I have a 2001 lexus is300, the chassis has 360,000 miles, the engine was just rebuilt with new internals. It has been converted from an automatic to a manual. Codes popping up are P0300, P0303, P0305, P0306. Spark plugs have been replaced, ignition coils have been replaced, plug wires have been replaced. only thing i can really think of is a clogged catalytic converters that’s causing it?
I'd look into the fueling next - if you have a high-level scanner, look for too rich/too lean (if you don't, read the spark plugs). Any chance there's crud in the fuel system making the injectors misbehave?
I forgot to say that i also replaced the injectors, The spark plugs are completely black and they’re almost shot from how it’s running. I think the only thing left to check is the fuel pump because i haven’t checked it yet
Test drive the car with a good scan tool attached to record the live data.
You know your car better than anyone so if your gut is telling you this may be a clogged cat why not test it to see if you're dealing with a clogged cat? It's easy enough. Might as well get it out of the way.
https://www.youtube.com/watch?v=qJTmddbOxMk
Check for clogged injectors and fuel pump pressure. 21 years old.
Did you get a guarantee with your "rebuilt" engine?
