Popping and Backfiring

I foolishly wash down my '92 525i engine three weeks ago, I clearly did not wet the computers. Now, about a week ago, my engine started to pop or backfire between 2000-3000 rpm, I assumed it was bad gas, so I add dry gas, injector cleaner, etc. now the start of the second week, same problem... a computer was hooked up to the engine,sparkpugs, coil, all components are working find, no sensor faults were found, after 3k rpm the engine runs fine...any suggestion on what to try ?