Misfire And Poor Running

What You Are Feeling

Erratic idle, a misfire that stays at every speed, a hesitation when you open the throttle, a stall at lights, or a car that simply will not pull. Scan first if you can: stored misfire or lean codes steer the bay work. On the V6, P0171 and P0174 usually mean intake gasket or MAF drift long before a computer swap (see A-047).

Petrol Checks

• Plugs and coils: One dead coil-on-plug pack shakes the idle and lights a misfire code on that cylinder. Wrong gap or oil-fouled plugs do the same across banks.
• Vacuum and intake: A leak at the throttle body, manifold or a split hose leans the mixture, hesitates on tip-in, and can stall when you lift off.
• Fuel side: Choked filter, weak pump pressure, or a blocked tank vent leave the engine flat through the rev range.
• Compression and timing: Uneven compressions or a mistimed chain are the mechanical end of the list, not the first guess after a coil swap fixed nothing.

Diesel Checks

• Injectors and filter: A failing injector or a packed fuel filter knocks, smokes, or simply refuses to accelerate cleanly.
• EGR path: A soot-choked EGR and intake tract idle roughly and feel gutless under load; blanking and cleaning notes are under A-001.
• Air and glow: Unbled fuel after filter work, or weak glow on a cold morning, looks like a running fault until the car is fully warm.

Do Not Chase Ghosts

Binding brakes or a slipping clutch can fake a power loss. Confirm the engine is the patient before you buy injectors. One change at a time, then a road test.