Hot start issue - The Unofficial BMW M5 Messageboard (m5board.com)

Gravy · 10th February 2012, 14:11

Hello all,

I've just returned from the MOT tester with a successful result but I still have an underlying issue that is becoming very annoying.

The car struggles to maintain idle for 30-60seconds when restarted from warm. If the car is left to go completely cold, it will start without issue (apart from the obligatory noisey aux pump), but once this cycle is over and I try to start it again (without the pump having to engage) the revs hunt around and shudder the whole car as if the engine is on the brink of cutting out. If a little throttle is applied to increase revs, it makes the hunting worse and virtually cuts out.

Could anyone suggest where I may need to look first, then second, third fourth and so on until I find the culprit to get this resolved?
If it makes any difference, I have just replaced the voltage regulator in the alternator but I doubt the two are linked as the problem remains.

Many thanks

uk_bm · 10th February 2012, 14:45

Air leak? Intake and/or vac hoses?

Could explain the behaviour.

Gravy · 10th February 2012, 17:15

Yeah I thought it could be that but I'm failry sure it isn't the intake otherwise it would be lumpy and struggle at all times?

The vac hoses could be the culprit as the pump is now very noisey again although they haven't been long replaced by the previous owner, unless one has pinged off somehow.
Perhaps an intake strip down is in order so I can inspect the vac lines and replace anyway just to eliminate/prevent any problems in this area.

Any other thoughts?

uk_bm · 10th February 2012, 19:35

I think the fuelling when cold might be open loop, or at least enriched, which could explain why cold isn't so badly affected. When warm fuelling is driven from the MAF reading, so if there's a leak between the MAF and the head then you are too lean.

Or something leaks more when hot.

Or something like that... Just chucking out ideas.

Gravy · 10th February 2012, 21:31

That's an interesting thought, thanks!

I actually spoke to Noel at NCB Autokraft today and he sounds like a useful chap to look after the car so I think I'll send it down there and ask him to see what's going on!

Although, in the meantime I will check out the perishables in the engnine bay and see if anything is obviously amiss.

Thanks for helping out