94racing

i dont have a way of testing for exhuast backpressure at the moment. i replaced the injector on 3 and im going to pick up a coil pack tommoro and swap the injector and coil onto different cylinders and see if i can get rid of the preignition im getting. i figure that if i cure that then the cat will be fine i assume. the preignition as im told is caused by a more explosive mixture(lean) causing the cat to be getting too much oxygen(says the gm mechanics)
so my questions just to make sure is...
the engine will preginite cold btw. just so you know its not the cylinderhead getting super hot.

1. there any way high backpressure could cause preignition
2. would either po121,po420 or po303 cause preignition or is there anything that would cause preignition?

EinST

These "gm mechanics," did they actually run diagnostics on your vehicle or are they just shooting the breeze? If the former, maybe you could post the freeze frames that go with all these trouble codes so we can see what they saw. That said, I really think you should consider maybe taking it to an exhaust shop. They should be able to check the exhaust back pressure at least.

As for my theory on your questions:

1. Yes, high back pressure can cause "preignition," or more correctly detonation. It interferes with the normal "breathing" and thus raises the combustion chamber temperature. This causes uncontrolled combustion, i.e., detonation. If left unchecked, it can get ugly with damaged pistons and valves.

2. As I stated in an earlier post, P0420 can coincide with the catalyst substrate breaking up and causing high back pressure. This in turn can cause the misfires (detected as uneven contribution to the crankshaft output) and possibly P0121. The latter says the TP sensor did not agree with predicted value which would be the case when you step on the accelerator but the engine bogs down instead.

94racing

Doesnt bog when you hit the gas at all. It runs out of power as you increase in rpms then slams into second as if somebody just rearended me. Service traction control and service stabilitrac comes on engine light flashes for 15 seconds then stabilitrac and traction control come back online.


Data from backpressure test
Driver.
Idle 0
2000 2psi
3000 4psi
4000 6psi

Passenger
Idle 0
2000 0
3000 2
4000 4
Also the air gauge was shaking from 0-4 so i put the high # readings down.
So are you saying that 6 psi i enough to cause a miss?
I put the new injector in 3 and moved coil 3 to 1. 1 to 5 and 5 to 3 so i will know if it is a coil out if the miss comes up as a po301.
I also had a freind ask if my alternator was good which was something that concerned me. It reads 14.7 volts at idle and 16 when higher in rpms.
My moms 08 denali does the same thing though so i have no idea.

EinST

Did that mechanic dude get the whole story (since your symptoms seem to keep changing)? If those pressure readings (pre-catalyst, I assume?) are to be trusted, you have a clogged bank 1 catalytic converter. A post-catalyst restriction would affect both banks. P0121 is because the throttle is open more than engine operating parameters would warrant, due to the "breathing" problem. You can expect random misfires from cylinders on bank 1 (odd numbered cylinders). I think you should quit wasting time and just replace the clogged converter. That's what I would do. Let us know how it works out. Good luck.

94racing

Ive just been trying to do this as cheaply as possible since im a college student which with if inreplace the cats wont be cheap on what i decided. Im going to drive the truck tommoro as i have got the codes cleared again today and so ill see if it has another po300 multiple cylinder misfire code pop up with the po420. If so then im ordering a new ypipe with both converters prewelded into it so when i sell it it will be done right. If the code thrown is a po301 then coil pack will be replaced and we will try that again so that im not welding something in that i cant turn around and resell on craigslist as an ls2 coilpack(i think thats what its identicle(spel) to) if you get what im saying. Also i got the freeze data from the scanner and i wrote down what i could.

Rpm 4947
barometric pressure 100kpa
Load value 59.6
Airflow rate 189.95 gr
Map sensor 92kpa/sec
Ambient air temp 19F
Coolant temp 174
Intake air temp 54
Ignition timing 16 degrees advance
Fuel rail pressure. 300kpag
Cat temp. B1(driver) 1180 B2(passenger) 1162
Controller voltage 13.8v


Thats all i got from it the fuel trends all said 0.

Thanks for all you help einst i really appreciate you putting up with my ignorance im used to carburated dirt motors, pushrod engines and racing quad engines and it takes me a little bit to catch onto things.
Ill keep you updated. Im trying to get the truck fixxed asap and its taking all of the time and $ i have at the moment.
Again thanks.

94racing

ok so will this fit my truck? or would i need to order one from
eastern catalytic company 2nd add down. it wont come up for an 08 so this makes me uneasy
07 2007 Chevrolet Silverado 1500 Catalytic Converter - Exhaust - Bosal, Eastern Catalytic - PartsGeek

or do i need to order the one from this site

3rd down catco brand

Discount Catalytic Converters - 2007 Chevrolet Silverado 1500 LT 8 Cyl 5.3L


i know they are cheaper than the rest but i just want it fixxed so i can get the truck to the point where no cel pops up and i can sell it. thats really all i have been trying to do it make it sellable.

quick responce would be appreciated as i am waiting to order it.
thanks
ty

94racing

i put a new ypipe on and its fixxed i have never had this much power before so i always thought it was a slug. my question is after driving it should the engine light go off or will i have to take it and get it cleared? ive drove it 60 miles and i was thinking the light would have went off. thanks for the advice also.