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P0172 & P0300 Diagnosis: High Fuel Pressure Issues in 1996 Chevrolet Cavalier 2.2L

Model: 1996 Cavalier 2.2L Fault Code: P0172 Posted: 2008-05-03 13:46

I've been troubleshooting my 1996 Chevrolet Cavalier 2.2L for several weeks now, experiencing persistent idle roughness and engine misfires indicated by DTCs P0172 (rich mixture) and P0300 (random/multiple misfire). I've already replaced the fuel injectors, spark plugs, ignition wires, and tested the fuel return line—no clogs or pinches were found. The original fuel pressure regulator failed, and the replacement regulator also popped its diaphragm. The car runs smoothly above 1000–1300 RPM and can maintain 70 mph without issues, but at idle it is extremely rough. Engine codes only appear when idling. After resetting the PCM, the vehicle runs better temporarily before relearning a rich condition within about an hour—returning to its original rough idle behavior. I'm now questioning whether the Powertrain Control Module (PCM) could be causing excessive fuel pump operation or outputting too much pressure. I've replaced injectors due to leaks, and the issue began after that replacement—suggesting a possible connection to fuel pressure regulation. I suspect high fuel pressure, given both regulators failing. I'm particularly concerned about whether the fuel pump itself is contributing to elevated pressure. While I understand fuel pumps can lose pressure over time, I've never seen them slowly build up pressure—this seems unusual. However, I’ve also read that some AutoZone injectors sold for 2.4L engines may be incompatible with the 2.2L Cavalier (a known issue). Additionally, some forums suggest that faulty ignition coils or PCM injector drivers can cause misfires and lead the PCM to compensate by adding excessive fuel—effectively turning off the regulator and allowing pressure to build. I'm currently awaiting a fuel pressure gauge from a family member for testing. I also want to verify if vacuum lines to the regulator are intact, as improper vacuum operation could prevent the regulator from opening and cause over-pressurization. I will confirm whether the new regulators were sourced from AutoZone and check if spark plugs are coming out wet—this may indicate fuel saturation. My main questions: Can the PCM cause the fuel pump to run too high? Can the fuel pump be outputting excessive pressure? Is there a known issue with AutoZone injectors for 2.2L models? Could a faulty vacuum system or regulator failure be causing this behavior?

Related fault codes
P0172P0300
Comments (4)
Anonymous 2008-05-03 21:16

The PCM is unlikely to cause excess fuel pressure—excessive pressure in the fuel system is more likely due to a failing fuel pump or blocked return line. A proper fuel pressure test is essential.

Anonymous 2008-05-03 23:36

As mentioned, high fuel pressure is the most probable root cause. If you noticed symptoms before replacing injectors, why not start with a new fuel pump instead? Testing fuel pressure directly will confirm whether it's the pump or regulator at fault. Once the fuel pressure is corrected, both misfire and rich code issues should resolve.

Anonymous 2008-05-04 07:40

Two key questions: Can a fuel pump be tested independently of the vehicle? I've traced the return line as per GM service documentation—curiously, the fuel pump isn't listed as a potential cause for high pressure in any official GM diagnostic sheets. Also, could a vacuum system failure prevent the regulator from opening, leading to uncontrolled pressure buildup? I’ve heard reports on Clubcav.com that some AutoZone injectors sold for 2.4L engines are incompatible with the 2.2L Cavalier—this may be contributing to poor performance. Additionally, some users report issues with ignition coils or faulty PCM injector drivers causing misfires. The PCM might interpret a lean condition and add more fuel, effectively turning off the regulator and allowing pressure to build up. This could explain both P0172 and P0300 codes. I’m still investigating whether the fuel pump is slowly increasing pressure over time—though this seems atypical—but I’ll verify with my fuel pressure gauge soon.

Anonymous 2008-05-04 08:44

AutoZone injectors being incompatible? That’s a real concern—I’ve never heard of such an issue, but it's worth verifying. Without actual fuel pressure readings and proper diagnostics, the root cause remains speculative. A thorough check of vacuum lines to the regulator is essential—ensure they’re intact and properly connected. Where did you source your new regulators from AutoZone? Are the spark plugs coming out wet? That could indicate a fuel-rich condition at idle.