Car is a B5 A4 nDBW 1.8TqM (AEB).

Had my ride dynoed several months ago. The AFR curve on top is from my 1st session. The curve curiously starts to spike at around 4.5T RPM. We stopped the run at a little past 5T RPM since AFR has already gone up to 13.5 & seems to be leaning-out more.

We decided to replace the stock fuel pump with a Walbro 255LPH unit. Result, I am now running pig-rich midrange as evident in the bottom AFR curve. I'm losing plenty of midrange torque because of this (check graph on the left). What concerns me is the 'shape' of the curve, w/c remains the same & only shifted downwards after the Walbro install. There is still this abrupt spike after 4.5T RPM.

I've contacted ATP, and they feel that it could be either the FPR or MAF sensor (both are stock in my car). Of course, it could also be their tuning.

The car still pulls well though, but it does bog-down once in a blue moon when it gets flooded with fuel @ WOT. Would appreciate any inputs.

note: 19.5PSI w/ a MBC on both runs.




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The graph is a little to hard to read on my computer. But I would start with the FPR & then check the MAF. You may need to install an adjustable FPR & then get it dialed in.

Did you ever change the fuel filter? It might be time to.

I would also check the coolant temp sensor.

 

There's a shop here that's willing to do a custom tune, but I'm going to try the simpler route first. An adjustable FPR would be a good start, but I'm a bit worried that it might simply shift the curve up or down. What troubles me is the shape of the curve itself: It leans-out abruptly after 4.5T RPM without any sign of letting-off. If ever, it should richen-out or at least hold steady approaching red-line. Instead, it's doing the exact opposite