Thread: Chasing P0300

Hey guys, hoping you can help me out here.

Past couple days I have been getting a recurring P0300 DTC.

Car is a 2004 GTP Comp G with 96k

New parts/tests:
Plugs Delco
Wires Delco
Coils removed cleaned and tested
ICM cleaned and tested
All of this was dont after receiving the code the first time

Symptoms:
Car runs pretty much normal, slight rough idle, seems to loose power under heavy load

Hooked scanner up (Actron CP9145) I know its not the best but it's all I have and here is what the live data reads as it shows on screen:

This was at a fully warmed up idle from a cold start and with code cleared. In closed loop.

ABSLT TPS(%) 3.5
IDLE 700-780 (fluctuates in this range)
CAL LOAD(%) 1.6
MAF(LB/M) 0.65
MAP("HG) 10.9
COOLANT 194
IAT 91
IGN ADV(DEG) 18-22.5 (fluctuates in this range)
LT FTRM1(%) -5.5
ST FTRM1(%) 0.0-4 (fluctuates in this range)
02S11(V) 0.160-9.45 (cycles up and down)
ST FTRM11(%) 0.0-3.1 (fluctuates in this range)
02S12(V) 0.850 (stays steady with very little changes

Using this can anyone point me in a direction I haven't gone yet?

I greatly appreciate any and all advice.

Seems like that vacuum pressure might be low. Normal is 20" or so. I would be tempted to say chase a vacuum leak but your fuel trims are normal.

Also your rear o2 bias voltage should be changing up and down. Pegged rich like that could indicate something is going on. Sorry im not being more specific, Im just pointing out a few oddity's.

Originally Posted by lewis1952

can be poor electrical connection to coil igniter, to an injector poor wiring connection to injectors or fuel pressure

Fuel trims are normal and there are no injector codes so we can probably eliminate injector connection issues, He also said that he cleaned the icm plate and coils / basically tuned up the entire ignition system.

Taken from ls1tech.com credit to joecar

Vacuum is related to MAP as follows: VAC = BARO - MAP


BARO is appox 29.9 inHg at sea level
Your MAP is 10.9 inHg
VAC = 29.9 - 10.9 = 19 inHg which matches your vacuum gauge reading.

So reading that I'd say my MAP is correct and I have no vac leak lol. Also i tested with starter fluid in the usual areas, no idle change. Also should mention I'm running 93 octane and have tried sunoco, shell, and citgo all with same results, still stumped. Luckily it is still under warranty so if I can't pinpoint it with your guys help they can figure it out haha

Originally Posted by frosty

Also your rear o2 bias voltage should be changing up and down. Pegged rich like that could indicate something is going on. Sorry im not being more specific, Im just pointing out a few oddity's.

The one that is pegged is labeled O2S12. Safe to assume the reader is calling this bank 1 sensor 2 which would make it the downstream correct? And if it is could I possibly be looking at a dying Cat?

Possibly but it also be acting like that due to the missfire. Most of the time you'll have a major power loss with a bad cat

Chalk it up to a clogging catalytic converter. Thanks for the help everyone in narrowing this issue down. She runs so much better with being able to exhale. By the wat the misfires were all on the rear cylinders, that's what made us pull the cat and crap just fell out of it all over the shop floor lol

Never noticed a lack of power since I have only owned the car for 3 weeks. And yes it runs smoother, cooler, more responsive and gained 1.5mpg. It's just a universal cat for now that my buddy had laying around in the shop. Once I put headers on Ill either run no cat or a Magnaflow High Flow cat since thats what I always used on my F-bodies. Now All I am waiting for is the trans to get rebuilt and Ill be in business. Warranty has that covered though Dealership is even covering the cost for upgrades to tougher parts and a shift kit