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2003 Chevy Blazer 4.3L Engine P0172 and P0300 Codes: Rich Running, Misfire Diagnosis

Model: 2003 Chev Blazer Fault Code: P0172 Posted: 2010-04-11 20:30

My 2003 Chevrolet Blazer with a 4.3L V6 engine and automatic transmission has recently set two diagnostic trouble codes: P0172 (Bank 1 rich mixture) and P0300 (random cylinder misfire). I understand that P0172 indicates an overly rich fuel condition on Bank 1, while P0300 points to a random cylinder misfire. The vehicle has accumulated 125,358 miles. Routine maintenance has been performed since the initial ownership at 33,000 miles. I consistently use high-quality gasoline—primarily Chevron—and only occasionally add Lucas fuel system cleaner every six months. On average, I drive approximately 300 miles per week (one tank of gas), with minimal weekend usage and frequent parking. The symptoms began yesterday: the engine runs roughly at idle and during light acceleration from a stop. Once the vehicle is moving and under load, it runs more smoothly. Additionally, I’ve noticed increased startup time—normally immediate upon turning the key—and my wife reported dark exhaust smoke this morning, which aligns with a rich fuel condition. Given that the issue appears specifically on the left side of the engine (Bank 1), could a faulty fuel injector be the root cause? Are there other common causes for a lean/rich imbalance in a 2003 Chevy Blazer 4.3L? I have hands-on experience with automotive engines, including building high-performance Harley-Davidson engines and restoring classic vehicles like a 1932 Ford coupe street rod. While I’m comfortable working on carbureted systems, I am less familiar with modern EFI (electronic fuel injection) systems controlled by the PCM. Any insights or troubleshooting steps would be greatly appreciated. Best regards, Mike

Related fault codes

P0172P0300

Comments (4)

Anonymous 2010-04-12 05:30

A leaking fuel pressure regulator is a common cause of rich mixture symptoms in vehicles like your 2003 Chevy Blazer. If the regulator is faulty, it can allow excess fuel to enter the system. In such cases, replacing the entire fuel injector assembly with updated, high-performance injectors may resolve the issue. Please update me on your findings after inspection.

Anonymous 2010-04-12 20:50

Thank you for the prompt response! I’ve reviewed several posts in the 'Trouble Codes' section related to P0172 and have completed the following checks: - Inspected all vacuum and PCV hoses — no issues found. - Cleaned the MAF sensor using electronic contact cleaner (removed, cleaned thoroughly, reinstalled carefully). The sensor is located near the intake manifold as per service manuals. - Checked fuel lines for cracks, leaks, or pinches — all appear intact. - Verified exhaust system integrity before the first oxygen sensor — no visible leaks. I was unable to measure fuel pressure due to lack of proper tools and gauges. I also did not test the injectors directly. As recommended, I sprayed both the MAF and MAP sensors with electronic contact cleaner and allowed them to dry completely. Update: This morning, startup became significantly easier—almost instantaneous. The rough idle and hesitation during acceleration have improved substantially. While the idle is still slightly rough, it’s much better than before. I’m currently researching the location of the fuel pressure regulator (FPR) on my 2003 Chevy Blazer 4.3L (X-code). Could you confirm its position? Also, what are 'poppett injectors' and where are they located? I thought this engine used a TBI system—do these injectors reside inside the TBI unit? Thanks again for your help—it’s very much appreciated! Mike

Anonymous 2010-04-13 05:17

Hi Mike, the fuel pressure regulator (FPR) is located underneath the upper half of the intake manifold. If it's leaking, you'll often see fuel pooling inside when you shine a light and prime the fuel system. The FPR can be accessed without removing the entire intake, though removing the intake allows easier access for gasket replacement. For detailed guidance, check out well-documented videos on Wells Ignition or D-tips.com—both provide step-by-step instructions with clear photos of the process.

Anonymous 2011-10-17 10:33

I’m experiencing exactly the same issue with my 2003 Chevy Blazer. I’ve already replaced all spark plugs (even though they were changed a year ago), but that didn’t help. I’m now planning to follow your troubleshooting list and test the system using the recommended steps. Do fuel injectors play a key role in this problem? Any advice on how to diagnose them would be helpful.