Audi A4 TDI battery...

[randall977]

My brake warning light regularly comes on when starting and today the car really struggled to start. I've never had to replace an Audi / VW battery before - even on a high milage 8 year old car! My current Audi is only 4 years old with 68K. Anyhow, how much for the real deal OEM part? There's one on eBay for £90(new)...

[a8 tech]

try to charge the battery firstly before anything else.Battery failure causes, if the electrics are fine then this can be a result of poor storage practise.
Sales pitch cars not being continually dripped charged can be fine at the time with a quick boost start but this reduces the life span.

[randall977]

Sounds like good advice, thanks. The car doesn't get driven that much so that probably doesn't help.

[randall977]

Well I charged the battery, went for a 4 hour drive - this morning it took a few attempts to start the car. Normal scenario is that the brake warning light comes on and the engine cuts out (when trying to start the car in the cold).

New battery required?

[kite]

have it checked before changing it

[a8 tech]

If the battery is ok then you need to check the pressure pump in the tank, common failure from contaminated oil clogging it up from the tandem pump leaking oil back into the fuel, further problems could lie in a failing egr flap.Neither will store a fault code.I presume you have checked for fault codes and found none.Its difficult to give a cause without cranking rpm and battery condition so these suggestions are speculation.

[randall977]

Thanks A8tech - there are no fault codes. I am going to get the battery tested tomorrow. I believe the EGR valve was replaced not that long ago under warranty... My gut feeling is that it's not the battery as I've never had a battery give up on any German car less than 8 years old - but I hope it is the battery!

[randall977]

Fault shown on two rear wheel speed sensors.... Glow plugs look like a possible culprit - below 5 degrees.

[randall977]

So the battery is fine but it sounds like there could be a problem with the plugs. I also noticed driving home that the car seemed to be misfiring - a bit lurching. Is this going to be an expensive job, could I do it myself easily?

[a8 tech]

Glow plugs will show a fault, I suggest you remove the fuel filter and empty into a clear container.

If there is a large oil mixture then this will indicate the tandem pump issue and further invest required.

Remove the lift pump from the tank to visually inspect, it could be blocked with oil.

The lift pump supplies the fuel to the tandem pump at approx .50 bar.

The engine will start without the assistance of the lift pump but a delay will be noticed.
When the engine is warm the holding pressure is sufficient for first time start.

Worth a look me thinks

[randall977]

Thanks for the recommendation - the fuel tank access is below the plastic cover in the boot (Avant)? So a misfire would show up a fault code, would poor starting as a result of worn glow plugs also show a fault code?

[a8 tech]

Misfire will only show if prolonged or open circuit.

Glow plugs are measure by voltage output and returning voltage so if the a glow plug was defective then the voltage supply will go to earth and return a fault code but if the circuit is complete a differing/expected current used then the ecu will see no fault as the glow plugs have consumed the power required.

The tandem pump is likely as this is a mechanical fault so no codes because the engine starts plus the injectors return a combustion difference to the ecu so self calibration activates to balance the fuelling.

Getting a bit deep here so start with the basics

coolant temp reading from cold compared to ambient temp
engine cranking rpm
fuel quality and pressure
air and egr %
valve timing

[randall977]

Okay so a few things to look at - I will work my way through them! Do you why the poor starting coincides with the brake warning light? It actually seemed to turn the engine off when it last came on???

I have just done another fault code search and there are none apart from; Auto HVAC, 01794-035 KWP1281 Control Module Incorrect Chassis Vin Number. I'm guessing this is not to do with poor starting!

[a8 tech]

Its simply a voltage drop from prolonged crank, sometimes the low oil pressure warning will show as well.I have seen a few crank senders cause this fault but they do not show a fault because the engine eventually starts so it returns no error, however the engine rpm when cranking can give this away in fact using a oscilloscope shows the the signal