A6 - Some kind of fuel delivery problem

[timwatsham]

Hi guys
Wondering whether any of you might have an idea about this...

I've got an A6 with the 115hp 2 litre engine (which I think is the same as the Golf GTi motor). There's a strange problem which has built up over the last 3,000 miles.

Initially when I ran the fuel tank right down, the car started to jerk under power - felt like it was spluttering, but could be electronic too I guess. As soon as I filled up with petrol, the problem disappeared. So I assumed it was dirt in the fuel tank and didn't run it low again. All fine for the next 750 miles.

Then with 1/4 of a tank left it did it again. I filled up immediately and it didn't go away with a full tank.

Drove down to France last weekend, and it did it all the way down there - luckily once up to about 100 on the autoroute it stopped doing it, only showing up when accelarating from toll boths. Continued doing it whilst driving around for the weekend, really annoying.

Then it cleared up all the way home until we got to the tunnel when it came back.

So in summary - the car is stuttering when idling, then jerking when under power (to the extent that I broke an exhaust mounting bracket). Sometimes it clears up (when it then runs incredibly smoothly), but then it comes back again...

Any thoughts? Dirt in fuel line? Dirt in injectors? Faulty ECU? Crack in an air hose somewhere?

Tim

[beano500]

What transmission?

My 2.4 has to go in tomorrow for some warranty work. Apparently there's a known issue on the Multitronic. Mine lurches at low speed, especially reverse, like a lurching lurcher taking kangaroo lessons in Lurchester.

Makes my already average manouevring ability atrocious....

[timwatsham]

Its a manual... (And its definately not my clutch control that's doing it...)

[beano500]

Well that's one thing eliminated then

I think it's a fuel issue then, dirt in the injectors as you already list.....

[timwatsham]

Hmm, which brings me to my next question...

Anyone any idea how to clean fuel injectors?!!!!

[SJobson]

You can buy tins of fuel system cleaner in Halfords - tip it in the fuel tank and run it through. Take care to pick the right one though.

And it's not guaranteed to do the job either, but it's a cheap fix if it works.

[IOLAIRE]

timwatsham said:

Hi guys Wondering whether any of you might have an idea about this... I've got an A6 with the 115hp 2 litre engine (which I think is the same as the Golf GTi motor). There's a strange problem which has built up over the last 3,000 miles. Initially when I ran the fuel tank right down, the car started to jerk under power - felt like it was spluttering, but could be electronic too I guess. As soon as I filled up with petrol, the problem disappeared. So I assumed it was dirt in the fuel tank and didn't run it low again. All fine for the next 750 miles. Then with 1/4 of a tank left it did it again. I filled up immediately and it didn't go away with a full tank. Drove down to France last weekend, and it did it all the way down there - luckily once up to about 100 on the autoroute it stopped doing it, only showing up when accelarating from toll boths. Continued doing it whilst driving around for the weekend, really annoying. Then it cleared up all the way home until we got to the tunnel when it came back. So in summary - the car is stuttering when idling, then jerking when under power (to the extent that I broke an exhaust mounting bracket). Sometimes it clears up (when it then runs incredibly smoothly), but then it comes back again... Any thoughts? Dirt in fuel line? Dirt in injectors? Faulty ECU? Crack in an air hose somewhere? Tim

Very briefly Tim,
these cars have two types of fuel pump system. The first is an immersed pump and is in the top of the tank. It has a very fine micronic filter which can clog up with sediment; but it can be so fine it actually looks ok. You have to remove the pump, take off the filter and clean it thoroughly with a solvent such as thinners.
Your car is displaying classic symptoms of that filter being choked.
The second system uses an underfloor high pressure pump to pressurise the injectors, but this is fed from a smaller, low pressure pump again immersed in the tank. It is a lift pump and lifts fuel out of the tank and supplies the pump under the floor.
On some cars the engine will still run even when this pump seizes because the high pressure pump will lift the fuel by suction, but in doing so it cavitates, (draws air) and you get bad running.
At high speed however the fuel flow is sufficient to cancel out the effect of this and hence the car runs ok at 100 mph or thereabout.
Check either of these out and hopefully it will solve your problem.
James.

[timwatsham]

James, thanks for that - glad that you all seem to think that it is fuel related too. I had horrible visions of something more sinister at work here...

I'll get my Haynes manual out and try to play around with it this weekend...

Cheers, Tim

[DeltaFox]

Id tend to go with what Iolaire said.
In addition it may just be that the pump is on its last legs or has a poor connection.