Where you check fuel trim matters as much as you read.

WHERE YOU CHECK FUEL TRIM MATTERS AS MUCH AS YOU READ FUEL TRIM IS PLUS 18 AS YOU READ LONG TERM FUEL TRIM IS PLUS 18 AT IDLE YOU'RE THINKING VACUUM LEAK BUT BEFORE YOU GRAB THE SMOKE MACHINE CHECK IT AGAIN AT CRUISE THAT NUMBER JUST CHANGED EVERYTHING THIS CHANGES THE DIAGNOSIS FUEL TRIM DOESN'T JUST TELL YOU THE ENGINE IS RICH OR LEAN IT TELLS YOU WHERE THE FAULT ACTUALLY LIVES AND IF YOU KNOW WHERE TO CHECK IT HIGH FUEL TRIM AT IDLE THAT IMPROVES AT CRUISE THAT'S A VACUUM LEAK MORE EFFICIENT AT IDLE WHEN MANIFOLD VACUUM IS AT ITS HIGHEST POINT LESS EFFECT ONCE THE THROTTLE OPENS AND THE VACUUM DROPS HIGH FUEL TRIM AT IDLE THAT STAYS HIGH OR GETS WORSE AT CRUISE NOW YOU'RE LOOKING AT FUEL DELIVERY WEAK FUEL PUMP RESTRICTED FUEL FILTER FAILING FUEL PRESSURE REGULATOR LOW FUEL VOLUME THE LEAN CONDITION FOLLOWS THE LOAD NOT THE VACUUM HIGH FUEL TRIMS AT CRUISE ONLY AND FIND THAT IDLE MIGHT BE A MASS AIRFLOW THAT'S UNDER REPORTING AIRFLOW UNDER LOAD AND THE PCM ADDS FUEL TO COMPENSATE ONLY WHEN THE AIRFLOW DEMAND INCREASES SAME CODE SAME SYMPTOM THREE COMPLETELY DIFFERENT CAUSES THE NUMBER IN THE SCAN TOOL IS ONLY HALF THE DATA WHERE THE ENGINE IS OPERATING WHEN YOU READ IT IS THE OTHER HALF AND THAT HAlf TELLS YOU WHERE TO LOOK NEXT ONE YOUR'E CHECKING IT WRONG