idahoz28

iTrader: (2)

Havnt been on board for a while so not sure if I'm posting this in right place. Have experienced an issue with my mostly stock Z28. Only mods are SLP aircleaner lid & exhaust cutouts.. Problem Im having (after developing a head gasket leak and having knock sensors, cam sensor, crank sensor, oil pressure sender, intake gaskets replaced).... is I get a P0300 code only at 80 mph and above. I can cruise all week... month... whatever in town and no codes. I can do same on highway at 78 mph. as soon as I hit 80 or above... code flashes, slow down to 78... flashing stops doesnt come back unless I hit 80. I did have a slight miss feeling in the pedal at idle. But since, I have replaced plugs, plug wires, coils it idles smooth... go on highway hit 80 mph flashing starts. Only other issue is the occaisional idle hunting when I turn AC on max. It doesnt do that all of the time either just now and again Any ideas why only at 80 mph and above?

idahoz28

iTrader: (2)

I know that exhaust that is rubbing against a heat shield can cause a knock sensor issue.... I do have a small exhaust rattle (vibration) that I need to fix... Would that cause a P0300??

LilJayV10

iTrader: (39)

Do you have anyway to log the misfire counter?

idahoz28

iTrader: (2)

I don’t

LilJayV10

iTrader: (39)

I would try and find something, if not you are just guessing and throwing parts at it.

There's a long list of possibilites it could be.

idahoz28

iTrader: (2)

I imagine local shop will be able to log misfires.. I don’t have much faith in guys around here. Maybe the shop at the dealer can help out.

JacobChevys

You can start with the basic diagnostics yourself, verify fuel pressure. Also helps if you can verify it under load.

A cheap diagnostic scan tool to monitor engine data I use is called DashCommand which is an app. I purchased a BAFX Wi-Fi OBD2 dongle and it can monitor basically everything a tech-2 can.

For $10 you can purchase the specified year GM PIDs on the DashCommand app and can monitor literally everything

idahoz28

iTrader: (2)

The BAFX isn’t available currently so I will look for something compatible. Thanks for the info

LS1Formulation

iTrader: (13)

Try doing a crankshaft variation learn. My '06 Silverado had the EXACT same symptoms and doing that fixed it instantly. You just need to find a scanner with that capability.

Rick Dorion

iTrader: (1)

Yes, the CPS relearn resolved my P0300 issue.

This was after a full tuneup, etc. I bought a relatively affordable bi-directional scanner (chinese made) and have done a few relearns.