GM cars of a dozen years ago had TBI, Throttle Body Injection. May still have, AFAIK. Instead of an injector at each cylinder intake, it has a single injector in the throttle body, curiously enought. The earliest EFI, on VW & Mercedes, had a throttle valve, and injectors on each intake. Fuel flow was controlled by a manifold pressure sensor that was a variable inductor. It was good, but the MP sensor was expensive.
Later EFI, about 1974-84, had a large vane that was moved by airflow through the throttle valve, and that was called AFC, Air Flow Controlled injection. It uses the same injectors as the MPC. In fact, the injectors in use today are almost identical to the ones on the 1966 VW.
Whenever an AFC equipped engine would backfire, it would damage the airflow sensor, souring customers and delighting mechanics.
As you know, the Mass Air Flow sensor upstream of the throttle valve is a hot wire that measures by the cooling effect of the air flow. That thing is very sensitive, and will cause grief when it gets dirty or contaminated. CRC now sells a special cleaner spray for MAF sensors. It leaves no residue. It didn't seem to help with my P171/P174 codes that came on at the same time, indicating a MAF problem. I'm in Illinois right now, and state law won't places like Autozone clear the codes.
If you opened the intake and squirted throttle body cleaner downstream of the MAF, then no harm should have come to it.
Did you look inside your cats when you got them out, or did the shop give you an "exchange price" for them?
GB | 
