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1999 Chevy Suburban PO305 Code: #5 Cylinder Misfire Diagnosis and Solutions

Model: 1999 Chevy Suburban Fault Code: P0305 Posted: 2014-05-15 12:45

I own a 1999 Chevrolet Suburban K1500 5.7L V8 with 148,000 miles. I'm currently experiencing a persistent PO305 diagnostic trouble code indicating a misfire in cylinder #5. I've already replaced the spark plug wires, cap and rotor, spark plugs (using NGK Iridium IX), and both front oxygen sensors. The truck runs smoothly with no hesitation, skipping, or rough idle—idle is so quiet I open the window to hear if it's running properly. I have not yet replaced the coilpack or fuel injector for cylinder #5. Despite these efforts, the PO305 code continues to appear. Given that the engine performs well under normal conditions, I'm wondering what other potential causes could be responsible—especially since a misfire is typically attributed to ignition, fuel delivery, or mechanical issues. Any insights or troubleshooting steps would be greatly appreciated!

Related fault codes

P0305

Comments (5)

Anonymous 2014-05-15 13:09

Did you use the correct AC Delco spark plugs? What did the old #5 plug look like—wet or black and gray? Try adding one bottle of SeaFoam fuel injector cleaner to a half tank of gas. Drive for about 3–4 hours on Friday, Saturday, or Sunday, then clear the code and report back. A misfire is commonly caused by ignition, fuel delivery (injector), or mechanical problems.

Anonymous 2014-05-16 07:05

I used NGK Iridium IX plugs from NAPA. The old #5 plug was black and gray with visible damage—dry, not wet. I've previously added SeaFoam through the fuel tank, but haven't done so since changing the spark plugs. I'll try this method again and update my results!

Anonymous 2014-05-21 08:12

If cylinder #5 is continuously misfiring despite all repairs—without replacing the injector—I'm wondering if it might be oil-fouled. Would switching to hotter spark plugs with non-foul extenders offer a temporary fix? I’m looking for a quick, low-cost solution since I can’t afford internal engine work right now. It's strange because the truck runs and idles extremely smoothly—almost too quietly to notice any issues.

Anonymous 2014-05-21 08:51

No—avoid hotter plugs or oil-foul extenders. If oil fouling were present, I’d expect O2 sensor codes, black exhaust, or a failing catalytic converter. Let’s troubleshoot step-by-step: - A 150K-mile engine is not necessarily worn out unless it has suffered catastrophic failure. - Is only cylinder #5 showing misfire? - Are you experiencing oil consumption? Suggestion: Perform a fuel pressure test and a fuel pressure leak-down test. - Turn key on, engine off—record baseline pressure. - Start the engine—record pressure. - Shut down the engine—turn key to 'on' and observe pressure drop. Report results (e.g., no drop, drops 10 psi in 3 minutes, or drops to zero in 3 minutes). If fuel pressure is normal, next step would be a compression test on cylinder #5—also consider cylinders #3 and #7 for baseline comparison. This could help identify whether the issue lies with the injector or another component.

Anonymous 2018-09-28 10:32

jcmnh, I'm experiencing the same PO305 code on my 1998 Silverado-style 5.7L engine. The truck is due for a smog test and now frequently throws the PO305 code. My engine has 200K miles, runs at 155 psi compression, fuel pressure of 60 psi when off, drops to 55 psi when running—no leak-down. Spark plug appears normal, not wet or oil-fouled. No visible smoke. Oil consumption is about 1.5 quarts every 5,000 miles. The PO305 code first appeared after extended idling (4–5 minutes), but now occurs more frequently—now even at 1681 RPM. I’ve archived the freeze frame data from my Actron scanner and am working to interpret it. This issue is becoming critical for passing smog inspection. I’d appreciate any insights or recommendations based on similar experiences.