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1997 GMC 1500 5.7L Misfire Cylinder 5 - Persistent Issue After Multiple Tune-Ups

Model: 97 GMC 5.7L Fault Code: P0305 Posted: 2009-12-01 19:22

I own a 1997 GMC 1500 pickup with a 5.7L V8 engine and 4WD automatic transmission. I've been experiencing a consistent misfire in cylinder 5. During driving, the engine shakes noticeably for a few seconds before the check engine light activates. This issue has persisted for over a year despite multiple visits to different repair shops. Each time, they replaced the spark plugs, ignition coil, distributor cap, rotor, and wires—yet the problem returns every time. I suspect the root cause may not be related to basic tune-up components.

Related fault codes
P0305
Comments (3)
Anonymous 2009-12-01 21:13

This suggests that a simple tune-up is unlikely to resolve the issue; the misfire in cylinder 5 is likely due to deeper mechanical or fuel system problems.

Anonymous 2009-12-08 18:11

I currently have the same problem on my 1996 Chevrolet 5.7L truck. I've already tried a full tune-up with no improvement. However, after running approximately five tanks of premium gas mixed with Lucas Oil fuel injector cleaner, the issue temporarily disappeared. My truck has around 255,000 miles and I believe GM's original injectors on this engine are prone to clogging or malfunctioning over time—especially at high mileage. I'll update this thread once I find a permanent fix.

Anonymous 2009-12-24 04:53

I'm currently working on a 1998 GMC 1500 Express with a PO305 code (misfire in cylinder 5). After testing, I discovered the issue was caused by a faulty valve in cylinder 5. I swapped cylinders 1 and 5, and my OBD-II scanner detected that cylinder 1 was now misfiring—confirming the original issue. Compression test showed 125 psi, which is within range. Upon inspecting the cylinder, I found heavy carbon buildup around the valve, likely due to heat and pressure over time. The valve appears to be degrading and spreading rapidly—this seems to be a known common failure in this engine family. Good luck to everyone facing this issue.