A8 D2 stalling while braking for junctions

Inexperienced Joined: Jun 26, 2004 Posts: 42

Hi,

my A8 D2 has started to stall while braking for junctions. I noticed that while braking the battery warning light would start to flicker and then come on solid when the engine had stalled - along with very heavy steering into roundabouts!

Because of this I have been putting the car into neutral just before braking.

I have now noticed that when rolling at low speed (<10mph) in neutral the engine hunts around tickover almost to the point of stalling with the battery warning light still flickering. At about 1 second after the car has come to a complete stop the engine stops hunting and ticks over rock solid at just over 600rpm.

Anyone know what could have an effect on engine idle speed when the car is rolling in neutral?

I just can't understand why the idle is rock solid when stationary.

Cheers
David

Posted: Fri Mar 22, 2013 9:43 pm

Is your check engine light on?
Have you had the codes read? ..OBD2 code scanner or VagCom?

A wild guess is a large vacuum leak, maybe even major leak in the intake plumbing, which settles into place when the car is still and at idle & maybe opens up under braking and steering.

Only a thorough inspection of all vacuum and intake plumbing and/or a code-read may tell.

Inexperienced Joined: Jun 26, 2004 Posts: 42

Check engine light is not lit.

VCDS reports no faults found with engine or trans

I had a quick look at vacuum hoses but they run everywhere. I can't see anything disconnected at all.

I did wonder about vacuum leak but couldn't think why the idle would settle down when the car stopped moving? It's rock solid when the car is stationary regardless of whether the foot brake is applied or not.

Posted: Sat Mar 23, 2013 11:38 pm

Wow...truly odd. Most anything that is "off" enough to actually stall the engine should trip some code...even if the code is just indicating an indirect symptom, not actually the guilty part.
Some common causes of stalling:
MAF going bad.
Engine speed sensor (crank position) or wiring failing.
Idle speed control valve sticking.
Fuel pump starting to fail.

All the above should trip some kind of code if a complete stall or misfiring occurred.
A failing fuel pump generates no code of it's own, but should trip some collateral code, as do other failures...like "lean mixture" codes.
Not sure if a sticking idle valve would trip it's own code, either(?)

Inexperienced Joined: Jun 26, 2004 Posts: 42

Thanks to everyone that responded.

There is no separate idle speed control valve on the ACK engine. The ECU adjusts the throttle position with a motor in the throttle body to keep it at the correct idle speed.

I had started to suspect the crank or camshaft sensors but couldn't figure out why they would fail when the engine was warm but be fine at cold start. I know they could be affected by temperature but my gut told me it wasn't these. I suspect OBD would detect problems with these?

I google'd idle and stalling issues for the 2.8 30V and just about everyone reported dirty throttle body as the cause.

It's not the easiest part to get off the A8! I spent the whole day taking off, cleaning and putting it back on.

I cleaned to TB mechanicals with carb cleaner. I even removed the butterfly to give better access. I opened up the throttle control valve electronics and gave it a good blast with contact cleaner and made sure to run the potentiometer wipers through their travel while the tracks were still wet.

While I had it apart I also took the MAF sensor out of its housing and gave it a good squirt with carb cleaner too.

After reassembling I used OBD tool to do the throttle body adaptation and then took it out for a drive.

Fingers crossed - its working fine now. Took it for a long drive to get up to normal temperature and then tried very harsh braking and normal braking under all sorts of driving conditions and there was no sign of the alternator light flickering and it never stalled once.

So, £10 worth of cleaning chemicals and a day's effort and it appears to be running fine again