Maybe I Found My Problem (One Of Them)
 

Today, I was checking the 420SEL project car. I started it up, it starts fine now due to the diode I added to the fuel pump relay, and it was running really poorly. So I checked the fuel pumps. One is easy to check and the other is back behind it. While I was checing the hidden pump, one of the wires came right off complete with alot of corrosion. Looks like I have some rewiring to do.

I'm not getting an O2 signal either. I'm going to clamp on a ground wire and then check the signal, using my O'scope.

One inop fuel pump on a dual pump set-up can cause problems. I don't recall if it is just starting problems or running problems or both off hand. Maybe after getting good connections at the pumps you can try running without the diode you added?

Yesterday, I replaced the broken wire on the fuel pump. Both pumps run but running is no better. When I jumped the fuel pump relay sockets 7 and 8 to test my fix, I heard fuel circulating in the hoses and pipes in the engine compartment. Maybe my fuel pressure regulator is bad.

If your problem is at idle, you can momentarily pinch off the return hose. You could also run a fuel volume test, then see how the fuel looks also after the volume test. Never heard of someone making a comment on hearing fuel circulating.
Gilly

The diode I added only works during starting to power the cold start valve. It serves no other function.

Why does it sould like there is air in the lines? Fuel pressure regulator, accumulator?

There is no sound in my 300SE. Maybe I need a fuel pressure tester.

Not sure on the noise.
What are the problems (symptoms) with the car?
Is that a single wire or 4 wire O2 sensor?
Gilly

The O2 sensor is a 3 wire. I can't get a signal on it after replacing it with a generic three wire, soldering the connections and using heat shrinkable tubing. I plan to add a test ground wire on the case.

Could the air sounds in the fuel lines be caused by a bad accumulator by the fuel pumps? By a bad fuel presure regulator?

My bad running otherwise might be caused by bad cap, rotor and wires.

During the day, I clamped off the low pressure side of the accumulator. The car seemed to run better, but not great. Now I need to measure the amount of fuel delivered. I'm off to the dealer to get an accumulator and a trans vacuum modulator, which seems to be bad given the amount of white smoke coming from the tailpipe.

Oxygen sensor
 

How do you check the oxygen sensor on your 420sel?

The connections are under the false floor on the passenger side. Take out the carpet, unscrew the plastic bolt and remove the false floor. The connections are on the left hand side. Unplug the green wire and hook up the VOM/DVM/O'scope of your choice.