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2009 Chevy HHR P0304 Misfire: Intermittent Traction Control, Black Soot on Intake Ports, and Fuel Type Impact

Model: 2009 Chevy HHR Fault Code: P0304 Posted: 2015-12-15 01:05

My 2009 Chevrolet HHR LT with a 2.4L engine and automatic transmission has been experiencing intermittent check engine light (CEL) activation and now displays 'Service Traction Control' messages. The issue appears to be linked to misfire code P0304, which comes on and off unpredictably—sometimes staying off for weeks before reappearing. I've performed a full tune-up, used Seafoam and Techron, replaced all spark plugs and fuel injectors, swapped coil packs, and conducted a compression test showing 150 PSI on cylinders 1 and 3, and 135 PSI on cylinders 2 and 4. Notably, there is significant soft black soot buildup at the intake ports of cylinders 2 and 4, which suggests incomplete combustion. The vehicle runs smoothly for a short period after starting but returns to its original behavior. I recently acquired a leak-down tester, though I'm uncertain about how to properly position each cylinder at TDC. The traction control light typically activates during stops when the engine is below 120°F (ambient temperature), and it does not appear if the vehicle is placed in neutral. This behavior is inconsistent with normal operation. I confirm that my car is stock, with no aftermarket modifications, and I use Mobile One Extended Performance 5W-30 oil. My OBD-II scanner reads live data and historical codes. The engine is currently at 103,000 miles. I've also confirmed that the ABS warning light turns off after the traction control message appears. I'm seeking expert advice on diagnosing this P0304 misfire—especially given the soot buildup and intermittent symptoms—and whether fuel type (E85 flex-fuel option) plays a role in the issue. I've switched from regular 87 octane to 89 octane, which may have improved performance slightly but hasn't resolved the problem. Additional questions: Is there a known GM TSB related to P0304 misfires on this model? What vacuum leak checks should be performed—particularly around cylinder #4? And how can I properly trigger the I/M readiness status on my TopTak TP-280 scanner?

Related fault codes

P0304

Comments (7)

Anonymous 2015-12-15 09:50

I don’t believe a leak-down test is necessary at this stage. Let me provide guidance later. Are you experiencing only P0304, or are there other related codes? Is the traction control light intermittent? Did you remove the intake manifold during any repairs? Can your OBD-II scanner read live data and historical fault codes? Is the vehicle stock with no aftermarket modifications?

Anonymous 2015-12-15 14:33

Yes, I only see P0304. The traction control light comes on primarily during stops—especially when engine temperature is below 120°F. If I shift into neutral at a stop, the light does not activate, which seems unusual. I did not remove the intake manifold; I only replaced the fuel rail when changing injectors. My scanner supports live data and historical code reading. The vehicle is completely stock with 103,000 miles. I follow a regular oil change schedule using Mobile One Extended Performance 5W-30.

Anonymous 2015-12-15 15:28

Traction Control (TC) often activates alongside ABS—check if the ABS warning light turns off when the TC message appears. If not, there may be an underlying ABS fault. With your scanner, check the history logs for any prior codes. When the TC light is on, perform a live scan to capture freeze-frame data including ECT (engine coolant temperature), manifold pressure (MAP/MAF), and throttle position. For P0304: Spark plugs are AC Delco; no K&N air filter or performance modifications. Thermostat is set at 195°F. Try a vacuum leak test on cylinder #4 using the old-school spray method—observe if the intake port appears wet, dry, or black/tan. Check for intermittent leaks near #4, such as in PCV or EVAP hoses. I recall a GM TSB related to misfires in cylinders 1 or 4—worth checking. Is this a flex-fuel E85-equipped engine? What fuel are you currently using?

Anonymous 2015-12-15 23:33

Found a relevant thread at http://www.chevyhhr.net/forums/problems ... tec-54425/ This is my own post from earlier—just sharing background for reference. Yes, this is an E85 flex-fuel engine. I’ve switched to 89 octane fuel (from regular 87), which has improved performance slightly. Spark plugs and injectors are both AC Delco; thermostat is standard at 195°F. The ABS warning light turns off after the traction control message appears. Cylinders #2 and #4 show heavy black soot and slight moisture at intake ports—this may indicate fuel delivery or combustion issues.

Anonymous 2016-01-01 12:55

Confirmed: This is an E85 flex-fuel engine, currently running on 89 octane. Previously used regular 87 octane; switched to better fuel for improved performance. Spark plugs and injectors are AC Delco, thermostat is standard at 195°F. ABS light turns off after traction control message appears. Cylinders #2 and #4 show heavy black soot and slight moisture at intake ports—this may point to combustion inefficiency or fuel delivery problems.

Anonymous 2016-01-08 21:46

Hi, can anyone advise where to properly install a vacuum gauge? I tried connecting it to the EGR line from the valve cover to the throttle body—but there’s no vacuum reading. There's a hard plastic hose that runs into the intake manifold but disconnects at a solenoid, so it doesn’t provide consistent vacuum. Is it safe to attach a T-fitting to the brake booster line? Also, on my TopTak TP-280 scanner, I can't get the I/M readiness status to show as 'ready.' How do I trigger that state?

Anonymous 2016-01-09 10:01

You can safely install a T-fitting into the brake booster vacuum line—just be cautious while driving. To achieve an 'I/M ready' status on your scanner: first, clear the codes and allow the system to go through a drive cycle. After clearing, perform a full drive cycle (e.g., city driving with varied speeds) to trigger readiness. This will show the I/M status as 'ready' in your scanner. Reference guide: http://www.obd-codes.com/trouble_codes/ ... -cycle.php