[Blown353]

I've experienced this problem on several engines with FAST EFI. Individual runner intakes seem to magnify the problem of exhaust pops during overrun especially if there are sync issues between each cylinder. Get a syncrometer and check each runner to make sure the throttle blades are adjusted properly.

FAST does not have a DFCO (decel fuel cut off) function so the injectors continue to deliver the amount of fuel that is specified in the VE tables during overrun as well as typically adding O2 correction during overrun if you're running closed loop. This causes the engine to run too rich during overrun and leads to popping in the exhaust.

Do some datalogging and while driving run the engine up to high RPM and then completely let off the throttle and coast back down to a near-idle. Look at the datalog see what cells on the VE table are used for overrun then play with the fuel & timing settings in those overrun cells. Usually adding some timing and taking away a lot of the fuel in the overrun cells will take care of the popping issue. Also limit the amount of positive O2 correction fuel in the overrun areas of the O2 limit tables (XFI) or O2 correction limit menu (FAST classic). That will prevent the ECU from adding too much positive correction fuel when the mixture starts reading max lean (15.9 AFR) when you let off the throttle to coast.

Also make sure there are no air leaks in the exhaust system.