VW Beetle (07) with intermittent surge when on full throttle

[Smitters]

Slightly irritating one I could do with some help diagnosing if I may:

2007 Beetle running the 150bhp 1.8T engine.

Symptom was noticed when flooring it in third on a motorway sliproad. Once past 3,000 rpm, the acceleration would build, then lessen, then build more powerfully again. It was always accelerating, but with more and less power. At the time it was noticeable in second and fourth too, but less so.

I did what the Haynes manual advised in terms of shoving the brake pedal about with the ignition on and off to check the hydraulics that are connected and it passed all those tests.

The garage has had a look, plugged in diagnostics and seen no fault codes at all, test driven it and of course, it doesn't fancy showing it today. bd thing. Worth noting it's much colder (and wet) today that when I felt it. They tested the intake system for leaks and found none and anyway, if it was a split/loose hose, I'd expect it to be a permanent fault and getting worse.

What else could I consider? I know nothing of fault code storage, but I would have thought if the fuel pump or injectors were causing issues, there's be a code, likewise if the spark was somehow intermittent and besides, that would sound different. The engine sounds fine, running on four, but like someone's got a knob to control the power and twisting it up and down.

I love intermittent problems, I really do. No, wait, that's not right. I f'ing hate them.

Any and all advise gratefully received. TIA.

[GreenV8S]

You don't say, so I'm going to guess it's a petrol engine. The symptoms suggest the engine is leaning out intermittently. At WOT I'd have thought the ECU ought to be open loop, and if so incorrect fueling might indicate it's getting a bad load signal. The best way to resolve that would be to get the garage to reproduce it while they record the ECU parameters. This will show what the sensors are telling the ECU and also what the ECU is doing in response. It ought to be clear from that what is going wrong.

[Mignon]

Could be dozens of things but fuel filter getting blocked is possible. Cheap to change anyway and see.

[Smitters]

Thanks both. Yes - petrol.

I shall have a chat with the garage and see if they have the facility to plug in whilst running the car and also look at a few simple parts that probably could do with a refresh anyway and may be the culprit.

[Kccv23highliftcam]

what were the revs doing at the time?

[Smitters]

Kccv23highliftcam said:

what were the revs doing at the time?

Slowing their increase proportionally. Were you thinking clutch? It is a manual, incidentally.