Exhaust Gas Recirculation (EGR) System

• When the EGR fails, fuel burns hotter and produces more nitrogen emissions. Check to confirm that all vacuum hoses are intact and do not have any leaks. Use your owner's manual to locate the EGR valve and EGR regulator. Attach a vacuum pump to the EGR valve. Confirm that the pump holds vacuum to 5 to 7 in-Hg. Finally, check the EGR regulator. Check for voltage to the solenoid from the EGR regulator electrical connector with the ignition key on, but the engine not running. Confirm 20 to 70 ohms on the EGR regulator with an ohm meter. Replace and repair any defective parts.
  
  

Ignition Timing

• To check for a defective ignition timing in your Explorer, you will need a timing light and tachometer. First, remove the shorting bar attached to the spout connector above the vehicle's timing chain. Attach the timing light and tachometer to the engine. Confirm the alignment to the timing marks adjacent to the camshaft. If this is not aligned, have the Powertrain Control Module (PCM) checked by a professionally trained technician. Remove the timing light and tachometer and reattach the shorting bar. Replace and repair any defective parts.
  
  

Defective Valves

• After confirming that the EGR and ignitions systems work properly, it is likely the backfiring problem is due to your Explorer's valve system. Remove the spark plugs one at a time and turn the engine over. The affected valve will "puff," showing a leak in compression. Any valves with these problems should be cleaned. You should also check the valve spring to make sure they work correctly, and replace them if there are signs of burning or damage to the valves.
  
  

Major Problems

• If the engine still backfires after performing these checks, you might have a major engine problem such as a cracked engine block or warped manifold head warping. These items require specialized equipment and training to repair, so it's best to consult with a professional mechanic.