Help! Just replaced a 4.0L engine in my 2005 Ranger with one out of a 2011 with only 7000 miles. The truck runs great except when you are cruising down the highway down a hill. It throws random multiple misfire codes - po300 and sometimes different cylinder misfire codes. I used all the peripheral parts off my old engine except the intake and EGR valve. Spark plugs are new, Fuel and air filter are new. I ran K44 through the injectors to clean them out. Anyone have a clue what would cause it to throw these codes and causing it to misfire when cruising down a hill at highway speeds?
Some history on the vehicle. The old engine blew and put a lot of oil in the exhaust. It took a while to burn it all out - tons of smoke. It did pass emissions so the cats should be OK. Would a O2 sensor cause this without throwing an engine code?