The engine was running fine until the runaway with the old pump? Installation of the new pump indicated the number five element had definate issues. Perhaps from long term storage. It was sent out for repair.

Now you have a problem of some sort on the number five cylinder after the repair. It seems to me you should have a talk to those that repaired the pump.

There is of course coincidence but there is also a chance the repair of the number five element was not totally successful. As the fuel gets hotter in the injection pump perhaps it is going or passing by in the element bore a little too much.

One test would be to run an injector on the end of the hard line with it not mounted in the head. Is there any output from that injector with the car warmed up?

You could run the engine up to temperature. Then install an injector on the hard line and start yout test. Since there is no differance when you loosen off the number five injetor feed nut after the engine is hot there may be no output from that injector.

For example if the bore of the number five injection pump element had some corrosion it might just seize up high in the bore once the elements piston heated up. There is a chance that after getting warmed up the delivery valve is staying open I guess as another even more unlikely possibility.

If so there would be no usable fuel out of the injector I would suspect. I just think it would be too coincidental if the engine had not exhibited this behaviour before your semi runaway you experienced.

I could see this pump testing well on a test stand in a service facility as it is obviously working well before your car gets warmed up. The internals of the pump heat up as well remember with running. I personally would not suspect a problem in that number five cylinder at this time that is not injection pump involved. Your hot compression check tomorrrow will be fine on that number five cylinder tomorrow I suspect.

I assume that the valve clearances have been checked in the last fifteen thousand miles. Or you are pretty certain that the number five cylinders valve clearance is there to some extent. In my books this is a long shot even as my common sense says it should not have a rapid onset as you have experienced.