throttle body cleaner, checked for vacuum leaks, and inspected the idle control valve (it was clean but upon inspection, it appeared to have been replaced at some point because the exposed part of the gasket looked new) Also checked the throttle cable and cam mechanism, all good. After buttoning it all back together, I then disconnected the battery as I read this might reset the ECU. Started it up and it was idling at 1300. I then took it for a short drive and experienced the same issues as the day before.
Where do I start from a diagnosing standpoint?
Thanks - Kevin