Jpearce

Hi all,

The other day I left the ignition on all day by mistake and ran the battery flat.

I had to jump start the car to move it off the driveway, and it ran just fine. I gave it a few minutes to get some charge back but didn't have time to go for a proper drive.

Later I tried to start it again - the battery had enough life to turn the engine over but it wouldn't start.

I assumed the voltage was too low, so I gave it an overnight charge, but it still refuses to start!

I've checked stuff like the fuel pump fuse - all OK.

Any ideas ??
Could it be anything to do with the immobilizer??

Any advice appreciated.

PaulW

What did you charge it with overnight?

Do you have a multimeter, and if so, check the battery voltage at the terminals and report back. If 11.7 or higher, it should start.

Paul

ttuling

Does the immobilizer light stay on for only a few seconds after you turn on the key, whether or not you try to start the engine? (That's what a 2001 car does anyway.)

Do the check engine and EPC (2000 onward) lights come on like they normally do before starting?

You might try disconnecting the battery negative cable for 15 to 30 minutes in case the instrument cluster (immobilizer) or engine computer didn't initialize correctly. After you reconnect the battery, turn the key on but don't try to start the engine immediately -- the engine computer needs some number of seconds with the key on to initialize itself (I think the owner's manual says how many).

If it's a 2000 or newer model and the battery really is well charged, you might try turning the key on for a minute or two without touching the accelerator or trying to start the engine. This should cause the engine computer to recalibrate the electric throttle actuator (with a quiet whirring and thumping under the hood, or at least I think that's what that noise is). It seems unlikely, but if this setting were way off after the power loss, maybe it doesn't open the throttle enough to start?

Instrument cluster and engine computer fuses -- there are probably several -- all matter, but it seems unlikely that any would blow between the first start and subsequent battery charging.

In any case, I'll be interested to hear what was wrong.

ttuling

so the computer can relearn more of the calibrations which were lost when power was gone.

Here's another low-probability guess: what if losing one of those calibrations caused it to run especially rich when you ran it briefly to move it off the driveway. Could it have fouled the plugs enough to make restarting difficult?

KMU

try the basics(fuel,timing,spark). I'm only telling you what I would do. I'm don't advise anyone to work on their own car. As Paul said, first, is it turning over well with no start, or not turning over well enough to start? Once good starter/battery performance is established, I check fuel. This is tricky and possibly dangerous for me without a fuel guage. Usually the pump relay is run for 2 seconds when the key is turned on to establish proper fuel rail pressure. I could verify the fuse and relay operation. I could spray throttle body cleaner in the intake hose (not on the Mass air flow sensor-some cars have heated wires) while turning the engine over to see if it tries to start. You would not try this because it could be dangerous.(poss.backfire if fouled or out of time) I might establish good spark by pulling a coil pack , putting a plug in it, grounding it against the engine block, and spinning the engine a couple of revolutions. I would never touch this HIGH Voltage output coil in this process for obvious reasons some of which have to do with heart health. In other words, "Don't do this". Disclaimer! If any codes were set in this process,I might have to go to a dealer to clear them.

Jpearce

Thanks for the advice - I'll try to answer your comments.

The car is a UK spec 1998 2.8 V6.

The battery voltage was low-11s when flat and high-12s after charging (mid-13s while the charger was still connected)

I've measured the voltage at the fuel pump fuse, and then also at the fuel pump connector - measured a good 12v when cranking the engine. Also could hear the pump whirring when reconnected. Haven't got round to checking fuel pressure yet - I'll disconnect a pipe at the weekend and see what squirts out.

My first gues is to suspect the fuel system rather than ignition. When cranking the engine for a long time it feels like its trying to fire - as if some fuel is building up - then it dies away again.

I've already tried disconnecting the battery for a while to clear the memory and then reconnecting.
Also allowed time for throttle calibration.
No joy. Also tried resyncronising the key fob/immobiliser and using the spare key. Still nothing.

I'll follow up with checks on the fuel pressure and ignition this weekend when I've got a bit more time and some daylight.

Thanks again - and any other advice gratefully received!

Cheers
Jp

PaulW

Then I would go after the spark.

If you don't smell fuel (which you should after cranking), then I would go after the fuel system.

pw

ttuling

due to carbon on the valves preventing them from closing. It may not be the problem, but I think I remember a mechanic saying it sometimes turns up after shutting off a cold engine which had run only a short time.

It prescribes various diagnostics, then possibly a new coolant temperature sensor and chemical cleaning and ten minutes of 4000-6000 rpm driving once the engine's fully warm. The starting procedure is "depress throttle fully and crank engine until engine starts."

You may find this bulletin, which was formerly published on the web by Audi of America, Inc. (and remains copyrighted by them), if you use google to search for Audi "carbon deposit buildup".

Good luck.

PaulW

Somebody sent me this link, thanks! I've put it up on audipages on the technical and maintenance page, it's the first hyperlink.<ul><li><a href="http://www.audipages.com/Tech_Articles.html">http://www.audipages.com/Tech_Articles.html</a</li></ul>

Jpearce

Ttuling - you're diagnosis was absolutely right, thanks.

The spark plugs were totally carboned up and sparking around the base.

Stuck in a new set (Denso, not NGK) and although it didn't start immediately it did finally go.

Gave it a good thrashing at about 5 to 6000revs in 4th gear for 10miles and it seems to be totally cured.

Its due a service in about a month so I'll take the opportunity to fit the 92degC thermostat and possibly chemical cleaning before the oil change too.

I'll still surprised at how quickly it went from being totally reliable to totally carboned up. Perhaps the coil packs are already breaking down and running near their limit.

Thanks again TTuling and everyone else.