P0300 misfiring when driving on the highway

[xjsc16x]

About two days ago my '02 Tahoe 5.3 Flex started misfiring. Only seems to happen when cruising around 60-70mph on the highway (that's when the light blinks at least). SES didn't stay on when it started happening but eventually popped a P0300.

I've attached a screenshot of me monitoring the misfires using the Torque Pro app after just jumping on the highway for 1 exit cruising about 60. Seems to be mostly cylinders 5 and 8.

Is there anything I should check right off the bat? Coil resistances? Not sure on what numbers are within spec. I'll probably go look at the plugs wires and coils on 5 and 8 to begin with but would like to know what else to check.

I don't see the misfires when idling / under load.

Thanks, Justin

 

[Tonyrodz]

Is that a free app?

 

[xjsc16x]

Is that a free app?

There's a Torqe Pro (paid) and a Torque Lite (free). I don't actually remember what the differences are tbh. I have the paid one, it was $5.

That plus the bluetooth obdii reader ($20) was basically the same price of a cheapo scan tool with 1000x the usefulness.

-Justin

 

[Tonyrodz]

Just got this. Hopefully I'll be able to figure out how to use the live data to figure out my issue.

 

[swathdiver]

Plugs, wires, coils, when was all that last checked? May need to do a compression test and or a leak down test if the easy stuff is inconclusive.

 

[xjsc16x]

Plugs, wires, coils, when was all that last checked? May need to do a compression test and or a leak down test if the easy stuff is inconclusive.

I just bought the truck a couple months ago, so we can assume never. This misfire just started this week.

I'm going to get some plugs and wires for it from napa but does anyone have a process for checking the coils? Would be much appreciated.

Thanks,
Justin

 

[cmc76]

I had an occasional misfire, then a constant miss when i had an injector go down. If i remember correctly it was #6. My cheapo scanner gave me an injector code, but had a mechanic check so i wasnt replacing them all. He said its pretty uncommon though.

 

[swathdiver]

Justin, I would recommend sticking with AC Delco plugs and wires. Some fellas on here can recommend good aftermarket stuff that doesn't throw codes if you prefer. Get a Tech2 so you can work on this thing. It will you save you time and money.

 

[xjsc16x]

Justin, I would recommend sticking with AC Delco plugs and wires. Some fellas on here can recommend good aftermarket stuff that doesn't throw codes if you prefer. Get a Tech2 so you can work on this thing. It will you save you time and money.

Honestly, after the hassle of chasing down the airbag code I just finally got rid of I'm seriously considering the Tech 2

 

[swathdiver]

There not that much money and will pay for themselves in precious time and money saved in short order. Because of the Tech2, I've been able to prolong the life of some parts, like a waning air temperature actuator and one of my catalytic converters. I paid just over $300 delivered (I think) and that included the factory hard case.