97 Mercury Cougar P0353 Misfire: After Replacing Coil and Injectors, Code Returns
My 1997 Mercury Cougar V6 3.8L started idling poorly and lost power suddenly. It was running smoothly before, then the check engine light came on, followed by a persistent misfire. The initial diagnostic code was P0353 — indicating a spark plug primary circuit fault in cylinder 3 (cylinder 3 is part of bank 1). I replaced several components to resolve it: - New spark plugs - New plug wires - New fuel filter - A new coil pack (single 6-pack, previously had a large crack on the bottom) - Vacuum lines were also inspected and replaced where needed. I tested the wiring from the coil to the ECM under the dash — no shorts or opens detected. After these changes, the P0353 code disappeared. However, new codes appeared: P0302 (misfire in cylinder 2) and P0306 (misfire in cylinder 6). Since cylinders 2 and 6 are on opposite sides of the engine but share a common coil pack, I replaced the fuel injectors for cylinders 2 and 6 with new ones. The vehicle still runs poorly — shaking badly — and has returned to showing P035-3. This makes me wonder if I'm just spending money without fixing the root cause. I’ve ruled out head gasket issues (oil and water are clean), and there’s no engine rattle or unusual noises. The problem seems to be a misfire, but the recurring P0353 code suggests an issue with the coil pack or its signal wiring. Since I don’t have access to a compression test tool, any advice on next steps would be greatly appreciated.
The P0353 code typically points to a faulty coil pack or poor signal wiring. Since the issue returns after replacing the coil, it's possible that the replacement coil is defective or not properly installed. Check for spark at cylinders 2 and 6 — even if the code shows P0353, misfires in those cylinders may still be present.