N55 idle weirdness

[Gad-Westy]

Driving home last night, 2011 535i, came to a stop, went to get going again and realised the engine had stopped. Had the radio on so didn't notice and no lumpiness or shaking. Restarted, set off. All normal. Car driving absolutely fine. Next junction, same thing happened. And it kept happening until I got home. It basically won't idle for more than a second or two. When you restart the car, you have to get quickly into drive or reverse and set off and then it is normal but if you leave it any longer, the revs just die off. No drama, no misfiring or lumpiness, it's as if you have just the shut the engine down.

Went out to the car this morning and started it from cold and it idled fine. Didn't have time to leave it going for ages but it did get warm enough to move from cold start idle over to warm idle. All fine. I did however get a warning on start up to the effect of 'high 12v battery discharge'. I get this every now and then if the alarm has gone off but I did not hear that last night or recently. Could it be battery related?

I scanned with bimmerlink and there are no codes relating to the engine at all.

I have a mental list of tests to do including the battery, MAF, MAP, maybe do a smoke test. But I would think that any vac issue or sensor issue would normally result in a very grumpy idle.

[danb79]

Gad-Westy said:

Driving home last night, 2011 535i, came to a stop, went to get going again and realised the engine had stopped. Had the radio on so didn't notice and no lumpiness or shaking. Restarted, set off. All normal. Car driving absolutely fine. Next junction, same thing happened. And it kept happening until I got home. It basically won't idle for more than a second or two. When you restart the car, you have to get quickly into drive or reverse and set off and then it is normal but if you leave it any longer, the revs just die off. No drama, no misfiring or lumpiness, it's as if you have just the shut the engine down.

Went out to the car this morning and started it from cold and it idled fine. Didn't have time to leave it going for ages but it did get warm enough to move from cold start idle over to warm idle. All fine. I did however get a warning on start up to the effect of 'high 12v battery discharge'. I get this every now and then if the alarm has gone off but I did not hear that last night or recently. Could it be battery related?

I scanned with bimmerlink and there are no codes relating to the engine at all.

I have a mental list of tests to do including the battery, MAF, MAP, maybe do a smoke test. But I would think that any vac issue or sensor issue would normally result in a very grumpy idle.

Could be cam/crank sensor; they don't always give codes unfortunately

[Gad-Westy]

danb79 said:

Could be cam/crank sensor; they don't always give codes unfortunately

Thanks Dan. Good point. I seem to remember having a crank sensor issue on another car that only showed itself when warm. Any way of testing these, or is it just a case of replacing them?

Looks like both sensors are cheap enough

https://www.autodoc.co.uk/hella/19913520?_gl=1*wkm...

https://www.autodoc.co.uk/hella/938027?_gl=1*av4w7...