random misfire 03 chevy 5.3l

[Kota2694]

Alright so I've got a 03 Chevy 1500 with the 5.3l Im fighting a miss truck has a brand new engine, new crank and cam sensor, fuel pump was replaced a year ago along with wires, has brand new ac delco proffesional platinum plugs. Just looking for suggestions. Being a random/ multiple cylinder miss I assume its not my coils,plugs, or wires. The downstream o2 sensors are no longer in place. Truck has headers going straight to a y pipe. Could that cause a miss? Messing up the fuel air mixture? I want to get the simulators for the rear 02s any thoughts on that? Hooking up a scanner I have like 11 codes coming up. All on the o2 sensors and then the miss fire. My rpms at idle bounce between 400 to 600.. could the purge valve cause any of this? Is there a way to test it? I plan to test my fuel pressure tonight. Along with finding a better scanner so I can see which cylinders are missing and how badly. Someone on here mentioned one cylinder missing could throw the rest off, is this true? I'm still learning on these engines and I'm stumped. Any help would be greatly appreciated

[Kota2694]

Also the truck stumbles under light accel but does okay if I open it up. Maf sensor was replaced a few years ago but is there a way I can test it? Trying to eliminate as many factors as possible till I find the cause. Had this issue with my old engine as well before the#8 piston skirt decided to fall apart. Also on another note the new engine I put in has a roller can and has been bored. The place I got it from said I need to tune the pcm in order to get full power. My question is can a tuner take care of this? Or what do I need for it? Trying not to have to take it to a shop.

[rdnkcpa]

Did you get the P0300 code? Are you getting cylinder specific misfire codes?

 

Not having the O2 sensors, I would think, be contributing to this.

 

Check with the easy stuff first. Fuel pressure, vacuum leaks etc.

[Kota2694]

I have the cheap little scanner that will just read and clear codes. The code was p0300 if I remember right, said random/multiple misfire. I'm hoping to hook a actual scanner up to it tonight to see which cylinders are missing and how often, checked fuel pressure tonight its fine.. just making sure I read your comment right. You said the 02s could cause a miss? I have no downstream o2s due to headers being put in, wires are just hanging there. I believe the fronts are fine just gotta get the sim's so my check engine light will go away. Do you know of a better way other then the simulators? From what I've found online that's my only option short of putting cats back under it.

[Kota2694]

Well my misfire is bouncing around all cylinders. Decided I'm going to send my computer in, have my downstream 02 canceled, minor tune up to accommodate for what's been done to the engine. Replace front 02s and hopefully my mis will be gone possibly step up my injectors as well. Fingers crossed.I'm ready to have access to the power this engine should have.

[Ghostrider66620]

you could get two o2 fittings and install the rear o2 sensors behind the cats location. if there's only one pipe ..put them both in the same pipe close to each other.

[efritz]

did you perform a crank relearn?