VSBB6

I was driving on the highway to break-in some more mileage on my rebuilt motor. The car was fully warmed up and I was crusing in 5th gear around 3K rpm.

Now all of sudden, the exhaust noise suddenly changed tone and the engine was bogging on me around 3K rpm while the exhaust pops violently under slow acceleration. I let go of the throttle and slowly made my way towards the nearest exit, the bogging was really bad under 3K rpm, the car hesitate to rev up & felt like something is choking up the exhaust.

From the wideband reading, the A/F suddenly became very lean (16 ish) under 3K. This is strange because my car has been street tuned for 14.5 partial throttle and settings have not been touched since. I double check my fuel/ignition map on my AEM EMS and confirmed the settings are the same.

However, it drives normal again when giving it about 30% more throttle and bring the revs higher. But with lighter throttle like 15 % or less it starts laging/popping/backfiring pretty badly. The noise is rumbling like what you hear from a washing machine or boat.

It sounds to me like fuel related problem. The things that came in mind are fuel filter (which was replaced 8 months ago), cap & rotor (with less than 20K), plugs (heat range -7 w/ just over 2K), injectors (RC with less than 10K).

Im kinda loss as to what may cause this, if anyone else has run into the same problem would probably have different setups as me.

From memory, one time when the motor was cold, sat outside for 3 hours or so, it bogged down heavily under acceleration, until it fully warms up. So I knew from that time to let the car warm up first, and so i did. But now it looks like there's more related into this problem.