What problems can a leak in the PCV system cause on a Ford Expedition?

A leak in the pcv system will cause a major vacuum leak because the hose connects to a fitting on the intake and its large enough that it sucks in quite a bit of air.All the symptoms you listed are typical of a vacuum leak too.The leak could be causing a misfire which in turn could cause the knock sensor to begin reacting.Fix the leak before you do any other trouble shooting.The rattling sound you're hearing is probably detonation.When a misfire occurs,the extra air that didn't get burned in the combustion chamber gets read by the o2 sensor as extra unmetered oxygen,so it tells the computer its lean.A lean fuel mixture will cause detonation.The knock sensor is supposed to kick in and tell the computer that the timing needs to be reduced in order to get rid of the detonation.Fix the vacuum leak first,then take the vehicle to Autozone and get it scanned for codes.It sounds like the knock sensor is malfunctioning.The scan should show a code if its faulty

SLUDGE can build up as teh OIL cannot breath right! ALL FORD engiens just L O V E MOBIL ONE 0W_30 engien oil and COME TO LIFE on it! Use AMSOI POWER FAOM to clean yoru PCV VALVE and HOSES OUT, and to REMOVE the IDLE AIR CONTROL VALVE whcih NOW wil be ful of SDLUGE and BRWON WAXY COATINGS! Never use VALVOLINE evver again! NO FACTORY uses it for GOOD REASONS! iT MAKES sdluge AND CAUSES ENGINE PROBLEMS OVER TIME! AMSOIL POWER FAOM wil remove ALL traces of SDLGUE inside the engien if you spray it DIRECTLY into the OIL FILLER CAP and drive for TWO DAYS eaily! These are GREAT engiens with UNLIMTED PWOER and MILEAGE< but BAD OIL will ruin them and give you GRIEF! MOBI ONE oil is good for ONE YEAR of use! AMSOIL gives a 25 k miel WRRANTY or ONE FULLYEAR! STop usign CHEAP OIL CHANGE SPEACIALS that RUIN engiens over time! Teh JUNK YARDS are full of BROEKN ENGIENS using VALVOLINE and other VERY CHEAP wax based oils! DO NOT rely ONLY on VISCOSITY RATINGS of oils! This is teh LOWEst rating of all! Rely on SPECIFICATION NUMBERS instead! MOBI LONE fulffils nearly ALL car makers needs, and once again NO OBNE uses VALVOLINE from the factory ANYWHERE on planet earth! GOOD LUCK TO YOU!

yes fix it