ARMY TM 9-6115-670-14&P 4-16 5. ENGINE RUNS ERRATICALLY OR MISFIRES. STEP 1. Check for clogged air filter (para 4.45.). a.  If air filter is clogged, SERVICE or REPLACE air filter as required. b.  If air filter is not clogged, proceed to step 2. STEP 2. Check for clogged or dirty fuel filter. a.  If fuel filter is clogged or dirty, REPLACE fuel filter (para 4.30.). b.  If fuel filter is not clogged or dirty, proceed to step 3. STEP 3. Check fuel filter sediment container for contaminated fuel, incorrect grade or fuel type. a.  If the fuel is contaminated, drain entire fuel system including vehicle fuel tank and APU day tank (para 4.32. or 4.33.).  Replace fuel filter (para 4.30.).  Service with proper grade fuel.  Bleed fuel system. b.  If the fuel is uncontaminated, proceed to step 4. STEP 4. Check for obstruction in fuel supply line. a.  If the fuel line is obstructed, remove obstruction or replace fuel line. b.  If the fuel line is not obstructed, proceed to step 5. STEP 5. Check for fuel leak due to loose injection pipe retaining nut(s). a.  If fuel is leaking from injection pipes, stop the leak by tightening the retaining nuts or replacing the fuel injection lines. b.  If fuel is not leaking from the injection pipe lines, proceed to step 6.