NGC 4151 is the brightest Seyfert 1 nucleus in X-rays .
It was the first object to show short time delays in the Fe K band , which were attributed to relativistic reverberation , providing a new tool for probing regions at the black hole scale .
Here , we report the results of a large XMM-Newton campaign in 2015 to study these short delays further .
Analyzing high quality data that span time scales between hours and decades , we find that neutral and ionized absorption contribute significantly to the spectral shape .
Accounting for their effects , we find no evidence for a relativistic reflection component , contrary to early work .
Energy-dependent lags are significantly measured in the new data , but with an energy profile that does not resemble a broad iron line , in contrast to the old data .
The complex lag-energy spectra , along with the lack of strong evidence for a relativistic spectral component , suggest that the energy-dependent lags are produced by absorption effects .
The long term spectral variations provide new details on the variability of the narrow Fe K \alpha line .
We find that its variations are correlated with , and delayed with respect to , the primary X-ray continuum .
We measure a delay of \tau = 3.3 ^ { +1.8 } _ { -0.7 } days , implying an origin in the inner broad line region ( BLR ) .
The delay is half the H \beta line delay , suggesting a geometry that differs slightly from the optical BLR .