W123 Fuel Door
 

I have question on the W123 Fuel door actuator. If after locking the car, the fuel door locks, then it unlocks in a while. Does that indicate the fuel door actuator is possibly defective or not? I read something once, that there were 2 different kinds on this don't recall the details.

I would suggest measuring the vacuum at the fuel door lock. You may have a leak in the line or the diaphragm in the actuator itself might be leaking.

If you don't have the time to pull it out I'd wonder what the delay between your door locks and the fuel door latch is. If it's significant then the problem is isolated to the fuel door actuator branch, otherwise you have a larger vacuum issue.

The Fuel Door Actuator line T's into the Trunk Lock line. Is your Trunk locking and unlocking normally?

It is!

Not all W123 year models feature the same actuator. Early versions were spring-loaded to the unlock position. Later versions were dual-acting to prevent the problem that you describe.

When I wanted to make quick determination of where my door lock actuator was leaking I put a section of vacuum hose on one of the nipples and stuck the actuator under water and put the hose in my mouth and blew into it.
There was an initial expulsion of air and in my case it continued to bubble air out of the nipple indicating the internal diaphragm was leaking. I took it apart and sure enough it was had a small rip in it.

The other place it could have leaked was on the Boot that seals the metal rod.

Personally I got fed up with the vacuum locking system and disconnected it and am happier with the reliability of doing it manually.

With no vacuum my Fuel Cap Door stays unlocked and that had not caused any issues so far.

If your fuel flap vacuum actuator has two vacuum lines to it, dismount it and remove the releasing spring on the pin .

Then it'll remain locked even after the vacuum leaks down .

If the leak is in a door pod, the flap will unlock due to the coil spring .