I have a beautiful 2000 Lexus gs300 with 247, 300 and just recently started missing, very badly, but not all the time, it's intermittent! I run this car on 87 octane and add a octane booster most of the time. The 2 front cats are hollowed out and I have a brand new 3rd cat on the rear drivers side. Sometimes I drive it just on 87 octane, is that bad for it because it's a supreme gas only vehicle? Most of the time, my car runs beautifully. I pulled codes of po300, 302, 303, 305 and clear them out with a handheld scanner and it will drive fine for a while until another miss happens. Not sure what to do?? Please help!!

 

My '02 gs300 250k+mi is missing worse and worse. I took it in for diag and found it was a head gasket issue. I should have known after trying to fill the coolant when replacing my thermostat. It kept bubbling up as I gave it gas. I just figured that since they are hard to fill all the way, it would be okay. I kept loosing coolant without my noticing, until it eventually overheated. The small amount of smoke at start up I chocked up to worn valve stem seals. These engines are beasts, but after they get tooled on for so long they will wear out. This one was sold to me with upgraded brakes, wheels, and exhaust. Somebody had fun with it, and it kept changing hands. It isn't in the best shape, but as soon as I get the head fixed, I will be happy to keep a hold of it.

 

P0300 is difficult to figure out because there are many possibilities. Individual cylinder are easier because you can test just the parts for that cylinder. I would assume that all cylinders are misfiring and would eventually throw codes P0301 through 6. So you need to look at what affects the entire engine IE: MAF, throttle, ICM, OCV, fuel pump. Worst case head gasket if it overheated. You could even just need to replace the plugs and wires and scrub out the throttle good and reset the system with a battery disconnect.

 

Start putting premium gas in it like you're supposed to and then address the problem instead of just clearing the codes. Start with the basics and do spark plugs and wires. In the process of that you may find the valve cover gasket is leaking onto the spark plugs or the wiring going to the ignition coils is cracked or one of the ignition coils is bad.

 

The 2jz is notorious for broken/exposed wiring and connectors going to the coils since they sit in the valley between the valve covers and are constantly exposed to heat cycles. Also the bottom of the air resonator can become brittle and crack allowing unmetered air to enter, also causing misfires.
In all honesty we can probably list 20+ specific items that can cause misfires. You need to start going over everything (vacuum hoses, ignition components, etc) to see if you can find anything that appears problematic. The whole regular vs premium argument goes on and on. If a car calls for premium I put premium, I’m perpetually broke and even I never understood the point of trying to save the minuscule amount by putting cheaper than recommended fuel.

 

Mine was doing this when I first got it, and the solution pretty much was just doing everything. I wasn't throwing the parts canon at it, the previous owner just legitimately abused it that badly.
Like everyone else is saying, just take things one step at a time, and stop clearing codes.
First, put premium fuel in it. In many states, 91 octane is pure gasoline, while 93 is ethanol, so try 91. It'll run better, which means better gas mileage, which means fewer fill-ups, so you're actually saving money doing that. However, if you don't, you're just putting extra stress on your engine. Should it cause misfires? No, it'll just cause the engine to retard the timing to avoid detonation. Could it lead to reduced performance that can eventually cause other conditions that CAN cause misfires if it spends its entire life getting 87? Absolutely.
If I was you, I'd buy a scantool capable of viewing live data. They're not necessarily super expensive, mine's an AutoLink scanner that's like $30. Check things like your fuel trims and such, which will narrow down if your problem is because of a fuel delivery problem or vacuum leak.
I'd also get one of those AutoZone head gasket test kits. The likelihood that your issue is a head gasket is fairly low, but it's $30 for eternal piece of mind.
If it's not, then like everyone else is saying, check your ignition coil wiring. If those haven't been replaced, you're never gonna be wasting money by replacing plugs/coils, since they should be done either way.
Next up, and this is the big culprit when you still have a misfire, how often do you change your oil? How often did the previous owner change their oil? If you go over on your oil change interval and end up with sludge in the engine, you could end up messing up your VVT system. Especially if you've never replaced your PCV valve, which causes sludge to build up much faster.
My VVT system was all kinds of messed up, and I'll bump this thread later when I've posted my full breakdown on how to diagnose and repair intermittent VVT issues. It definitely would cause misfires without tripping the VVT code at certain points, so it's definitely something to look into. For now, here's a text troubleshooting guide.
Welcome to the world of engine diagnostics. You always enter it in the most miserable ways, but trust me, you'll start having fun when it's not your daily driver you're doing it on.