P0300 code driving me nuts

Jorgeliebes817 · August 13, 2016

Hi, everyone. So I have a 2003 4.8L Vortec. It has a P0300 code. The symptoms, other than the misfire, are: will start to idle rough in the morning for about 2 minutes then will straighten itself out. Can drive all day long with no issues. But, as soon as I come to a stop (traffic light, gas pump) once I take off it will start misfiring, backfiring, etc. I can hold the gas at 100% and the RPM's will not go above -/+ 1500. After about 5-6 seconds it takes off like nothing was ever wrong. When it misses, the check engine light will start flashing, signaling a rich mixture, but when it straightens out the light goes completely off. It doesn't stay on. So it's as if the computer has short term memory and forgot all about the misfire. Another thing that has me scratching my head. Only bank one misses. When it's rough idling at its worse, I can unplug coils 1,3,5, and 7 and it will not change a thing. I plugged them back in and as soon as I unplug either cylinder on bank 2 it runs even more rough.

I've replaced spark plugs, boots, fuel filter, fuel pump, tested fuel pressure, tested coils.

Any ideas? Thanks in advanced..

anumber1 · August 13, 2016

Look for a leaking intake manifold gasket or other source of un-metered air entering the intake.

That you have narrowed it down to missing only on one bank makes my guess the manifold or gasket.

mawint · August 14, 2016

I have a 5.3 but I had similar symptoms once before. Turned out to be the upstream O2 sensor in the side having issues.

Oreillys I believe can show you the freeze frame of the O2 sensors when the code was thrown with their code reader. I can't remember if it showed a negative for upstream or that they were both flat.

Jorgeliebes817 · August 15, 2016

Anumber1, i appreciate your advice. I tried testing for vacuum leaks starting post MAF all the way to the brake booster vacuum hose. Nothing seems to be leaking. Tried it cold and at operating temperature.

Jorgeliebes817 · August 15, 2016

Mawint, i crawled under the truck while it started to miss. I moved wiggled the upstream O2 sensor pigtail and it seemed to want to straighten out. I left it running for about 20 mins trying to get it to miss so I can hook up my DTC reader since it also has freeze frame feature but it wouldnt. I revved it, red lined it, it wouldn't rough idle the whole time. So I'm guessing the O2 had a false connection or something. I pulled the pigtail off to check for corrosion but I didn't see anything. State emissions Inspection is due and I don't want it to decide to start acting up once I take it. But I don't want to buy a $75 sensor "just in case".

anumber1 · August 15, 2016

Good to hear you narrowed it down!

ChevroletTough · August 16, 2016

I had a very similar issue, checked everything, literally everything.Truck's CEL would only flash above 2500 RMPs though, when in drive and when in park (reving and holding steady). Ended up being the crank shaft sensor. Needed a new one, and it needs a crank shaft sensor relearn. Keep us posted.

I have a thread on hit.

Jorgeliebes817 · August 17, 2016

ChevroletTough, I have something similar. Sometimes the check engine light will come on, but when in park I barely hit the gas and it'll flash. I'm assuming that's a dealer fix as far as the relearn.

ChevroletTough · August 17, 2016

ChevroletTough, I have something similar. Sometimes the check engine light will come on, but when in park I barely hit the gas and it'll flash. I'm assuming that's a dealer fix as far as the relearn.

That is correct. The dealership needs to do the GM Tech II crank shaft relearn.

The sensor was cheapest on amazon.com if I recall. I double checked the part # with delphi.com

https://www.amazon.com/gp/product/B000C9JXZ2/ref=oh_aui_detailpage_o03_s01?ie=UTF8&psc=1

This was for my 2004 Silverado with a 5.3L. With prime shipping I couldn't find a better deal.

The sensor is easy to replace (you have to drop the starter). The dealership did it all for me for around $80 as it requires a relearn. Otherwise I would have just replaced the sensor myself.

Best of luck, this issue took me a year of battling to resolve. A GM dealership can check that specific sensor to see if it's reading incorrectly as well.