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NEED HELP! OBD II Code P0300

Hello, I have a 1997 GMC K1500 w/ the 5.7 Vortec engine. Lately I have noticed a slight random misfire at different rpm’s. Most of the time it occurs at highway cruising speeds. At WOT the engine has normal power and acceleration with no noticeable hesitations or misfires. I preformed a tuneup about 10,000 miles ago and until now everything was fine. Hooked my scanner to the truck because of on illuminated CEL and found trouble code P0300 which is “Multiple Random Misfires Detected”. At the time of tuneup I used all OEM parts (ACdelco).
Parts changed or performed the following at time of tuneup:
~spark plugs ( gap .060 )
~plug wires
~Distributor cap
~rotor button
~air filter
~fuel filter
~cleaned MAF sensor
~clean fuel injectors (gm dealer)
~clean throttle body
I am almost positive I am missing a few items. I am sure I could fix the current problem however, I do not want to just throw parts/money at this without knowing exact cause. Could really use some help! I do have an OBD II scanner that can read fuel trims and such if you need that info. I myself do not know how to use the info to diagnose any problems!
Thanks in advance for help and/or replies!

One thing that can cause a DTC P0300 is a stretched timing chain.

On your engine, remove the distributor cap.

With someone turning the engine over by hand at the crank bolt in the normal direction, watch the rotor in the distributor rotate.

Now have that person turn the engine over in the opposite direction while watching the rotor.

If the engine can be turned over in the opposite direction more than 5 degrees before the rotor in the distributor begins to rotate, that indicates a stretched timing chain.

Tester

Thanks Tester. Would a stretched timing chain cause any other symptoms as well? or just the misfire & the code to set? I have watch the timing advancement through live stream data while driving the truck an all seemed well. ( No severe fluctuations that I noticed).

If there’s a lot of miles on this truck, check for a stretched timing chain.

Tester

Truck has 300,000. Engine and tranny have 30,000

That would be called a Minor Detail

Tester

I would strongly suggest that you remove all of the spark plugs and perform a compression test. If the compression is dropping (entirely reasonable considering 300k miles…) then nothing will cure it short of a new engine.

As a precursor, you might connnect a vaccum gauge to an intake manifold vacuum nipple and see what’s going on pre-compression check.
If the manifold vacuum is low (say 13-15") then that’s a sign the engine is worn. A compression test will simply verify the bad news.

If the compression is good, my first guess is there’s a gremlin in the ignition system. Maybe remove all the spark plugs and take a look at the tips, comparing to photos of what slightly used plug tips should look like (dry, and a light gray color). Without any other codes, beyond that it’s hard to say what the problem is. The engine if I understand what you said above is fairly new, recently installed, right? If I had that problem I’d o’scope the ignition signals, and probably measure the intake manifold vacuum, the fuel pressure, and check the fuel trims if nothing else pans out. If the intake manifold vacuum level looks problematic, that could be caused by an exhaust restriction too, which could cause misfires.