A4 B7 1.9 TDI rough running midrange after EGR & AFM replaced

[ojsmith]

Hi,

My 2007 1.9 TDi (209,000 miles) developed a major fault a few weeks ago, it would start and idle Ok, but had very little power, ran very rough and smokey and couldn't get up any kind of hill.

EGR was replaced, which got us back on the road, but there was a residual problem of significant hesitation in mid-range. Seems fine at idle, or under full throttle, but would engine would cough and jerk a lot in mid-range revs under moderate load; driving at legal speed up our 20-30 MPH mile-long hill was about the worst scenario.

Air flow meter was diagnosed faulty and replaced, which I'm pretty sure has helped bottom end grunt, but the roughness was still there in the middle. Suspect failed air meter contributed to EGR valve getting clogged.

Mechanic reckoned the turbo was sticking (some kind of variable geometry?) as there was one error code saying overboost. He gave it a dose of Mr Muscle oven cleaner to help clean the crap away. It does seem to be about 80-90% better, but the roughness is still there sometimes, though less harsh than previously as well as much less frequent.

What I find slightly odd is that fault disappears as soon as I give it full throttle, no apparent waiting for revs to get over certain level.

Any suggestions for other things to check/try? Failing that I might be tempted by another dose of Mr Muscle!

Cheers,

OJ

[Rob69]

Check the operation of the V157 throttle flap upstream of the EGR valve, this is linked to EGR operation and throttles intake air at light or part load to help draw in exhaust gas but goes wide open when you floor it. I've had trouble with these on a few 1.9 engines and they didn't log a fault code. An overboost code would put the car into limp mode and you wouldn't get full throttle. This tends to happen when flooring the car to overtake or climb a hill. You can check turbo VNT operation while driving if you can get hold of VCDS, it's possible you still have fouled VNT mechanism at that mileage.