P0300 and Weird Engine Issues
 

Just picked up an 04 Deville(base I think) and it has a P0300. The engine is idling rough at idle, but gets a little better when applying gas in Neutral. Also, it spells like fuelOriginally it has a P0302 but I switched the coils and it became a P0300. I've replaced the spark plugs, sprayed the MAF sensor behind the air filter, replaced three coil packs that tested bad with an ignition spark tester.

At this point, I figured I'd post up here for some troubleshooting steps since it seems this code is pretty widespread and I'm hoping I fall into the category of easy fix. Anyone have any suggestions or somewhere I could start with the troubleshooting?

Thanks in advance!

Scanner would be handy fuel trims maybe useful. Post if you have scanner. I would check for vacuum leak at underside of boot between intake and EGR housing.

I do have a fairly in depth scanner(I think it's in depth). I have this: http://m.harborfreight.com/can-obd-ii-deluxe-scan-tool-62119.html

What's the best/easiest way to check for a vacuum leak at that location?

https://imgur.com/gallery/t87lu

I also included the one spark related test because it was fluctuating anywhere between 6 and 20. Not sure if that's pertinent but wanted to include it anyway. Is this scanner giving the info I need?
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Any ideas?

I'm unable to open file. If you spray flammable aerosol below boot does rpm change.

https://imgur.com/gallery/t87lu

Does making it this link above help?

The boot between the air filter housing and throttle body or between the throttle body and "big black spiral housing" (sorry, brand new to Cadillacs)

I sprayed around both areas and no change to the RPM. The engine is still shaking like crazy like it wants to come off the motor mounts :)

That worked. MAF seems too low which could indicate a vacuum leak. Does it change with throttle? Do you know what fuel pressure is? Should be in mid 30's running. Do bank 1 and 2 sensor 1s go from .1 to .9? You could try to disconnect injectors momentarily to see which cylinders a not firing. If it's the same cylinder(s) as coil previously replaced ignition module may have failed.