Hello all i am new to this forum and am looking for some help.

2002 chevy-suburban 5.3 4wd p0300 Problem

Parts That we have replaced with new OEM.

All Spark Plugs
All Spark Plug Wires
All Ignition Coils
All Fuel Injectors
Fuel Filter


I had not done those and the car has 200k Miles so i figure its about time anyways .
I do not want to just buy parts and hope for a fix.

I have an Autel mkx808 car scanner and can do live data and several other things with it.



What i have found is the misfires are happening on BANK 1 only

Cylinders 1 , 3 ,5 and 7 Misfire
Cylinders 1 and 7 Misfire the most.

When i drive the car it has a major loss of power and the CEL flashs at me.

here is what i have tested so far.



Live Data shows

Car at operating temp and at idle
Bank 1 Heated o2 sensor 1 Moves up and Down from 100 mv - 900 mv like normal

Bank 2 Heated o2 sensor 1 Moves up and Down from 100 mv - 900 mv like normal

Bank 1 Heated o2 sensor 2 Moves up and Down from 100 mv - 900 mv THIS IS NOT Normal

Bank 2 Heated o2 sensor 2 Stays steady 750ish mv

I thought maybe the cats where bad so i did a temp test and it seems fine im going to do another temp test tonight to make sure ill edit in the exact temps later.

Could it be the bank 1 cats are bad and causing a misfire on bank 1?


My fuel trims stay around 0 and move to -10
Bank one fuel trim moves towards -10 more then bank 2





Fuel Psi
Checked after car sat all night 0 psi
checked when key is turn but engine is not running 58-60 psi then starts to drop
Checked when car is running stays stead at 60 psi

fuel injector psi ( i have this feature on my scanner)
All Injectors Shoots up to 67 psi then down to 44 psi ( they are all new)


now some times after the car sits i have to prime the fuel lines to start the car so i know the pressure is dropping.

I dont see how that would cause a misfire one bank 1 only tho.

A new fuel pressure regulator is my next item but may need a fuel pump?


im building a smoke tester and have not done a compression test yet.



what could this p0300 random misfire be. its driving me crazy and i need the truck for a family trip soon.

any and all help would be great thank you

 

how about you start by not using an unreadable light grey color and size 100 font.

what is the fuel pressure while misfiring is occurring?
you may also want to post your freeze frame data.

 

60 psi while misfire is happening.

idk why the font did that but i cant edit it now.

 

you can pull the plugs out and visually check that they're actually sparking properly.

bad crank/camshaft sensors can also cause misfire.

 

any way to test the crank and cam sensors?

 

probably with a scope. I don't know if there's a DIY way. Try Google.

 

I had the very exact problem on my son's Pathfinder. I changed many of the same parts you did with no success. I then found that the rear cat on the drivers side was clogged. After fixing this problem it ran good again. I unbolted the rear cats and left the pipes open, then started it and it ran good again with power. This told me that the rear cat was bad.

 

good point. If you have cats on only one bank then yeah that's a possibility. Smack it with something to see if it rattles, or you can test the theory by either drilling a small hole before the cat, or taking out an O2 sensor if there is one, and see if it runs better.