Mechanics

CHEVY MULTIPLE CYL MISFIRE DTC P0300

2006 Chevrolet Cobalt

Engine Performance problem
2006 Chevy Cobalt 4 cyl Two Wheel Drive Automatic 90500 miles

Drove the car to work last Thursday morning and everything was working great. When I went out after work it started but the engine was shaking like it had a misfire, and my check engine light was flashing. Rented a code reader from Auto parts store and I got a DTC of P0300 (Multiple Cyl misfie). Did a basic tune-up replaced plugs (were bad), replaced air and fuel filter (fuel filter was dirty), checked for vacuum leaks (none), checked PCV (clear).

After I did all of this I still had the really bad idle and a smell of gas in the exhaust. Next step to me was to Replace the Crankcase Position sensor and I noticed that there was no longer a smell of gas in the exhaust but the issue is still there. I have a lot of pressure out of my exhaust pipe, and I banged on the CAT to see if it sounded hollow or solid, sounded hollow.

I did notice that when reviving the engine to see if the issue still presisted at higher RPMs the engine dies at 4K RPM. Not sure if it's the CAT producing enough back pressure to kill the engine or if it is something else.

I used a higher grade of tester and noticed that while the engine is running the timing kept advancing and retarding many degrees at idle. I saw up to 13 degrees on the high end and 6 degrees on the low end. Is this normal? I have never heard of an engine trying to adjust it's timing so much at idle.
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Jgomez123
November 9, 2010.



I think the next step for you is to have the injectors professional tested and flushed. The code leads to injector issues. Also check the injector harness to see if the wiring is rubbed through anywhere.

Gmtech
Nov 10, 2010.
I couldn't find a shop that would test and clean the injectors today so I went and individually disconnected the injectors starting from left to right. Here is what I got.

1. Disconnected first injector connector--engine idled about 500 RPM and died. This tells me the first injector is working.

2. Reconnected first injector connector and disconnected second injector connector--Engine ran rough but idled at 1K + and erratic. This tells me this injector is suspect.

3. Reconnected second injector connector and disconnected third injector connector--Same results as second injector. Suspect this injector is also malfunctioning.

4. Reconnected third injector connector and disconnected fourth injector connector. Same results as disconnecting the first injector.

This all leads me to believe that the second and third injectors are clogged or malfunctioning. So to test this theory I swapped the third and fourth fuel injectors and expected the engine to run really rough. This was not the case, engine ran the same (1K + and erratic). So I disconnected the third injector and the engine ran the same. Reconnected the third injector connector and disconnected the fourth and the engine ran the same. I did the same with the first and second injectors (swapped injectors and individually disconnected them) engine is still running the same.

Tiny
Jgomez123
Nov 10, 2010.
I am having the same issue with my 2007 cobalt. I have tried all the previously mentioned steps. Did you ever find a solution?

Tiny
Tessasargent
Jan 24, 2013.
Did you ever get the injectors checked by a shop to see about proper spray

Gmtech
Jan 30, 2013.
Another possibility could be your ignition module and or wiring. Check the wiring where is is routed near the trans cooler lines and see if it is ground out.

Gmtech
Jan 30, 2013.