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1998 GMC Bravada P0300 Misfire: Diagnosis and Repair Steps After Multiple Part Replacements

Model: 98 bravada Fault Code: P0300 Posted: 2011-04-05 17:28

I own a 1998 GMC Bravada with a persistent P0300 misfire code. I've already replaced the ignition coil, MAF sensor, distributor cap, rotor, spark plugs, and wires. No vacuum leaks were found, and I've checked the fuel filter and replaced the catalytic converter. The engine runs after these replacements but still stumbles noticeably at idle. I’ve also performed a ground resistance check, but results are inconclusive. Despite all these fixes, the issue remains. Could it be related to cylinder compression, fuel pressure, or a blown head gasket? Any suggestions on how to properly diagnose this common P0300 code in older GM trucks would be greatly appreciated.

Related fault codes

P0300

Comments (5)

Anonymous 2011-04-05 18:54

You've replaced quite a few components—start by reviewing detailed discussions about misfires on GM trucks and Blazer models. Verify fuel pressure is within specifications. Check for fuel pressure drop past injectors or the fuel pressure regulator. Also investigate low cylinder compression, binding distributor, or other mechanical issues. The P0300 code is very common and generic—it often requires actual diagnostic testing rather than just part replacements. Can you identify which cylinders are showing the most misfire symptoms?

Anonymous 2011-04-05 22:35

Initially, the car wouldn’t start. After replacing spark plugs, we noticed water or fuel residue on cylinders 1 and 3—mostly in those areas. We cranked the engine to remove some of the moisture and then used an air hose to blow out the cylinders. It started right away. I suspect a blown head gasket; I applied Blue Devil treatment, and white smoke stopped. However, I’m not fully confident in that fix since the car runs better after each part replacement but now stumbles slightly with P0300. Tonight I drove it to work and noticed a small amount of antifreeze coming from the reservoir—no overheating, oil is clean and not milky. I may need to remove the intake manifold and inspect for leaks or internal damage.

Anonymous 2011-04-06 06:37

That sounds like significant engine problems. If you’ve already removed the intake manifold, consider removing the cylinder heads as well—this could reveal underlying issues such as worn valves or internal leaks that may be contributing to the misfire.

Anonymous 2011-04-06 18:09

I took it to a mechanic for a compression test. Cylinder 4 shows significantly low compression, and they identified a burned exhaust valve. The spark plug in that cylinder was completely dry—this suggests no spark or fuel delivery. Could this be due to suction loss or lack of fuel injection? Is the issue mechanical or related to fuel system performance?

Anonymous 2011-04-06 19:01

Fuel delivery is unlikely—the dry spark plug and low compression point more toward a mechanical failure, such as a burned valve. Removing the cylinder heads for inspection and repair would likely be the next logical step.