4.0L Engine Backfire

[steamer84]

I have a 2003 Ranger and have a problem whenever I turn on the A/C. Within a 2 or 3 second time frame, the engine backfires. Sometimes its quite noticeable and other times its just a little pop. Otherwise the engine runs extremely well. I do not know if it's a sensor problem or a control module problem. Has anyone else had this problem?

 

[RonD]

Could be a Lean condition occurring when computer opens IAC Valve to increase idle when AC is turned on.
Coupled with a lower flow fuel injector.
Also check PCV valve hoses very carefully, the elbows wear at the bottom causing a slight vacuum leak at first, which gives you a Leaner condition, then coupled with IAC valve opening you get lean enough mix for back fire

Lean fuel mix can self-ignite when entering a hot cylinder on intake stroke so back fires though intake.

The lean mix is seen by O2 sensor on that bank, which is why back fires don't continue.

If possible do an OBDII live scan and see if one bank of engine has higher fuel trim numbers than the other.
Fuel trim number tells you if one bank might have an air leak or injector with lower flow

 

[steamer84]

4.0 Engine Backfire

Thanks Ron,
I appreciate the info.