Over-run valves were fitted to the XJ12 and XJ-S with D-Jetronic fuel injection to limit vacuum that can be produced when the throttle is closed and engine is at high RPM (such as going down a hill). Without these valves, the engine can backfire in this overrun condition.
A common problem is that they can leak and cause high and even erratic idle. I recently checked their operation by removing the hose to each valve and sticking my hand over them at idle. There was too much suction at idle when they are supposed to be closed, so I removed them, cleaned them up and tightened the spring on each before re-installing. This didn’t improve the leak, so I realized that the leaking was occurring around the mating surface where the valve sits in the housing (yellow arrow). I added some gasket maker to this surface and re-installed. Now there is only a very light suction at idle.
