Update - I have decided to opt for a fuel distributor rebuild and fuel injector cleaning svc/test via Ebay. Today I removed fuel distributor and injectors on 90 420SEl with 87k miles for reference. Car is original Seattle bought life long resident so you can imagine based on our climate that it's friendly to rubber, interiors and not prone to rust. In fact car is completely rust free and many of the rubber items like axle boots are in incredibly good shape.
Anyhow upon removing fuel injection items here is what I found interesting - job was pretty easy (one fuel connection bracket had to be slightly and temporarily moved via pliers to remove 1 fuel injector holder allen bolt) with total time to remove fuel distributor, injectors and plastic nozzle holders about 70 minutes. No injector felt secure in it's plastic nozzle holder after removing clamps. I understand they should have still been secure in their fitment. This tells me I probably had one or more injector that wasn't completely sealed and thus letting in unregulated air. Some of the plastic nozzles were also loose in intake manifold and could be spun freely which would cause potential additional air leak issues. After removing I certainly wouldn't trust a 30 yr old car with original injector nozzles and fuel injector seals to be sealing properly so I ordered all knew ones this evening as they are very inexpensive.
Boxed up fuel distributor and injectors to send for service and will update in a few weeks on outcome.
(For those following my other thread on differential I'm still trying to figure out best option, used replacement or rebuild Will update on that thread upon conclusion. Diff comes out this weekend, so much for low key and relaxing on Fathers Day).
I've only done the injectors and cups once but I thought they were kind of loose after replacement, too. I think the new orings afford that movement. I did find some vac leaks before the project by spraying carb cleaner or something around the injectors.
Looking forward to your report on the diff. I just had the car in the air this week and noticed what could be lube coming out of the left side at the axle.