328 'grabs' when stopping at slow speed.

I have an N-reg BMW 328 SE Auto. My local garage recently replaced a broken air hose somewhere which had suddenly started causing stalling (wouldn't idle). And I took the opportunity to have a routine service
done, as the 'Inspection' indicators indicated it was due.
But now, when stopping at slow speed (e.g. edging forward in traffic), I can no longer brake to a truly smooth halt. At the very last moment, there's a short but sharp 'grab'.
Idling is now about 750 rpm instead of the 600 I recall, but I'm assured both by my garage and from replies to a similar post over at uk.rec.cars.maintenance that this is correct. But this definitely makes my car less pleasant to drive. I took it back; mechanic came out with me, acknowledged the behaviour, can't explain it, and says he'll speak to Vines in nearby Crawley. Meanwhile, can anyone suggest a likely cause please?
--
Terry, West Sussex, UK


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In typed:

I'd try ACE Automotive near Billingshurst, West Sussex
Sorted out my E34 525i a treat.
Give Dale a call on 01403 701845
Jason Russell
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I gave you the likely cause last time you posted this. Why start a new thread? And did you have the check I suggested done?
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Here's your reply from uk.rec.cars.maintenance in full, so others know what we're on about: --------- "Is this the roll out change to first gear? If so, sounds like the idle sensor is out of adjustment - on my 528 if you come to a halt with your foor still on the throttle even slightly - by left foot braking - you'll get a thump into first.
Try pulling the throttle *up* with your foot and see if this makes a difference. It might just be cable adjustment - if indeed it has a cable." ---------
I wasn't sure I understood some of your points. For a start, what is a 'roll out change to first gear'? The behaviour I described is a short 'grab', as if brake pads suddenly lock, not a gear change. As I mentioned, it's when edging forward in a traffic jam. Typical speed probably well under 1 mph. And, as I said, I'd been told by the garage (who said they'd checked with BMW) that idling speed was correct.
Also, I can find no cable adjustment. The pedal rests on some sort of roller, but the lever is out of sight and I see nothing emerging at that (very low) location inside the engine compartment. The mechanic who came out with me muttered something vague about 'all done by computer', but frankly I'm no longer confident in him.
There's no way I can pull the throttle up. Trying to do so makes no difference to idle speed, and indeed I couldn't detect any movement of the pedal; seems to be prevented from moving backwards.
I suspect it's just my ignorance of the terminology. I'll try quoting your reply verbatim to my garage - but sooner or later it looks as if I'd better get to a BMW dealer, which I'm beginning to regret not doing at the outset. (The local garage is literally 5 mins walk, so very hard to resist, especially when they claimed 'very familiar with servicing BMWs'!)
--
Terry, West Sussex, UK


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It's the gearbox changing down as you roll to a halt.

But that's exactly what it feels like. It 'thumps' into first gear.

Now hang on - edging forward, or slowing down?

It's not exactly to do with that.

The gearbox is told in some way when the engine is at idle. And it's not by the actual speed, as this of course varies slightly - cold or AC on.

It was just a thought - but I'm not sure where the sensor that tells the gearbox the throttle is closed is - quite likely part of the TPS etc.

You might do better with an auto specialist.
But make sure you go out in the car with a mechanic and demonstrate the fault.
All I'm saying is your symptoms are *exactly* the same as I get on my ZF auto if you left foot brake and keep a whiff of throttle on. It thumps *badly* as it comes to rest and changes to first.
Thinks. Is it a Steptronic? Or can you select a second gear start? This would soon show if my theory is correct.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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OK, understood now thanks.

Well, both! The 'grab' obviously happens after 'accelerating' to about 1-5 mph from the nth stop during a couple of miles worth of M25 traffic jam, and braking seconds later to the (n+1)th stop.

TPS? Transmission Programming System?

The failure of the mechanic to mention any of this clearly means he's not truly qualified on the BMW finer points. When I took it back they 'cleaned the carbs and jets' (I think those were his words), and 'checked the idle speed, and couldn't find anything wrong.' As I mentioned, he didn't argue that something was amiss when he accompanied me shortly after for a run. His stated intention (Monday) was to speak to Vines. However, he was apparently off ill for a day, so I'll give it till Monday before chasing again. Don't really want to let the local garage off the hook, as that cost me 258.

Pretty sure I can start in 2nd. I'll check and report back.
When I tried it this morning (to check your 'pedal up' suggestion) one thing I did establish was that I could virtually eliminate that final 'grab' by quickly easing off the brake at the very last moment. So maybe I'm making heavy weather of this? Or maybe it really is nothing to do with idling, and somehow a brakes or brake pad issue? But then, what has changed? This routine 'Inspection Service' didn't touch the brakes.
One other possibility is that maybe I've had a small split in that 'air hose' for years, and that somehow reduced the idling speed. Only after its complete failure and replacement id my idling speed now 'correct', necessitating a new driving style?
--
Terry, West Sussex, UK


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But it only happens when slowing, not speeding up? Obviously you can't slow without speeding up first, but I'm interested in when the grab actually occurs.

Sorry - throttle position sensor.

I hope not as it has neither. It has fuel injection.

Yes. It's always annoying when a fault isn't fixed.

Well, the symptom I'm describing on my car requires both the brake and accelerator to be depressed at once - and the accelerator only by a tiny amount. Release either and the thump doesn't happen.

I'd have thought you'd have seen an alteration of the idle speed on the rev counter?
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FWIW, it starts OK in second.
I had another conversation with the local mechanic this morning. The gist was that the air hose split could have caused the computer to change its settings. My obvious question was "So, after you replaced the air hose, why didn't the computer change the settings again to match the new fuel/air proportions?" He said something about the computer possibly being faulty, but I didn't really follow. Make any sense?
--
Terry, West Sussex, UK


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And does this stop the snatching as you slow to a rest? Just that if it does, this means it *is* the box changing to first causing the problem.

Computers get blamed for everything?
In practice, the vast majority of faults put down to the engine ECU turn out to be something else like a sensor etc. In your case I'd be looking at the idle control mechanism to make sure it's clean and functioning correctly.
I remember reading a report from a company which supplies re-con ECUs that over half those returned on exchange had no fault found.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Took it to Vines this morning. Paraphrasing verdict: - not much if anything wrong - idling speed is OK, but probably was lower before, due to gradual effect of split air hose (until it failed completely), so you're seeing a difference that's hard for others to see - the control unit (ECU?) may well adapt over next 1,000 miles or so to new conditions
One factor that muddies the water a bit is that idle speed does appear to change. Does it get slower when hot? And when it is running at say 600 instead of the 'correct' 750, the 'grab' is much reduced.
I tried with '2' selected, and I'd convinced myself that did not grab - but mechanic claimed to detect no difference. He pointed out that at the speed we were doing (1 mph in garage forecourt), it would remain in 1st gear anyway, whether I selected D or 2.
My conclusions: --------------- - At the idle speed of 700/800 I was seeing throughout a 280 mile journey over 2 days, half of it in traffic jams, prompting my posts, there was a distinct grab on the final few inches of braking; it had not been present before.
- At 600, which is what I was seeing this morning, the grab is much less. I'm hoping that future idle speed (hot) remains at this level.
- Perhaps the ECU is *already* beginning to 'adapt' to revised fuel/air proportions?
Thanks for your patient help.
--
Terry, West Sussex, UK



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I don't know your actual box, but on my Steptronic if you select 2nd gear it will start in second. As did my previous E34 with a 'conventional' auto.
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Not sure there's any definitive way to check (a string of LEDs would be nice!) but I suppose an rpm check at say 10 mph would probably establish whether I was in 1st or 2nd (or whatever speed would ensure it didn't change up before that).
--
Terry, West Sussex, UK


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It should be obvious as it will be slightly sluggish off the mark compared to in first.
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*You sound reasonable......time to up my medication

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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When edging forward in traffic...?
--
Terry, West Sussex, UK


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It is to me, but might not be to everyone.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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