engine sounds like jake brake on decel

[bombastic1323]

so here goes. i have perused this forum and bimmerforums for any indication of the problem ive been having, and with no codes to pull this is driving me up a wall. for reference i have the M44B19. it all starts when i put in AKG 85a poly mounts for engine and trans. about a week later i had the unfortunate occurance of clipping a curb lightly on the passenger front wheel. after this incident the engine seemed to be underpowered during wot pulls, similar to a vacuum leak in feel and sound. if i did a half-throttle pull it would function normally, full power and no odd sounds. after thoroughly checking all vacuum lines with an unlit propane torch and starting fluid, i replaced the disa valve as it was pulling air. no change to symptoms. cleaned MAF, throttle body and IAC, again to no avail. changed vcg due to oil in spark plug wells. spark plugs came out looking like a cross between hot and carbon deposited, possibly ashy. changed plugs with vcg, and to address the lack of power feeling i replaced the coil pack and wires shortly after. after all this the car pulls like new again, but when decelerating in gear at engine speed (no brakes) it will sound like a diesel jake brake has been applied for about 1k rpm. in second it occurs between 4&3k rpm, in third its down like 3250-2250 rpm. fourth it disappears and i havent been able to drop 5th from the top to bottom to determine its occurance. my last resort ideas fall into the fuel pressure regulator and the fuel pump themselves but id like to know what ideas i havent thought of/tried for this yet.

[xxxJohnBoyxxx]

Clogged cat converter or maybe broke off the upstream O2.

My internet guess

[bombastic1323]

So I've finally thrown codes, and pending are misfires random, 1, 3, and 4. Confirmed is p1174 long term fuel trim. I checked data and it's defaulting to lean from a rich condition. Per your reply I checked the front o2, which is there and working based on all evidence (scan data/graphs). I then removed itfrom the bung to see how that affected the misfiring, per the clogged cat suggestion, and it seemed to do nothing or slightly slightly alleviated the misfiring. I'm stumped.