1 year ago I started getting P300 Random misfire code on my 2004 Tahoe. Over year I replaced or eliminated Air Manifold Gasket/Leak, Air Pressure good, Smoke Test Pass, Fuel Pressure good, NOID Test Pass, Fuel Injectors replaced, new Plugs and Cables, new Coils, new Throttle Body, new MAF sensor, new CAM Position Sensor, new cable harness to coils, cylinder pressure tests all passed (C1 slightly low). Basically everything except O2 Sensors, CAM, Lifters, or Computer.
UPDATE 11/26/19 - over summer kept testing and purchased new CAM, Lifters, Spings, Head Gaskets, from 1AAuto. Was waiting on cooler temps (now) to do work but in August added an oil treatment called “RESTORE” to oil change. Dealers does changes as I have $10 lifetime changes. Dealer left off a plug (think on purpose) and after getting home found it arcing bad to block. I plugged it back on plug.
WOW FACTOR- Its my wife’s car and I did not do much until end of September. We had to get state inspection but EL was off. I cleared codes to get inspection and it worked. EXCEPT now EL has never returned and today I plugged in Torque Scanner and after 20 miles of testing not 1 misfire and STFT perfect near 0, and LTFT and O2 ranges good.
It’s almost as though RESTORE fixed my issue but I just cannot believe it!!! I wonder if the arcing plug caused a computer reset that might have worked? It makes no sense to me and I have $700 of parts in a box....
PLEASE ANY FEEDBACK ?? I feel
I am losing my sanity on this as I just cannot believe an oil additive was the fix!!!
Always start with the simple stuff first. We have a similar fuel additive here called redex, some people swear by it. Can cure what seems like major engine issues just by cleaning out valves and injectors etc. They do one for diesels too to help clean the DPFs out, a new DPF can cost about £1000, a fuel additive can cost £10. Sounds like it worked for you, congrats if it did!