Persistent code P0300 being thrown, what next?

Ken,

Unfortunately, I don’t entirely disagree. I was already in process with the injectors, so I put six remanufactured Carquest injectors in, with the result of no real change.
I’m just very surprised at how well it runs, except for throwing P0300 at about 2600 RPM.
Am going to get into bank 1 again and verify those compression numbers. Yesterday, after installing the six injectors, I took "dry" compression readings off bank 2. Results: Cyl 2: 95 psi; Cyl 4: 96 psi: Cyl 6: 99 psi. I had heard anything over 90 psi was minimally acceptable for an old engine. You obviously feel that minimum should be at least 100?
A mechanic had done leak-down tests on those three and reported all "passed" the leak-down tests. I will have further discussion with him re your thought to just give up on this engine. I am not yet ready to give up, though, especially now that I have tackled getting to bank 1. That had put me off previously.
Do you think that there is any chance I could eliminate the P0300s by, say, installing all new (though cheap) cats? How about the chance of eliminating the P0300s by replacing the complete fuel pump, including fuel filter and pressure regulator? If either might eliminate the P0300s, which would be more likely to eliminate them?
Please fill in more detail, if possible, on why the misfires happen in a "worn out" engine, assuming you’re right and being just plain "worn out" is the real culprit.
In the meantime, I have my eye on another Sienna that looks really nice, and "only" has 144,000 miles on it. Probably a good alternative but I would still hope to get this current one running at least enough for someone to drive it around town.
Thank you, again for any and all feedback.
Sincerely,
Roger

Ken,
Thank you, again, for your answer.
As an aside, I rechecked dry and wet compression. Final numbers:
Cyl #1: 101 psi dry; 105 psi wet
Cyl #2: 95 psi dry; 105 psi wet
Cyl #3: 99 psi dry; 103 psi wet
Cyl #4: 96 psi dry; 103 psi wet
Cyl #5: 98 psi dry; 104 psi wet
Cyl #6: 99 psi dry; 112 psi wet
You may be right, that the engine may be worn out, but I don’t think so. The reason is that I have been making progress with it. At first, it was throwing 3 codes consistently, at about 2600 rpm, under load (going uphill and stepping on accelerator. The codes were P0300; P0302 and P0305. After I put in 3 new coils on Bank 1, and plugs all around, it stopped throwing the last two, but was still throwing P0300 under same conditions. I considered that progress, so went back in and installed 6 remanufactured Carquest injectors (and replaced the coil connector on cylinder #4, which had disintegrated). I then went to start it, and it would crank but seemed to have no spark. I figured I had accidentally done something wrong on that last upgrade. About four days later. After a bunch of searching online, and asking former mechanics, etc, to no avail, I finally went to a person I had identified as a mechanic who is very savvy, especially in figuring out specifically WHY things might be happening, and explaining the causes to me in detail. He told me two things:
1) He would next install a $29 crankcase position sensor, because the ECM (computer) uses data from that sensor as part of its determining whether or not there are any misfires. Yesterday, I installed the new crankcase position sensor, even though at that point the engine still wouldn’t start. Then I went back to that very smart mechanic and asked him what he would suggest, as I had determined I had no spark (after I grounded coil and plug on #4 to the front Cat shield, cranked it while iPhone was set on RECORD, watching that grounded #4 plug and coil just sit there, emitting no spark whatsoever). I asked him where the electricity would come from to create the spark on #4 coil and plug. He checked out the coil circuitry online and showed me the functions of each of the 4 wires going to coil. I then asked him to look back further to see if there was any fuse or fusible link before the actual coil. Shazam! The diagram showed a 15-amp fuse in that circuit, and that the fuse was under the dash. I went back home and checked that "ign" fuse (I think it was # 48). It was blown! I replaced it and the Van started right up. I then took it on my "standard" test-drive route, where it had thrown the three codes previously, on cue, at 2600 rpm, and most recently had thrown just the P0300 code. This time, I made it almost all the way through my test drive before, as I pushed it hard uphill, at 70 mph and about 3000 rpm, it threw the dreaded P0300 again. But it now threw it at a more stressed stage of operation. So, I consider that as progress in this case, where most mechanics are telling me they have no clue what’s going on, and you are telling me to give up on this engine. But I know it’s running too well, under almost all conditions, for me to give up on it.
I consulted with my go-to thinker, and he gave me suggestion #2
2) Either try switching the three coils I had replaced, but had saved, with the three "original" ones that were now on bank 2. The goal was to see if one, or more, of those original coils, was crapping out under the load conditions I have described. Alternatively, he suggested I pop for three additional, new, Denso coil packs. After considering the options, and at this late stage in this saga, I decided to get the new coil packs.
I will pick them up this morning, install them in bank 2, and make my test run. You may end up being right, and I may need to give up on this engine, but not ready to do so yet!
Sincerely,
Roger

Ken,

Still throwing P0300.
Can you explain what causes a "worn out" engine to throw P0300 code? I continue to read up on how the computer comes to the conclusion that there are random misfires. Could be clogged Cats, could be an as-yet undiscovered vacuum leak, could be partially clogged fuel filter. I have been using gas with 15% ethanol, and maybe that set this up. Have read "carbon tracking" could be the culprit. Or, maybe you’re right and the engine is just "worn out."
After I installed the three additional, new Denso coils, yesterday, it seems to be noticeably stumbling more than the day before. I will try to figure that one out as well.
Will keep you posted.
Sincerely,
Roger

Ken,

I do appreciate your straight, honest answer. After some further assessment today, including some feedback from Kenny Kase, who says Hi, I am finally pretty clear that the low compression itself is likely the problem. Kenny explained that the low compression can lead to incomplete burning of the fuel, which, in itself, could be, as you say, the root of the problem. So, I went out and changed the oil. I had been in the habit (foolishly with hindsight) of continuing with the 5/30 oil that had always been used in this vehicle. With hindsight, I should have shifted to a heavier oil a while back. Anyway, I am also going to switch to higher octane fuel. My thinking is both of those moves might bump compression/more complete burning (at least by a little). Ideally, occurrences of the P0300’s decrease. In the meantime, I’ll get what miles I can out of it, and plan for replacing the engine. Such is life.
Thank you, again, for your input.
Sincerely,
Roger

Ken,

Have been driving my 2004 Sienna. It is so consistent with throwing P0300 (and sometimes, also, P0302 and P0305), as soon as I put it under any load, that I noodled to the thought it could be that VVTi valves were clogged, so not advancing timing when needed. I pulled both VVTi Valves. Bank 1 was covered in varnish. Bank 2 looked good. I replaced the VVTi oil control valve in Bank 1. No change! Still throwing P0300. I watched VVTi video on The Car Care Nut on YouTube last night. It explained how that system works to advance timing. Really helped my understanding! That got me wondering if the filters in that system might be so clogged, they are not allowing enough oil to flow, even when computer is telling VVTi control valve to let oil flow to advance timing. In that scenario, the control valves would be working, so no code for them would be thrown, but they would not be advancing the timing, due to low oil flow. Incorrect timing would then throw the P0300 (302; 305) codes I am experiencing.

Questions:
1) What do you think of this theory?
2) How would you suggest cleaning the oil so it would flow better?

Note: I had a mobile mechanic check compression on bank 1 on March 30th, and in the process he found that the bank 1 valve cover gaskets had completely failed internally, filling the plug wells. In replacing the valve cover gaskets, he/we noticed the significant build-up of varnish-looking gunk on the inside of that valve cover (even though oil has been changed judicially every 3000-400 miles). That’s what started me thinking about clogged VVTi system components...

Thank you for any thoughts.
Sincerely,
Roger

Ken,
Put in new VVTi filters, but it made no difference.
Thank you for your feedback, though.
Sincerely,
Roger