A6 Quattro 2.5TDI Starting/Fuel fault

[The Fiend]

Hey Folks

Please help with a problem that I've got. I've had my 2001 A6 for about 4 months and it's had a fault since I bought it. It's done 105000 miles. There was an annoying judder/vibration felt through the car whilst driving and intermittently a lack of power around 3000 - 3500 rpm as if there was fuel starvation. I started an elimination process and removed the EGR valve and intake manifolds to clean. Since rebuilding the car, it takes about 15 seconds of cranking before it started, followed by a load of white fumes from exhaust, not unlike a smoke machine.

After much head scratching I found air bubbles in the clear fuel feed pipe to the injection pump. Disconnected the fuel feed pipe at the filter and nothing coming through. Removed rear seats but cant hear the pump priming. Using a test lamp a cant get an electrical feed at fuse no 28, the one for the lift pump.

Has anyone come across anything like this before. I dont want to assume that the lift pump is fault as it a couple of hundred quid. Is there a relay which feeds the fuel pump circuit.

Any help greatly appreciated

Cheers

Graeme

[zak2005]

Don't bother about your pump under the rear seat - it's role is only to move fuel from right part to left part - fuel tank is split in two ( actualy three, adding the part from the trunk ) by transmission line to back ( quattro ). It is a mistery why audi didn't put a lift pump but this is the reality ( I changed fuel meters from fuel tunk and I realise the fuel line is directly in the fuel not through the pump !) - Injection pump is the only pump. Check injection pump timming adjust - you need VAG-com .

[The Fiend]

Thanks Zak but from what I've read on the forums there are 2 pumps in the tank. One which pumps fuel from left to the right side and then another which feeds the fuel filter only. This ensures that there is sufficient fuel at the filter for the injection pump to work. I've been in at the pump in tank which for a UK car (right hand drive) is in the RHS of the tank with fuel pipe leading off toward fron of car.

If i'm wrong then I eat my words, I just need to figure out why there is no power getting to tank pumps.

CHeers

[Peter D]

There is only one electrical pump in the tank, the second is a suction jet pump whiich it powered by the recirculation flow of fuel from the electrical pump. You need to detail the year and model ou your car as they did have different pumps, one produced a flow to the engine and the earier ones did not. Regards Peter

[zak2005]

Peter D is right - there is one electric pump and one mechanical pump. The electric one is designed for fill in with fuel the cavity where the fuel line stay and ensure functionality for mechanical pump ( suction pump ). Injection pump ( VP44 bosch type for 2.5 tdi engine ) is in charge with suction the fuel from tank. Inside Injection pump there are two pumps - one for low pressure for suction the fuel from tank and ensure presure for timming adjust cylinder and hidraulyc cap - high pressure pump for ensuring pressure for injectors.
Check the fuel filter - it may be old and dirty.

[The Fiend]

Thanks Peter & Zak. The model of car is A6 Avant 2.5TDI quattor 180BHP year is 2001 with an AKE engine code. Car was registered in June 2001

I am still trying to get my head round how this works. So there is no pump in the tank which is responsible for getting fuel to the fuel filter.???

The fuel filter has been changed as has the regulation valve atop the filter housing. The car has had a running fault for some time although only yesterday I noticed the bubbles in the clear fuel pipe to injection pump.

I was concerned that there is no power supply at fuse 28.

Cheers

[Peter D]

Right upto serial number 4B_2_109430 the pump does not prime the main pump so it is not a lifter. Serial number 4B-2_109431 onwards and all allroads the pump was a lifter and should prime the main tandem pump. F28 is in fact the pump supply and you could remove the fuse and use a meter to see if you can see the feul pump resistance only a couple of ohms. You can apply 12 volts directly and you should here the pump running. Relay 17 in position 6 of the 13 relay carrier behind the drives storage box would be your next port of car, either hold it to see if you can detect it clicking or remove it and test it out of the car. Regards Peter

[Peter D]

Sorry got called away. When the pump is not running but the engine cranking the tandem pump pulls fuel but pulls air in through any minor crack in a flexi hose, crimped connector, fuel filter seal, temp regulator, fuel lines or tank connections and this could be your starting problem. Regards Peter

[The Fiend]

Hey Peter

My serial no is 4B82n004636 so am I correct in recapping that fuel is drawn from the tank by a low pressure pump housed inside the main injection pump.

I am unable to detect the relay clicking. Have swapped it with another 373 relay from a seperate relay carrier above the main one. If I try a basic test lamp on the 4 spade connections on no6 relay carrier I get a positive feed at the bottom one only.

I am afraid that testing no 28 fuse slot for ohms is beyond me. All I can tell you is that If I touch the metal spade fuse in position 28, the light in my test lamp does not illuminate so I can only assume the relay is not clicking as it's getting no power feed.

Dont suppose you know where the fuse takes it feed from?? I could try running a 12v wire from battery to fuse 28 to see if the injection pump will turn over.

Cheers

[The Fiend]

Just to recap. This starting fault has only come about since the fuel lines were off car as I was cleaning out intake manifolds. Prior to having fuel lines off, the car has always had an annoying sensation of a slight judder/vibration felt through the vehicle(not steering) and upon hard acceleration when engine not warmed up you get a hesitation which will clear if throttle lifted for a few seconds then re-applied. I am assuming that having fuel lines off has allowed more air into the system and now is a fuel blown starting fault but I am starting to wonder if the pump has been a bit dodgy for some time as from what I read on forums, it's a common failure.

[Peter D]

Have you checked the fuel pump electrical connector on the tank. Inspection cover uner the rear seat. All Allroads have a lifter pump. One of your fuel line connectors is perished/crack and is letting in air and without the lifter working it would make it worse.
If you do not get 12 volts as F28 for 2 seconds when igition is turned on and then when cranking then J17 relay position 6 is not working or is not being commanded to operate. You need to check the operation of relay 3 in the electronics/ECU box under the scuttle cover beneath the windscreen.
If the pump circuit had failed earlier and there were no air leaks then you wold not have noticed any engine problems but you would have run out of fuel when the gauge said you had fuel, if you never went below about 1/3 tank then you may not have realised. After that then the Diesel direct injection system control unit may be the problem and a system diagnostic check may throw up a problem. Regards Peter

[zak2005]

Peter please tell me if lifter pump is present in allroad including quattro type or only in allroad ? ( my audi : WAUZZZ4BZYN096448 )

[Peter D]

I beleive all Allroads and Quattros Have the prime function so it is a lfter. Post you exact model, engine code and year and ideally the date of manufacture and I'll check you you. Regards Peter

[zak2005]

A6 quattro sedan year 2000, WAUZZZ4BZYN096448, engine AKE ; Thanks

[Peter D]

It appears that for that early model and the low serial number the pump was not a lifter as such, however allroads and quattro's employ a suction jet pump and this creates a small positive pressure at the pump outlet pipe. From serial number 109341 all 2 wheel and 4 wheel drives had a lifter type pump Regards Peter

[All-rod]

There also might be blockage in the tank making pump harder to suck fuel in.
This is what my mechaninc thinks could be problem with my car.

[Peter D]

So disconnect the feed pipe from the tank and use a suction pump to see how easy it is to pull fuel from the tank, remember that the rate of flow as well as the vacuum required is what you need to measure. Once you have obtained a flow rate i.e. litres/5 seconds with a know vacumm then repeat the test with the same diameter peice of pipe into a container and replicate the list height. Remember that the tank flow will be a little slower as you have some junctions and you are pulling fuel through the pump. Have you been using junk fuel or home made deisel. ? Remember if you need to lift the pump out then reduce the tank to below a 1/4 and obtain a new seal for the sealing ring. Regards Peter