Infamous P0303 with no obvious cause

[02Z71Raven]

New to me 2008 tahoe ltz, 0 vin engine code, with 172k on the clock.
I've swapped injectors, coils, plugs, wires, and replaced injector connector.
Rough idle at cold start, runs smooth(er) when warm, but still not smooth. After a while it does produce a code for evap purge, I think it's a 0496 or something.
Compression test is good and no audible ticking.
What next before I abandon this 3 week old project?

 

[iamdub]

Other than the EVAP purge code, the P0303 is the only code? New to you- what was done prior to you buying it? Is that why the PO sold it?

 

[02Z71Raven]

Shady used car dealership. I called out the code when I bought it, they said it needed a coil.
That is the only code and I have given it a couple of hundred miles since purchase and no other codes.

They said they fixed a rear main oil leak, but that's all they mentioned.

 

[Fless]

If you have the P0496 evap code the purge valve could be staying open (or not be responding to commands) allowing too much vacuum to be applied to the evap system. That could allow gas fumes into the intake and make it run rich. Something to look into...

What do your short- and long-term fuel trims look like? Do the STFTs change much between idle and 1,500 rpm or so?

 

[02Z71Raven]

You surpassed my level of observation there. I see the purge valve opening, but I wouldn't think it's staying open. It only shows up after a bit of a drive, but i dont know that much about how it works.
I can probably get fuel trim data with what equipment i have but i haven't had the truck for the last 10 days to try.

 

[swathdiver]

New to me 2008 tahoe ltz, 0 vin engine code, with 172k on the clock.
I've swapped injectors, coils, plugs, wires, and replaced injector connector.
Rough idle at cold start, runs smooth(er) when warm, but still not smooth. After a while it does produce a code for evap purge, I think it's a 0496 or something.
Compression test is good and no audible ticking.
What next before I abandon this 3 week old project?

Purge valve has nothing to do with the misfire on number 3.

You may need to perform a crank sensor relearn.

Verify that each injector is connected to the correct harness and cylinder.

This is usually a broken valve spring or collapsed lifter.

Get yourself a subscription to AllDataDIY for the shop manual and trouble shooting guide. Get hold a Tech-2 or similar tool to aid with diagnostics. You cannot properly diagnose this without a scan tool.

If the car is driven too long with this code, it could very likely take out a catalytic converter.

 

[02Z71Raven]

Purge valve has nothing to do with the misfire on number 3.

You may need to perform a crank sensor relearn.

Verify that each injector is connected to the correct harness and cylinder.

This is usually a broken valve spring or collapsed lifter.

Get yourself a subscription to AllDataDIY for the shop manual and trouble shooting guide. Get hold a Tech-2 or similar tool to aid with diagnostics. You cannot properly diagnose this without a scan tool.

If the car is driven too long with this code, it could very likely take out a catalytic converter.

Connectors are all in the right place from what I can tell. None seem long enough to reach the wrong place and no other cylinders are producing codes.
I assume a crank sensor relearn would require a tech 2? I don't have access to one right now but had considered purchasing one since I own and maintain 4 vehicles.
And after looking into the evap signal, I'd say you're probably right that it's unrelated, but I'll work on it later after this misfire is addressed. Thanks

 

[iamdub]

You may need to perform a crank sensor relearn.

This is why I was asking if any other work or parts replacement was performed. But, I've learned to not ask any leading questions when diagnosing through text!

 

[02Z71Raven]

Went ahead and took it to an LS professional and found out I have a lifter with a flat spot on the roller. All parts are on order and we will be deleting the dod and throwing in the cam and then tune. My 10k truck just became a 15k truck, but I'm not mad, im going to find the silver lining in this and grin and bear it as grandpa would say. Besides, it's gonna be a beast when it's done.
So now I'll be looking at performance mods though I never expected to build a street truck out of an LTZ w/ 4X4.

Thanks for all yalls help. I want yall to know that I haven't been on this site for at least 6 months or more, but I knew yall wouldn't let me down. You guys are awesome!! I'll try to be more active and not give out too much bad advice.