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1997 Pontiac Bonneville Won't Start with P0300 Code: Fuel, Spark, and Coolant Issues

Model: 1997 Pontiac Bonneville Fault Code: P0300 Posted: 2008-10-13 18:24

My wife's 1997 Pontiac Bonneville will not start. The starter engages and turns the engine smoothly, but there is no consistent combustion sound—only a constant cranking noise. I've tested all coil packs and confirmed they produce sparks when checked with a screwdriver and ground wire. Spark plugs have been replaced, along with the cam position sensor, crankshaft position sensor, and MAP sensor. Fuel delivery to the cylinders has been verified: upon removing spark plugs, visible fuel residue was present on all plugs. I also measured fuel pressure at approximately 40+ PSI. Despite these checks, the engine does not start. The vehicle occasionally makes a faint clicking sound resembling cylinder firing, but this is inconsistent. After further investigation, I discovered coolant seeping into the combustion chambers—likely due to a failed plastic upper intake manifold. I replaced the upper intake manifold last night, and while the engine now cranks, it still won't start. It appears the engine may be flooded with fuel and coolant, leading to misfires and potential internal damage. I am now concerned about hydrolock risk—excessive coolant in cylinders could cause rod bending during startup. To clear out contaminants, I disabled both spark and fuel systems, removed all plugs, and manually spun the engine several times until no fluid came out of the plug holes. After this procedure, the engine started but runs roughly with intermittent misfires. I've observed smoke or steam coming from the tailpipe during operation, which appears to be due to coolant migrating into the exhaust system. This has since subsided after a few drives, though the engine still misses under load. I suspect ongoing issues related to fuel and coolant contamination are causing the P0300 code (random/multiple cylinder misfire).

Related fault codes
P0300
Comments (8)
Anonymous 2008-10-13 18:27

Have you checked your fuel pressure? Low fuel pressure can cause intermittent combustion, where injectors deliver enough fuel to produce a few sparks but not sufficient for reliable ignition.

Anonymous 2008-10-13 18:30

How do you confirm fuel delivery to the cylinders? What is your measured fuel pressure? How exactly did you test for spark?

Anonymous 2008-10-13 21:26

Fuel pressure reads around 40+ PSI. I confirmed fuel presence in all cylinders by removing spark plugs—each had visible fuel residue. Spark was tested using a screwdriver to loosely ground the plug wire, and a clear spark was observed.

Anonymous 2008-10-18 09:38

What engine type is installed? If it's the 3.8L V6, and the plugs are wet with fuel while you confirm spark, remove the intake boot, take off the throttle body screen, open the throttle plate, and perform a pressure test on the cooling system. Look for a half-inch tube at the bottom of the intake—coolant leakage from there may indicate a failing plastic upper intake manifold.

Anonymous 2008-10-18 12:48

Thanks for the tip—I found this information on another forum and replaced the upper intake manifold last night. The engine still won't start, but I now believe it's due to flooding and coolant in the cylinders.

Anonymous 2008-10-18 15:14

Be cautious of hydrolock—more than a small amount of intake leakage can flood the cylinders with coolant, potentially bending cylinder rods during startup. Always ensure the engine is fully drained before attempting to start after such repairs.

Anonymous 2008-10-18 15:26

The safest method to clear out internal contamination is to disable both spark and fuel systems, remove all spark plugs, and manually rotate the engine several times until no fluid comes out of the plug holes. This helps purge coolant and fuel from the combustion chambers.

Anonymous 2008-10-19 11:25

The car now runs but with a rough idle. Cranking the engine with open intake valves cleared most of the debris. I saw smoke/steam coming from the tailpipe during initial operation, likely due to coolant entering the exhaust system. This has since stopped after a few drives. The engine still misses under load, suggesting ongoing misfire issues related to fuel or coolant contamination.