Weird Fuel Problem

Standard (as good as) 1.8 GTI.

Runs fine on a light throttle.  As soon as any power is asked for, it behaves as if it's got an ignition problem.  i.e. missing.  Sometimes this can be sorted for a few miles just by opening and closing the fuel filler.

Gets worse the longer the journey.  Gets TERRIBLE if I floor the throttle for an overtake, and takes many miles to get back to only missing on bigger throttle openings.

My guess is it's the pump, or accumulator, or filter.  I'll probably change one at a time (I think I've got spares somewhere in the garage) but would appreciate anyone else's take on it.

Cheers,

Al

Sometimes this can be sorted for a few miles just by opening and closing the fuel filler.

Try driving around for a short time with the filler cap off. It is possible that your vent for the tank is plugged and the pump is having a hard time drawing fuel. I have seen metal tanks get crunched in some from the suction, rare I guess, and the plastic ones do also but come back to form when they get some air back in. Because you say it clears for a while when you crack open the filler cap this could be your problem.

I'll try this.  But I don't understand how it can cure the problem.  Surely, even if it works, then when I put the cap back on I'll get the same thing happening?

Also, looking through the book (Haynes) it looks to me like the tank's supposed to be a sealed, non venting system…..  Though I can't imagine why….

And what I didn't mention above is that, yes, it clears for a while when I crack open the filler cap.

But not always…..

As I said, I'll try what you suggest.  I'm also attempting to understand more the relevance of the accumulator, and what might happen if it fails….

Al

Have you checked the gravity valve in the rear wheel arch ? It may have fallen down and give these symptoms . Located above the filler neck

AlTwoLitreBigValve said

I'll try this.  But I don't understand how it can cure the problem.  Surely, even if it works, then when I put the cap back on I'll get the same thing happening?

Of course it is not going to "cure" your problem. It is just a suggestion on pin-pointing the actual problem. Nobody in the world, unless they are very lucky, can tell you the exact problem from what was so far written. You did however state something that indicates an unvented fuel tank and this is a way to see if that is the case.

AlTwoLitreBigValve said

 Also, looking through the book (Haynes) it looks to me like the tank's supposed to be a sealed, non venting system…..  Though I can't imagine why….

Sealed, yes. At least to the point that gas vapours are captured and burned in the engine rather then set free into the air. But air is allowed into the tank to replace the fuel use, it has to be or the fuel will stop flowing

AlTwoLitreBigValve said

 And what I didn't mention above is that, yes, it clears for a while when I crack open the filler cap…But not always…..

Important bit of information to leave out.

AlTwoLitreBigValve said

 I'm also attempting to understand more the relevance of the accumulator, and what might happen if it fails….

Almost any time it goes bad it will cause a hot start problem. It is nothing more really then a pressure holding device so a small amount of pressure is retained in the fuel lines when the engine is shut off. This helps to stop heat soak, vapor lock, which can cause the engine to not start until the fuel cools down again. It also is a buffer for the sudden pressure the fuel pump builds when it first starts pumping so no damage is done to parts futher down stream. I can't see it causing an issue like you have.