CODE PO300 Random Misfire Detected - @ Wits End!

[Sideburnz]

Okay, here it is 1999 Tahoe 2DR, 2WD, L31 Vortec! Modifications include Volant Cold Air Intake (15957), JBA Competition-Ready Headers (6830SJT), TWG High Flow Catalytic Converters, Flowmaster 40 Series Muffler, Complete 2 ½” Dual Exhaust with H-pipe and a Black Bear Performance Tune!

This past week in order to remedy the problem I installed ACDelco Distributor Cap, Rotor Bug, AC Delco Iridium Spark Plugs and Summit 8mm Wires! I’ve even used a can of starter fluid to check the intake for leaks! No leaks!

Any help would be appreciated!

 

[RawbDidIt]

Okay, here it is 1999 Tahoe 2DR, 2WD, L31 Vortec! Modifications include Volant Cold Air Intake (15957), JBA Competition-Ready Headers (6830SJT), TWG High Flow Catalytic Converters, Flowmaster 40 Series Muffler, Complete 2 ½” Dual Exhaust with H-pipe and a Black Bear Performance Tune!

This past week in order to remedy the problem I installed ACDelco Distributor Cap, Rotor Bug, AC Delco Iridium Spark Plugs and Summit 8mm Wires! I’ve even used a can of starter fluid to check the intake for leaks! No leaks!

Any help would be appreciated!

Hook up the scanner: what cylinders are misfiring? is it limited to one bank, or across both banks? Is it at startup, or after the O2 sensors warm up and it goes into closed loop?

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[stutaeng]

Hook up the scanner: what cylinders are misfiring? is it limited to one bank, or across both banks? Is it at startup, or after the O2 sensors warm up and it goes into closed loop?

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Yes!

Need to narrow it down to engine bank/cylinder.

Check vacuum with vacuum guage. Could also be intermittent sticky injectors. Do injector balance test.

Check fuel trims on scanner. If fuel trim gets better with increase in RPM, then most likely vacuum leak. If fuel trims stay bad with increase in RPM, then most likely injectors.

 

[TheAutumnWind]

Spider issue?

https://www.gmt400.com/threads/mpfi-spider-injector-swap-upgrade-info-and-questions-thread.38867/

 

[Schurkey]

Is this a REAL P0300; or is your scan tool a lying bastage? Cheap-junk "consumer" tools may list generic codes instead of specific codes. They'll take a "real" P0305 for example, and display P0300. Are you getting the REAL code, or the generic substitute code?

What is the cranking compression of all eight?

What is the distributor (cam sensor) offset figure, in degrees?

Will the ignition coil RELIABLY fire an HEI spark-tester?

How old are the O2 sensors?

 

[98 Nitro]

Could be bad new parts. Try old cap/new rotor...old rotor/new cap, swap the Volant for the stock intake, etc. Sometimes it is very simple, on my 94 Tbi truck I found a little oil on a spark plug used a defouler and it solved a misfire/poor running truck.
I agree with @Schurkey on finding out which bank(s) are misfiring, and if it is due to cam offset I recommend the Dash Command app.

 

[Schurkey]

I agree with @Schurkey on finding out which bank(s) are misfiring,

It's not a matter of bank-to-bank. It's a matter of individual cylinder(s). Which means the problem could be narrowed-down rapidly and perhaps resolved.

Truly "random" misfire won't be any fun to figure out.

 

[stutaeng]

Do a "Cylinder Balance Test." It is fairly easy to perform with a test light. This should reveal the missing cylinder and go from there:

https://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/how-to-trobleshoot-a-hard-to-diagnose-misfire-1

Here is a video showing procedure. It is on a distributerless ignition system, but same principles: https://www.scannerdanner.com/chapt...cylinder-is-misfiring-cylinder-drop-test.html

 

[Schurkey]

Do a "Cylinder Balance Test." It is fairly easy to perform with a test light. This should reveal the missing cylinder and go from there

IF (big IF) this is a hard misfire, the code wouldn't be P0300. A cylinder-balance test would pick it up easily.

IF (big IF) this is a genuinely-random misfire, the code would be what we were told it is--P0300--and a cylinder balance test may or may not pick it up. In fact, a cylinder balance test could lead you on a wild goose chase by improperly identifying a specific cylinder when the problem is...random.

Thus it would be nice to know if the scan tool used to generate the P0300 is a consumer-grade piece of crap, or a decent, trustworthy tool.

 

[Sideburnz]

Scan tool was from AutoZone!