P0304 Engine Code
 

Two weeks ago, P0304 (#4 misfire) set on my '09 HHR SS with 142K miles. Not having access to a scan tool, I applied some backyard mechanic troubleshooting, and swapped the #4 ignition coil with #2. I then cleared the code (by disconnecting battery for a few minutes). Lo and behold, the CEL came on again after a few days. I had the local parts store get the code again. I was hoping the problem would have moved to cylinder #2, but it didn't. P0304 reset again. Additionally, P0300 set this time, which I am guessing is just the byproduct of #4 acting up. I'm not sure where to go from here.

All spark plugs were replaced at 100K with AC Delco iridium type, so I don't think that's the issue. While swapping the ignition coils last week, I inspected the #4 spark plug. It did not show any signs of problems (I. e., it was not damp from oil or coolant saturation). It was the normal tan/light brown color from a properly firing plug. The electrode looked okay. I took pictures of the plug and can share them here if anyone wishes to see.

The engine is noticeably misfiring on cold startup. The issue seems more prominent when the ambient temperature drops below 30°F. I was going to add Heet to the gas tank, wondering if I got water in the fuel somehow. Beyond that though, I'm not sure where to go from here to troubleshoot this. Any thoughts?

Thanks in advance,
M Ridzon

P0300 means that the computer has not collected enough data to pin point which cylinder is misfiring.

Next step is compression and leakdown. Fuel pressure.

Don't be so quick to rule out spark plugs. Do the same swap with the spark plugs that you did with the coils.

It also could be fuel related(injector), or like donbrew suggests, mechanical.

This weekend, I will troubleshoot further. I have a few questions:

1. What pressure range should I be in for the compression test?
2. Where is the fuel pressure test port? Is it the Schrader valve located just in front of the valve cover, near the tranny end (i.e., not the serpentine belt end of the engine)?
3. What is the testing procedure to check fuel pressure? Do I merely cycle the key to the Run position a few seconds and check the reading? Should it then hold pressure for a certain time?
4. What is the expected fuel pressure?

After doing these, assuming good results, I'll then swap spark plugs from another cylinder to see if the code changes.

1. Most important is that they are nearly the same; if #4 is more than 10 PSI lower the problem has been found.
2. yes.
3. that will test the low pressure pump, which is of more interest. Engine running will give the high pressure, that is variable and get very high in the 1500 PSI area, best to check with a scantoo or RPD if you have itl. Be aware of gas spraying all over.
4. My memory thinks around 150 PSI.

Key on, engine not running, about 70 psi.
Cold engine at startup I'll see about 2200 psi
Warm engine at idle, around 350-450 psi.
(all these readings are from my RPD)

So let me get this straight...the car has 2 fuel pumps?? One low pressure and one high pressure? Which one does the Schrader port test (i.e., which one is upstream)? How would I know if one or the other is the problem?

Electric pump in the gas tank that supplies fuel to the HPFP (high pressure fuel pump)
Mechanical, cam driven HPFP.

Disregard the red circle:
https://cimg8.ibsrv.net/gimg/www.che...4da7fba40c.jpg

So is the Schrader valve port upstream or downstream of which pump? In other words, I want to know which pump is giving me the pressure reading at the Schrader valve port. From looking at the engine, I'm going to guess that the Schrader valve port only reports the pressure from the electric pump in the gas tank. Please clarify.

That schrader valve is before the HPFP, so yes, you would get the in tank electric fuel pump readings from that port. Not sure but I think the only way to check the HPFP pressure is through the OBD port. A Tech II scanner or maybe even one of those apps and your phone can monitor the HPFP.
I have the Torque app and it will monitor fuel pressures. Not sure if the Torque app measures the high side or not. Luckily I have the RPD unit on my SS that I can monitor a lot of stuff.