'03 Misfire problem- Next steps?

So, I have an 03 with just about 160k miles. It's been having some idle quality problems, mostly when the engine is cold, as well as stalling frequently when cold.
CEL is on.
Recently the IAC valve was replaced, along with the MAP Sensor. The idle got
better after replacing those.
I have a code scanner which is showing codes P0301, P0302 and P0300. So Misfire in Cyl 1, in Cyl 2 and Random Misfire.
To resolve this, I first swapped the ignition coils from the two misfiring cylinders to the two that weren't misfiring, to isolate the coils as the cause. The misfire stayed with the cylinders, sam OBD codes.
I then swapped the plugs with new ones. The old plugs were not fouled, and generally looked OK, although the plugs from #1 and #2 were noticeably darker (soot?) then #3 and #4. No change in the Misfire codes, but there was a little more improvement in the idle quality.
Next, figuring I've mostly, if not completely, eliminated the ignition system, I replaced the fuel injectors. I tried TecRon fuel system cleaner (2 helpings) with no change.
Replacing the injectors made a noticeable difference in idle, and definitely more responsive, but after several hours of driving, the CEL came on and I got the same codes again.

So, now, before I go back to the dealer with my head lowered in shame, I'm hoping for some advice on where to look next for a misfire issue.

Would worn/misadjusted valves cause this? It's a Gen 2 which supposedly have much better history with valve issues than Gen 1.

Any advice?

Thanks

Just to be on the safe side, I'm going to ask. I'm sure you did this already but nothing was said in the post. Did you clear the codes after each process? Because the codes don't clear themselves. I'm going to add a PDF file on the Radom Misfire Codes, maybe this will help.

Sorry, I just noticed you had a Gen2 and the file was for Gen1. I did read up on the Gen2 troubleshooting. There could be a short in the wiring between the ECU and cylinders and poosibly a bad ECU. It sound like you covered all your other bases.

Sorry, I just noticed you had a Gen2 and the file was for Gen1. I did read up on the Gen2 troubleshooting. There could be a short in the wiring between the ECU and cylinders and possibly a bad ECU. It sound like you covered all your other bases.
Double faults are difficult to isolate and pinpoint. It seems that resolving more than one condition has improved the problem

Nothing was mentioned about compression or perhaps I missed it. While Baystatemac's debugging was systematic and well thought out, I do believe that if the valve adjustment is off (too tight) that it can lead to misfires intermittently. If the problem is at the marginal stage before it gets really bad enough to burn the valves. Just in case I would make sure that the exhaust valves are not too tight. With too little clearance it will not take much before one or more valves burnout. Burned valves are a 'bee-otch'.

BTW the intake valves do not normally exhibit this tight clearance problem unless erroneously adjusted that way. The intake valves should be checked too anyway.

Another problem could be a very small compression leak between #1 and #2 cylinders that for one reason or another only shows itself when cold. It would help to verify this with a compression test when cold. A visual analysis of the coolant for oil and the oil for coolant should be done even though this is probably unlikely. A compression leak between #1 and #2 can occur without fluids leaking. Only combustion gasses can leak in some cases depending on where the leak is in the head gasket. If it is not near the coolant jacket or the oil passages and at a point where the cylinders are adjacent the two cylinder's gases can leak directly from one cylinder to the other.

Good luck.

-Rg

Just to be on the safe side, I'm going to ask. I'm sure you did this already but nothing was said in the post. Did you clear the codes after each process? Because the codes don't clear themselves. I'm going to add a PDF file on the Radom Misfire Codes, maybe this will help.

Yes, the codes were cleared after each successive repair attempt. I should also note that the codes did not get set immediately. Each time after clearing the codes, they would get set after several hours of operations. That includes basic normal driving, highway, etc. The OBDII Monitors are almost all set by the time the misfire codes come back. (The EVAP monitor is the only one not set, but my impression is that's that the hardest one to make ready).

Double faults are difficult to isolate and pinpoint. It seems that resolving more than one condition has improved the problem.

-Rg

Agreed. I spend my life doing troubleshooting (on computers, not cars, anymore), so It made sense.
Compression check- Not done. Only because I don't have the necessary tools, and that's because it's been about a million years since I've had to do this level of my own repair work. And then it was on a big 1967 Chevy engine, which was a lot easier.

So, how difficult is a valve adjustment? I have the valve specs that Honda has published, but are they the "real" specs? (I'm again thinking of the horror stories with the Gen 1 Engine, which I know don't apply here, but...)

Looks like it should be pretty straightforward, but not having done it before, I just wanted to get some thoughts about it. It has been done before by the dealer during regular maintenance, but it's been awhile now. At least 50-60k.

Agreed. I spend my life doing troubleshooting (on computers, not cars, anymore), so It made sense.
Compression check- Not done. Only because I don't have the necessary tools, and that's because it's been about a million years since I've had to do this level of my own repair work. And then it was on a big 1967 Chevy engine, which was a lot easier.

So, how difficult is a valve adjustment? I have the valve specs that Honda has published, but are they the "real" specs? (I'm again thinking of the horror stories with the Gen 1 Engine, which I know don't apply here, but...)

Looks like it should be pretty straightforward, but not having done it before, I just wanted to get some thoughts about it. It has been done before by the dealer during regular maintenance, but it's been awhile now. At least 50-60k.
Me ignoramus. :D
I have never done it on a Honda but other than access to the valves removing the valve cover, one DOC is pretty much the same as the next. I am not sure but the V-TECH is probably not involved. Or is it?

Following the FSM procedure would help one from getting into any OOPS situations!
That is what I would do if I had to do a valve adjustment.

All that is required is the appropriate go-no-go feeler gauges besides determination, some basic level of experience and the tools to remove and replace the valve cover.

Then again it is a Honda! So all bets are off! :D

-Rg

OK, Long time without words.

I ended up bringing into the dealership to get this resolved. I had run out of time, I needed a sticker, and I was traveling 2 weeks later, so I hadn't much choice.

In any event, the problem was severely maladjusted valves. I paid them $360 to adjust, and all is well. New sticker, nice smooth idle, no stalling, runs great.

Of course, now a month later, I'm getting a new code, P0420, but that's a different story...

Great! You might have saved yourself a big expense if it was the exhaust valves too tight. That condition can burn valves! Big bucks to fix these days. The days of the $350 burned valve repair are gone!!! It is three times that to fix now.

Be advised that on the the older Honda aluminum heads the exhaust seat can sink into the head. Reducing the valve clearance rather than the anticipated which is increased clearance due to wear.

Do you know what the before state of the valve adjustment was for grins and giggles? That is how we learn. Well I do anyhow.

-Rg

Do you know what the before state of the valve adjustment was for grins and giggles? That is how we learn. Well I do anyhow.

-Rg

Unfortunately I didn't ask, and they didn't offer. The service writer was somewhat put out by the work I had already done. He had initially stated, before I convinced him otherwise, that they would have to start from scratch, with plugs, injectors, the whole shebang. I was happy enough to get him to just send it in to the tech, and have the tech talk to me if there were questions.

I'll keep it in mind for the next time...