I can get the SES light to come on for about a minute by getting the engine RPM over 3000 RPM and no code will be stored. If the light stays on then I get code P0300. The directions for my code reader say to not start the engine while it is connected.
I don't think it is a COP. I've had a couple go bad and it is a very distinctive misfiring. The lack of power is probably a fuel flow/mixture issue, clogged exhaust or general ignition control issue.

 

I'd start by cleaning the MAF sensor.

 

I did that already.
Don't know if Pro DIY did.

 

That's because the Misfire Monitor is a Continuous Monitor and the light turns off when the misfire goes away. But if you had a 'scan tool' instead of a code reader it would be able to display what are called "Pending" codes. Pending codes are stored in the PCM's "Pending" file for 40 "trips" and if within that time the PCM doesn't detect that code again it will be erased from the PCM's pending file.

The fact that you need to get the RPMs high to trigger the light is a clue that it is not typically a COP issue. It is more likely that it is a fuel ratio problem. Might be a lazy MAF as Alloro indicated, or with all the injector cleaner you used, it may have cleaned out the tank and deposited the gunk in the fuel filter.

The P0300 is a random misfire and is stored in Continuous memory. That is typically the clue that it is not a COP issue, as all COPs wouldn't go bad at the same time. A lazy MAF might cause random misfires, as could a bad load of gas, a plugged fuel filter or marginal fuel pump (at higher RPMs when extra fuel flow is required), or really worn out sparkplugs.

Hope you pin it down . . . . .

 

The first thing I thought of was a dirty MAF, but after I cleaned it there was no difference (at first I thought it was smoother at idle, but it's not). The MAF had not been cleaned until I did it at 126k. I put a bottle of injector cleaner in one tank, ran a tank without, and a 2nd bottle in the 3rd tank. The misfire was happening before I changed the fuel filter (100k), the spark plugs/COPs (120k), injector cleaner (125k) and cleaned the MAF (126k). Is the only way to tell a lazy MAF by swapping in another? They're pricey. Still looking for my fuel pressure gauge.

 

Alright, what's a MAF and how do you clean it?

 

Mas Air Flow sensor
It's in-line with the intake duct coming from the air filter box. You have to disconnect the duct and look inside it to see the MAF sensor. When you look inside you'll see a tiny coil of wire. Gently spray this coil with carb cleaner. Do not touch it with anything like a rag or brush. Just spray it and if you have compressed air you can blow it clean and dry. Otherwise just spray it a few times until it looks clean.

97-03 models looked like this:

 

Our 2003 5.4l Expedition is still lacking power above 3k rpm and seems to have gotten a little worse around 2800 rpm.
Idles smooth and runs great up to 2800 rpm.
Got a new fuel pressure gauge and took some readings.
After sitting overnight there was no pressure.
Turned the key on and the pressure went to 50 psi. (seems a bit high or gauge ??)
Started the engine, idling - the pressure flutters between 50 & 55 psi.
Increased the rpm to 3k and pressure holds at 50 psi.
Short jump up to 4k and pressure still 50 psi.
After sitting for 10 minutes with everything off the pressure was still 50 psi and after 30 min was 48 psi.
Took it out for a test drive and runs nice and smooth up to around 2600 rpm and starts to have no more power.
No chugging/misfiring/hesitating, just no more power.

 

Hmm, perhaps the cats are plugged up and restricting the exhaust flow.

 

I had the cat fail on an '89 460 E-350. The honeycomb inside crumbled and clogged the output hole. You could hear stuff rattling inside when tapped. After pulling out the pieces it ran fine. But, that was a single cat. The Exped has 2 cats and a single tailpipe. What are the odds both are clogged and if one side was clogged what would that do?
What's in those precat chambers? Also, I don't hear any weird noises/rattles from the cats when tapped hard. I'm considering using a chain tubing cutter to open the pipes for inspection and band clamps to reassemble.

 

Got another OEM Ford MAF to try.
Cleaned out the throttle body while I was in there.
Took it out for a test drive and no difference.
When trying to accelerate to get up to speed on the freeway the power just dies at 3k rpm.
If I accelerate slowly it will go right up to 70, just keep the rpm below 2800.
I'm thinking one of the cats is clogged.
Anyone know how the '03 cats are built?
Do they use a ceramic honeycomb to create lots of surface area like the one in my '89 van?

 

So who's problem are we trying to help here? Both? ProDIY started it, his problem isn't fixed yet and now he seems to taken a back seat to R&T's problem. Just wondering.

 

...............................

 

Our symptoms seemed similar and I thought our troubleshooting efforts might benefit each other. I can start another thread if it's appropriate.

I've been reading about precat and cat construction and I may have a partially clogged one. That might explain why it runs fine under 3k rpm, and at higher rpm restricts the exhaust flow on one side of the system. There's a few tests that can be done with a vacuum gauge that might help, particularly #14 here: How to Use and Interpret a Vacuum Gauge

 

Doesn't matter to me. It could get a little confusing when responding to the posts and who to respond to. Your problems are definately similar. Why the 2800 RPM magic number? I am interested in what is causing this problem. Keep us informed, both of you, to what you are finding. Nothing like chasing a little worm that keeps avoiding capture!