engine running rich and dying, help please!

[w123jab]

hello again,

i really need some help with my 280 te 1985. I had the car tuned because it was running very rich and was continually cutting out, i guess from choked up plugs.

after the tune up it ran very well for a week or so, with a tickover at about 6-700 revs when warm, and a very clean exhaust. then i noticed that slowly over a period of days this figure was falling, and over the next week once the engine got down to about 300 revs it died and became a beggar to start.

the soot appearing out of the exhaust suggests that it had returned to running rich again.

i took it back to the tuner who said that it needed more air, he fiddled. i suspect all he did was alter the idle screw, infront of the engine by the radiator.

once again it ran very well for a few days, although now at a higher tickover, about 1,100. over the last week it has once again started to drop away and is not starting again.

it seems to be alright when the engine is cold but as soon as it is warm the problem appears after about half and hour.

the choke seems to be working as the engine reduces tickover after a couple of minutes of warming up

does anyone have a clue what could be making this happen.

could it be something to do with the oxygen sensor, i doubt this has been changed on the car at all? would a non-functioning sensor create this problem.

this is the last problem (so far) that needs to be sorted and if i can fix it i will have saved this nice old girl from the wreckers, and proved all my doubting thomas friends wrong.

very grateful as always for any suggestions

justin