320E M104 engine, Idle problems, Part 2

[Mike Gilson]

320E M104 engine, Idle problems
To recap my on going problems the car has been running beautifully for a number of years and then all of a sudden it started to have a rough idle and finally stalls. With increasing frequency when cruising the power will shut off for several seconds and then come back on again. Also when waiting at a road junction and accelerating away there will be no power when accelerator is depressed. This engine runs very smoothly and these problems seem like a fuel shutoff.

I get the following codes:

code 4 hot film mass air flow sensor
code 13 O2S(lambda) control operating at rich or lean limit

Plugs and coils are fine. The EGR pipe has been cleaned, Purge valve is clear. Fuel pressure regulator is not leaking. The EGR diaphram appears to be ok, as I can actuate it by applying vacuum. I can find no obvious vacuum line problems. I cant see it being a temp sensor problem. I have cleaned the MAS.

My question is where do I go from here? It would seem to be a fuel problem: Are the symptoms typical of a faulty MAS sensor, do they switch on and off? Is it a faulty O2 sensor? Is it a fuel pump or fuel relay problem and how do you tell? You help would be appreciated.
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1993 320E 140,000km