This is something Jeff posted some great photos and a description of a while back in
this post.
This is the photo from the above linked post;
The middle valve is about where most I have checked are popped out to. The valve on the right is how it should be (the retaining collar sitting flush).
This valve is on the timing belt side of the injection pump on top of the lower case. I believe it relieves the pressure in the lower case to the top half of the pump (where it eventually can get returned to the filter). Over time the retainer will walk itself out a little (Jeff says commonly if the car is driven hard under cold conditions).
The thing is, the pump relies on the case pressure to help keep proper start of injection timing. So if this valve starts to relieve this fuel pressure too early, the timing piston works overtime to keep correct start of injection timing. When corrected, it does make a noticeable difference!
I did a short presentation on this at Fest this year (unfortunately to a small crowd!). I then did a couple more for people after the presentation before handing my tools to my friend, Tom Salvetta, so I could go on the road rally while others wanted their case pressure relief valves checked.
I am not as intimately familiar with the pump internals as Jeff is. For a better explanation of how everything works together he is a good person to ask.
