No acceleration Read codes, PO300, PO303?

2011 CHEVROLET EQUINOX
195,000 MILES • 2.4L • AUTOMATIC
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FREERIDER78
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  • 28 POSTS
Read codes, PO300, PO303, coil swap with 2, found oil in plug wells and carboned up plugs. Replaced valve cover gasket and well seals and plugs. Gapped. 035 iridium. No CEL flashing or codes but permanent PO300. When driving no acceleration. After repair initially seemed OK. Then checking deeper for codes PO302. Swap with 1. When pressing gas pedal several times while parked and idling a large cloud of smoke, oil, emitted and then dissipated. Acting like a plugged cat which was replaced with aftermarket unit. Exhaust smells like sulfur. Oil doesn't smell like gas. Suspecting plugged cat. Don't think it's HPFP, no code and no injector code. Lastly there seems to be both vacuum and pressure at the oil cap. Is that normal? Haven't run compression test or fuel pressure test but there's definitely pressure at the rail when I depressed the Schrader valve. Stumped.
Jul 26, 2026 at 9:27 PM

What to check next

Diagnostic guidance. Suggested checks and possible repairs are separate from the owner's reported results.

First record current misfire activity and code status, and confirm whether the fault follows a correctly identified coil swap. Check for continued oil contamination and assess the reported engine noise before further driving or repeated throttle tests. Have exhaust backpressure measured before condemning the converter. If restriction is confirmed, also identify the misfire or oil-burning problem that may damage a replacement. Compression testing and a measured fuel-pressure comparison can follow the findings; pressing a fuel-rail valve is not a valid pressure test, and noise alone does not condemn the high-pressure pump. Testing both the loss of power and its underlying cause offers a better chance of a lasting repair than replacing the converter alone.

Guidance references and applicability

Walker: Exhaust-system diagnostic testing https://www.walkerexhaust.com/support/tech-tips/simple-diagnostic-testing.html General support for measuring exhaust restriction and distinguishing it from other engine faults. Odor, soot or poor acceleration alone does not identify the restricted component. HELLA: High-pressure fuel-pump diagnosis https://www.hella.com/techworld/en/technical/sensors-and-actuators/checking-a-high-pressure-fuel-pump/ General support for comparing diagnostic data and checking the supply side before condemning the high-pressure pump. Use the actual vehicle's fuel-system procedure and specifications.

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AI-ASSISTED GUIDANCE · REVIEWED BY 2CARPROS
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STRAILER
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It would help if you can upload a video of the engine running so I can see it. The oil can be when you changed the spark plugs and valve cover gasket and should burn off. The spark plug being black can indicate the cat is plugged and lowering engine vacuum when will richen the mixture making the problem worse. Here is a video to help you check the catalytic converter.

https://www.youtube.com/watch?v=kNXh1Lphzr4

Also, if the misfire follows the bad ignition coil change it out with a new one. If the engine makes a tapping noise at idle the HPFP could be going out or the camshaft lobe that drives the pump might be worn as well. If there is a noise, (this is why I want the video) I would remove the HPFP and inspect the camshaft lobe. Please let me know what happens.
Jul 27, 2026 at 11:00 AM
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FREERIDER78
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Ok next time I'm there I will get a video. There is definitely a noise but doesn't sound like chain rattle. More like lifter or such. Passenger side of engine on top. I didn't listen directly on the HPFP. I'm also not positive if it followed the coil or not. I'll be back there in a few days and will update you. They're supposed to be talking to the shop that did the cat and having it checked.
Jul 27, 2026 at 12:13 PM
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STRAILER
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Sounds good but yes the engine noise can be the high pressure fuel pump or the camshaft lobe is worn down. Let me know.
Jul 27, 2026 at 5:32 PM
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Discussion summary

No acceleration Read codes, PO300, PO303?

The owner reported misfire codes, oil in the plug wells and poor acceleration. Plug-well seals and plugs were replaced, but symptoms returned with smoke and engine noise. The owner suspected a restricted aftermarket converter and planned to have its installer check it. Whether a misfire followed a swapped coil remained unclear.

Reported outcome: No converter-test result, confirmed pump fault or successful repair was reported.

The coil-swap result, source of smoke and exact engine noise were not established. Sulfur odor and the absence of an injector or pump code do not settle the diagnosis.

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AI-ASSISTED SUMMARY · REVIEWED BY 2CARPROS
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