P0300 code at 80 mph and above only
P0300 code at 80 mph and above only 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?
Last edited by idahoz28; Jun 24, 2024 at 01:54 PM.
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??
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Joined: Apr 2015
Posts: 154
Likes: 36
From: West Coast
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
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
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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
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
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Joined: Apr 2006
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From: Grand Rapids, Michigan
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.
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.
This was after a full tuneup, etc. I bought a relatively affordable bi-directional scanner (chinese made) and have done a few relearns.
BINGO!!!!! you guys with the last two posts were correct....Crank sensor relearn solved that problem right away. Now just hoping the new IAC valve solves my hunting idle when the AC is on full blast.
