Compression test today - 420SEL 1896
 

Sadly, the results were very bad... I might have to sell my project car as a parts car. This has also been my daily driver for several years but recently started idling roughly.

PSI right bank: 90, 55, 82, 90
PSI left bank: 65, 55, 75, 70

I was wondering why the O2 sensor light was coming on.

:-(

Was that with a hot or cold engine?

Cold
 

Cold but then I did the test again at operating temp.
Marginal improvement - the 90's went to 105. The 55's went to 67.
Still a long way from optimum 123...

engines wont even start under 90lbs, i think your compression tester is not reading correctly,does it use a hose screwed in or rubber seal pressed into hole? double check readings throttle open, warm engine , fully charged battery?

123psi is optimum? For what?

My near 300000 mile M103 still was in the 160-170psi range.

Try a new tester. Are you sure you are operating it correctly... plugs out, throttle fully open, etc

Agreed, thats way too low, you'd have way worse than a rough idle.....new tester time!

How many miles on the motor? My dad's 420 has almost 300k and runs great....the bottom ends of these engines are extremely durable.

OK, you've convinced me...
 

I'll get a new tester immediately.

The rough idle paired with the O2 sensor light coming on (nearly every trip), suggested to me that perhaps it was a burnt exhaust valve. I think I eliminated electrical by changing the O2 sensor, all the spark plug wiring, the distributor rotor and the ignition coil - all new - still running roughly.

Initially I thought vacuum. So I have been all over the vacuum system too. I have changed nearly every rubber connector - no change in the rough idle.

Worn injectors can cause rough idle, as well as low fuel pressure(failing pumps), clogged fuel filter, bad pressure regulator, bad spark plugs (did you change these also?) Distributor cap can cause problems too.

Does your economy gauge work properly or seem to behave normally? I had a rough idle one day and it was because the line to the econo gauge fell off the engine.

Yes I changed the plugs, and the fuel filter. I will check the economy gauge but it appears to be working normally.

I have thought about injectors - maybe that's another avenue to pursue, along with a new compression tester!

Yes I'm sure; I did it how they taught me at school.

AllData stated in the specifications section (for my engine) that the normal compression is 8.5 bar (@~14.5 psi/bar = ~123 psi). Perhaps I miscalculated the conversion from 8.5 bar to psi...