Probable failing Crank Position Sensor

[Stephen Edwin]

My recent query about how to change the crank position sensor was not just idle curiosity. The 240 SE B200F has for a while been sometimes not quick to start. As if it had damp. Once under way it has behaved well. Other times it has started on the button.

This afternoon to come home it took about half a dozen tries. Every time it fired but would not hold the revolutions. And then it did work so I drove the car and it behaved basically perfick. None of which fits with damp.

(No signs of damp in the atmosphete this lovely sunny day. I confess I didn't do a dispersant spray on the HT.)

I remind myself that:

Originally Posted by Clifford Pope

In my experience they [Note by Stephen now: The context was the ignition amplifier] either work or fail totally.
I've had two fail, on different cars. One was nearing home when the engine suddenly stopped dead, and nothing would make it restart. I swapped in a spare amplifier and it started first shot and was trouble-free afterwards.

The other cut out while the car was idling in the drive. Again, a replacement fixed it.

I've once had a AMM fail. The car suddenly started running badly, spluttered, the lambda light came on, and it cut out. It started again, and ran reasonably well, not quite up to par. Diagnosis showed it was the AMM.
I think when they fail the system switches to a limp-home mode, which is a default setting which is almost OK - useable, but poor.

Poor or erratic starting I've found is usually the CPS. I had trouble with one over a period. Mostly the car started instantly, at other times it refused to start, but always did eventually. Once started it ran fine. I think it might be the wires or the insulation fraying, so might depend on engine movement, rather than a fault actually in the sensor body.

The fuel pump relay can go at any time, either spasmodically or suddenly. So too can fuse contacts. As in my experience this is easily the most common cause of poor starting or misfiring, I'd always;
1) - twiddle fuses 4 and 6
2) - jump terminals 4 and 6 with your handy short lead with a clip on each end. This bypasses the relay and runs the pumps continuously.
3) swap the amplifier

Those are quick tests, done in seconds, with a good chance of getting you moving.

The air mass meter was new less than a year, less than 1,000 miles ago.

The fuel relay fuses are good, blade fuses newer than the air mass meter. And once started the engine ran good so I'm not doubting the relay, yet.

Not a total engine stop as would usually be for the amplifier.

So I reckon, reasonably likely to be the crank position sensor?

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