Misfire at high speed

[twitcher]

My 2000 1.8 Avant has started to misfire at speeds of 70+, particularly on slight inclines and when overtaking. It doesn't happen on day-to-day running around towm, just on the motorways.

Diagnostics have shown no fault at all, all plugs are firing, and unless it reaches about 4000rpm there's no problem. My local indie found the pipe from the intake to the fuel pressure regulator was collapsing under vacuum, so replaced it, and this certainly improved matters, but hasn't completely eradicated the problem.

Anyone with any ideas or similar experiences?

[iwaters]

I experienced something similar a while back, not on an Audi but the principal is the same. It was basically misfiring under load, so over certain revs, up hills ect as you have been experiencing.

It turned out to be the insualtion on the HT leads breaking down and were shorting across each other.

You havent given your engine code so I can't be sure but if your engine is a normal aspirated 1.8 you have an double ignition coil with a direct connection to two of the plugs and the HT leads to the other two plugs. Inspect the leads carefully for signs of glazzing. Especially where they run close together.