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1997 Mercury Mountaineer 5.0L V8 P0303 Misfire: Exhaust Manifold Leak and Injector Diagnosis

Model: 97 Mercury Mountaineer Fault Code: P0303 Posted: 2006-02-07 18:27

I own a 1997 Mercury Mountaineer with a 5.0-liter V8 engine that is running rough and has triggered a diagnostic code P0303 — indicating a misfire in cylinder 3. Upon inspection, I discovered a crack in the passenger-side exhaust manifold specifically affecting cylinder 3. Raw fuel is visibly leaking from this crack, which I can confirm by smell and observation. I tested the situation by disconnecting the spark plug wire for cylinder 3 to prevent ignition — when I cranked the engine, raw gas began flowing out of the crack in the manifold. After wiping the area and smelling it directly, I confirmed it is fuel. I have already replaced the spark plugs, plug wires, and the cylinder 3 injector with no improvement. Compression tests show all cylinders reading between 120–140 psi, indicating good mechanical integrity. The engine starts easily but runs roughly with noticeable misfires. Given that raw gas is leaking from the exhaust manifold, I am concerned about potential internal fuel delivery issues or a faulty ignition component such as a coil pack or ignition module. My next step was to perform further diagnostics at a shop after towing the vehicle. A fuel pressure gauge revealed inconsistent readings — pressure drops immediately and does not hold steady. When all spark plugs on the right bank were removed, cylinder 1’s plug was found soaked in fuel. Tapping the key with the plugs out showed gas spitting from cylinder 1 even when the injector was unplugged. All other cylinders appeared dry. This indicates that cylinder 1's fuel injector is stuck wide open continuously, causing excessive fuel delivery into the intake and exhaust system. The leaked fuel then travels through the header and drips down the cracked manifold at cylinder 3, where it escapes visibly. This explains both the raw gas leak and the P0303 misfire. I replaced the faulty injector in cylinder 1, and since then, the issue has been fully resolved. The exhaust leak stopped, the engine runs smoothly, and no further misfires are present. I believe this case highlights that a malfunctioning fuel injector — even on a different cylinder — can cause misfires and visible fuel leaks through the exhaust system. This may be helpful for others experiencing similar symptoms with their 1997 Mercury Mountaineer or other V8 engines with P0303 codes.

Related fault codes
P0303
Comments (6)
Anonymous 2006-02-07 21:44

Are you certain it's raw fuel leaking? If so, that suggests no spark in the combustion chamber. However, visible raw gas from an exhaust crack is unusual — could this be a misdiagnosis or a different issue? Have you reviewed this OBD-II code explanation? https://www.obd-codes.com/p0303

Anonymous 2006-02-08 05:38

Yes, I can see and smell the fuel. It ignited when the vehicle was running for about an hour — the manifold was hot at that time. To prevent ignition, I disconnected the spark plug wire and then cranked the engine. As a result, raw gas began to flow from the crack in the exhaust manifold. After wiping it clean, I confirmed it is fuel by smell.

Anonymous 2006-02-08 05:43

I used a spark tester and observed clear sparking at all cylinders — including cylinder 3. The presence of spark suggests that combustion isn't failing due to lack of ignition. However, the spark may be weak or insufficient to ignite fuel properly. Given that I've replaced plugs, wires, and the injector with no improvement, a faulty coil pack seems like a plausible next step.

Anonymous 2006-02-08 05:45

The initial fire occurred because the exhaust manifold was hot after driving for about an hour — heat likely caused the fuel to ignite. This doesn't mean there was spark from the plug; rather, it's a result of high temperature and fuel presence.

Anonymous 2006-02-09 21:24

After towing to my shop, I conducted additional diagnostics. A fuel pressure gauge showed inconsistent readings — pressure rises quickly but drops immediately with no hold. When all right-side spark plugs were removed, cylinder 1’s plug was heavily soaked in fuel and spitting gas when tapped, even without the injector connected. All other cylinders were dry. This points to a stuck-open fuel injector in cylinder 1. The leaked fuel travels through the header and drips into the cracked manifold at cylinder 3, causing visible leakage. This explains both the P0303 code and the raw gas leak. I will update if this resolves the issue.

Anonymous 2006-02-11 08:10

I replaced the fuel injector in cylinder 1. The problem is now fully resolved — no more misfires, no exhaust leaks, and engine runs smoothly. This case may help others facing similar issues with P0303 codes on V8 engines.