General Maintenance & Repairs - Weird blinking check engine light..

Need some help..just finished the 5.3 swap in my 98 formula and it's been running fine the past 2 days. However tonight on my drive home (30 min drive) the check engine light started flashing occasionally once I got towards like 60 mph..anything under like 40 and it was just a solid light but started lighting up at like 60-65. Plugs and wires seem fine..what could be causing this? Motor seems to be running fine, no issues

Blinking ses light means random multiple misfires.

Pull the codes for specifics. Although...I guess you're probably not running a stock PCM are you?

Blinking ses light means random multiple misfires.

Second this. If your computer is reading everything right it means you're misfiring every time it starts flashing like that.

I got that much, but that's why I said plugs and wires were fine and the car runs fine while blinking. Just odd that it's only doing it at certain speeds. It's a stock 98 pcm with a tune loaded on to it.

You have to pull the codes but my best guess is o2 sensor on the way out.

You said that there is a tune flashed to the computer. How long after it was tuned did it start doing that? If it is predictably doing it at certain RPM/speed, maybe they messed something up with the spark or fueling tables in the tune.

Only code was a P0300 random misfire..starting to think it just needs a case relearn..well I had the tune put on before I started it up for the first time which then leads back to my OP. Tune was just a basic 5.3 tune thrown onto my 98 ls1/t56 pcm

Are you sure none of the plug wires are touching any parts AT ALL of the metal exhaust manifolds?

Random misfires can be SO many different things.

Does it only do it at 60mph? Or always now.

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Only code was a P0300 random misfire..starting to think it just needs a case relearn..well I had the tune put on before I started it up for the first time which then leads back to my OP. Tune was just a basic 5.3 tune thrown onto my 98 ls1/t56 pcm

Ah ok, so its just the stock 5.3 tune? I thought you meant someone actually tuned the car. Can you cause the misfire by bringing it up to the problem RPM while in park/neutral. If so maybe you can try to isolate which cylinder you're getting it on.

Yeah, none of the plug wires are touching the manifold. It only does it between like 60-70 ish.

Well it is a custom tune but basically the stock 5.3 timing tables, cylinder volume?, etc. with some correction for gears, tires, etc loaded onto my pcm. Bringing it up to an rpm in neutral won't set off the flashing light, only doing 60-70ish mph will..

Yeah, none of the plug wires are touching the manifold. It only does it between like 60-70 ish.

Well it is a custom tune but basically the stock 5.3 timing tables, cylinder volume?, etc. with some correction for gears, tires, etc loaded onto my pcm. Bringing it up to an rpm in neutral won't set off the flashing light, only doing 60-70ish mph will..

I would clean the MAF to rule something simple out. Then get a scan if you can to see how the 02 sensors are switching with the engine running.

Someone here recently had a misfire at a certain speed....it was a dirty MAF.

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Okay, I'll try cleaning the MAF and see if that helps and I might be able to get the case relearn tomorrow to see if that gets rid of it