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Starting about a week or two ago, my 3.4L engine (2004 Chevy Monte Carlo LS) has thrown a p0300 code, random misfire. Here are the things I've done:

  • Changed spark plugs and wires
  • Knocked fuel injectors (tapped them with screw-driver and hammer)[it is a mess of liquid under the engine cap, but I'm not concerned with those - should I be?]
  • Reconnected bank 1 upstream oxygen sensor (it was unplugged and I had no idea that it was)
  • Had both my battery and alternator tested (volts, CCA and CA). All checked out.
  • Changed oil 1.5 months ago
  • Replaced many parts of the PCV and other air-intake valves. A lot of them have been melting apart so I've changed to a mix of plastic and rubber heat-shielded hoses. Last time I had a series of misfires, it was because my PCV broke in half. I don't see anymore leaks or damages here.

More specifically, when the RPMs get about 500-600 (about normal idle) the engine will ... I believe "misfire" and the rpms shoot up to about 1000-1200. Then about 10 secs later, they drop back down to 500-600 and repeat this 'shoot' bakc to 1000-1200. This only happens when I am stopped.

When I am driving, I seem to feel a "jitter" when I get in speeds near 50-70. As if the car struggles to go faster. It will decelerate fine.

A tech at O'Reilly's suggested I check the engine ground wire. I haven't done this yet, but he said it wouldn't hurt to see if it's got any wear or damage to it. Is this worth looking at? I'm not sure where the ground wire for my engine is at, and it's a pain to look through things under my hood (tight space, small car, low to ground).

I also have a question as to what this open port might be near the PCV (pics below). The whiter pic has the port in quesiton circled.

Any suggestions or comments? Need more info? Is it a coil pack that needs changing in the higher-numbered engine cylinders? I have three coil packs (top-back of engine casing)

Images of Engine Engine layout

Unknown port

EDIT

It might also be worth mentioning that the car doesn't always start when I turn the key UNLESS my foot is pumping gas (in cold or hot weather). Without pumping the gas, it's a risk if it turns over or not (else it just leaves the battery on and engine never starts). With pumping the gas, it always will turn over and engine starts,

EDIT 2

Upon further listening of the engine, I can hear either a "sucking" or a "blowing" sound (like the sound high-pressure liquids make or an air can might make, but not a hissing sound) near the EGR and PCV valves (look in the images to see where they are). Also note, the EGR is heavily rusted. Should this be changed out? There is very fresh liquid around one of my fuel injectors (closest to this sound I'm hearing). Closing in as this being the problem.

  • How's your vacuum line? Is the rubber hose perished/cracked? The idle-hunting and hissing symptoms definitely make it sound like you have a vacuum leak. – Zaid Aug 14 '14 at 6:36
  • There isn't any "hissing" except for a low-pressure sound running through a hose at regular intervals - is what I meant. But yes, still suspecting some form of air-related problem. Going to take apart the air filter housing and hose and clean them out. – Bashkir Aug 14 '14 at 16:24
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Have a gas test carried out on your vehicle, it is often forgotten just how much information a gas test provides. Among the causes for P0300 are bad plugs, bad plug wires, bad ignition coils, leaking fuel injectors, massive air/vacuum leaks, and EVAP system faults. If you can have your vehicle scanned using an OBD scanner and also by a scanner compatable with the vehicles manufacturers. The OBD scanner will show the engine sensors and actuators actual values, whilst the OEM scanner can go deeper but may include substituted values.

  • A bad ignition coil was it. For some reason I never bothered on a spark test. Two wires were receiving no spark along them. Traced it back to a completely dead coil. – Bashkir Nov 11 '14 at 4:18

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