Symptom: When coming to a junction, slowing down and trying to turn left or right, then engine speed dropped very smoothly towards 0 and stalled. The transition between engine running and engine stopped is very very smooth with no sign of the ECU trying to correct when dropping below idle speed. This was also an intermittent issue.Over the years, there have been many things wrong with my car, some of which I have investigated and fixed.
I meant to post these findings as a reference in the hope that it may help someone (and myself, because after a long time I might not remember it anymore). It seems that some of the problems are not very common (but some are, I think).
Cause: This problem seems to be electrical in nature. This is a phase I car with no ASR hence no steering yaw sensor. The possible contributions to the issue when turning the steering wheel that I could think of would be: 1, increased load to the engine and 2, slight change in lateral force during cornering which affects the connection (connector) somewhere.
I started with checking the lambda sensor output at idle. After warming up the fueling system would enter closed loop, at which point the lambda sensor output should cycle between a maximum and minimum value. This oscillation should be quite consistent and in my case the it was not. Every now and then some the output stays high or low and the values were a bit all over the place. So with a possibly defective sensor, I ran the car without lambda sensor for a while. That did not solve the problem, so the next item was the MAF sensor. I don't have reference values on the MAF sensor output, so I cleaned the sensor and the connection but the issue persisted. I decided to bite the bullet and replace both the MAF and lambda sensor. That did the trick.
I suspect the culprit is the MAF sensor. But the way the issue manifest itself is really unexpected.
Remedy: Replace MAF sensor (and possibly lambda sensor).