← Back to list

P0307 Fault Diagnosis and Fix for 1999 Chevrolet Suburban K1500 - Misfire in Cylinder 7

Model: 99 Suburban K1500 Fault Code: P0307 Posted: 2008-03-15 09:08

I've been experiencing a recurring engine misfire issue on my 1999 Chevrolet Suburban K1500 (K1500), which has now settled into a consistent pattern. The problem began with difficulty starting—engine would turn over several times before firing, and once running, it would stumble and vibrate. By the next day, the truck failed to start altogether. I initially suspected a failing fuel pump, so I replaced it. No improvement. I then changed the plugged fuel filter, but the issue persisted. Eventually, the check engine light came on with code P0300 (Random/Multiple Misfire). I cleaned and replaced the Mass Air Flow (MAF) sensor, but no change was observed. A key observation: every time I attempted a repair, the truck would start briefly—only to fail again after being allowed to run. The pattern became clear: it starts reliably when cold, but begins misfiring once warmed up. This behavior repeated the next morning, confirming the temperature-related nature of the issue. Another mechanic suggested a faulty sensor, such as the coolant temperature sensor. After installing a new one, there was noticeable improvement—engine now starts consistently, though it still stumbles occasionally during startup. The misfire has since isolated to cylinder #7 (the rear driver-side cylinder, against the firewall). I performed a diagnostic test by swapping both the spark plug and plug wire from cylinder #7 with those of another cylinder. After resetting the code each time, I received P0307 in both cases—confirming that the issue is specific to cylinder #7. Vacuum system checks (including PCV valve inspection) showed no leaks or issues. What other components should I inspect? How can I properly test them? Update: After further investigation, we discovered a loose Ignition Control Module (ICM)—a component similar in function to an old distributor. When the truck was running and turned toward the driver’s side, it caused engine shutdown. The issue appeared linked to improperly installed plug wires on that side, which created excessive tension on cylinder #1's wire, possibly pulling the ICM out of position. After swapping a longer plug wire from the opposite side for better slack, we were able to reposition the ICM slightly and observed improved performance. However, this adjustment triggered a new code: P1345 (Ignition Control Module Position Mismatch), which relates to camshaft/crankshaft timing or module positioning. Final update: After turning the ICM fully counter-clockwise and driving 20+ miles, a P0421 code appeared—'Warm-Up Catalyst Below Threshold, Bank 1'. This was linked to retarded spark timing. Adjusting the ICM slightly (by turning it clockwise) resolved this issue. Since then, the truck runs smoothly, starts reliably, and no service engine light is triggered. This case highlights that even after replacing fuel components and cleaning sensors, underlying mechanical or electrical issues—like a loose ICM—can cause persistent misfires, especially in older vehicles with complex ignition systems.

Related fault codes
P0307
Comments (5)
Anonymous 2008-03-15 10:27

Update: After driving the truck for a while, I received P0300 again. Following reset, four additional codes appeared during my drive home—among them P0306 (misfire in cylinder #6), which is the cylinder I swapped with cylinder #7. The system then returned to P0300 for remaining faults.

Anonymous 2008-03-15 11:30

When was the last time your truck underwent a full tune-up? Ensure you're using high-quality components. Have you tested fuel pressure? These are key diagnostics for misfire issues on older diesel or gasoline engines like the 1999 Suburban.

Anonymous 2008-03-15 12:53

The vehicle was previously serviced by a dealer shortly before purchase. While parts aren't brand new, they appear to be relatively recent. Fuel pressure has not yet been tested—though we've already replaced the fuel pump and filter, which should ensure adequate line pressure. However, during troubleshooting, we found that the Ignition Control Module (ICM) was loose and possibly misaligned. When turning the module toward the driver’s side while running, the engine shut down. This coincided with using overly short plug wires on that side—creating significant tension on cylinder #1's wire, likely pulling the ICM out of position. After swapping a longer plug wire from the opposite side to provide slack, we were able to reposition the module slightly and improve performance. A new P1345 code (ICM Position Mismatch) appeared after adjustment, which relates to camshaft/crankshaft timing or ICM positioning. More updates will follow.

Anonymous 2008-03-16 12:14

Final update: The root cause was confirmed as the Ignition Control Module (ICM). After turning it fully counter-clockwise and driving approximately 20 miles, a P0421 code appeared—'Warm-Up Catalyst Below Threshold, Bank 1'. This issue was linked to retarded spark timing. I then adjusted the ICM slightly clockwise and drove another 40+ miles. The truck now runs smoothly, starts reliably, and no service engine light is active. This case demonstrates how a loose or misaligned ICM can cause persistent cylinder-specific misfires in older vehicles like the 1999 Suburban K1500.

Anonymous 2008-03-17 11:54