2002 chevy-suburban 5.3 4wd p0300 Problem Need help

[kamelred408]

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

 

[kbuskill]

I am not super familiar with the GMT800 platform.

On the GMT900 trucks there is a harness that goes between the coil packs and the main engines harness, so that you could unplug it from the main harness and remove all 4 coil packs while still attached to the bracket as an assembly.

I have seen that harness go bad and cause the same issues you are having.

I assume the older LS engines use the same type of harness.

Looks like this...

Hope this helps.

 

[kamelred408]

ok thanks im doing a valve cover job on it now so ill swap them and see if miss moves to bank 2.... that is a great idea thanks bud

 

[Fless]

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?

Suspect the check valve in the fuel pump, with it not holding pressure overnight. Can you monitor the pressure after you shut it off, to see how long it takes for the pressure to go down? Document elapsed time and the resulting pressure.

 

[Larryjb]

Intake manifold gaskets are a common failure point, and would certainly cause a misfire on one side only.

 

[kamelred408]

10 psi dropped in 5 min and the rest took about 30 more min



ill spray the intake with some brake cleaner im making a home made smoke tester as well


ill keep updated


i took the valve cover off to make sure no lifter where stuck . all are moving the pcv was toasted so i ordered one and i broke vacuum line for it as well so have to wait until tomorrow

 

[swathdiver]

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

I don't think your fuel pressure problem is related to the misfires and catalytic converter performance. You can swap downstream O2 sensors but I think you have a bad cat. Now, what caused it? Might want to do compression test and maybe a leak down on Bank 1 to eliminate a bad head gasket. Any oil or coolant loss or contamination?

 

[Larryjb]

I see now what might be happening. If you have a few injectors on the one bank that are leaking, the following may be happening.

Both banks move to negative FT, and your fuel pressure falls. Your 2nd O2 sensors also switches. This is suggesting to me that you are getting extra fuel injected, so the fuel trims are trying to back that extra fuel. Because your FP is falling, that suggests this is real. The extra fuel is creating a rich mixture that leaves unburned fuel that the catalytic converter is unable to deal with, which is why you see the 2nd O2 sensor also switching.

What are the long term and short term fuel trims?

 

[Larryjb]

Oh, just in case you have two problems, pull the vacuum hose off the fuel pressure regulator and smell the hose. If you smell gasoline, that is one reason why your FP is dropping.

 

[kamelred408]

I don't think your fuel pressure problem is related to the misfires and catalytic converter performance. You can swap downstream O2 sensors but I think you have a bad cat. Now, what caused it? Might want to do compression test and maybe a leak down on Bank 1 to eliminate a bad head gasket. Any oil or coolant loss or contamination?

I broke my pcv valve hose during the valve gasket repair i am doing atm. im going to swap the o2 sensors and see if any change tomorrow when i get the new pcv and hose. i have oil leak from both valve cover gaskets but nothing else that i can see. i am going to do a compression test as well do you know the specs for this car?

I see now what might be happening. If you have a few injectors on the one bank that are leaking, the following may be happening.

Both banks move to negative FT, and your fuel pressure falls. Your 2nd O2 sensors also switches. This is suggesting to me that you are getting extra fuel injected, so the fuel trims are trying to back that extra fuel. Because your FP is falling, that suggests this is real. The extra fuel is creating a rich mixture that leaves unburned fuel that the catalytic converter is unable to deal with, which is why you see the 2nd O2 sensor also switching.

What are the long term and short term fuel trims?

all the injectors are new and sealed. i tested them one by one with my scanner i posted the psi in org msg. but i could see how that would be a problem if they were leaking....

my FPR and F Pump are both bad. but when the car is on it has good fuel pressure. i am going to replace both but i want to get this misfire figured out first. i like your thinking on this tho. if i had leaking injectors on that bank i think you could very well be right.

I am not super familiar with the GMT800 platform.

On the GMT900 trucks there is a harness that goes between the coil packs and the main engines harness, so that you could unplug it from the main harness and remove all 4 coil packs while still attached to the bracket as an assembly.

I have seen that harness go bad and cause the same issues you are having.

I assume the older LS engines use the same type of harness.

Looks like this...
View attachment 238474
Hope this helps.

i swapped this harness tonight while doing the the valve cover gaskets.
once i get the car running again tomorrow i will be able to see if you are right. great thinking and thank you for this idea... i do hope it is something simple like this harness