lower fuel trim with weak power and slowed speeds?

Hello, you're thinking about the fuel trims backwards, with a long term at -14%, the ECM is taking fuel away from the mixture due to what it is determining to be a Rich mixture not lean. Where did you buy the fuel injectors? This is becoming a more common occurrence with people buying cheap fuel injectors that are not the OEM injectors, it sounds like the injectors you may have purchased are flowing too much fuel, and the ECM is taking fuel away to compensate for this issue. When you look at fuel trim numbers, that is the action the ECM is taking to deal with a fault, not the cause. So if it was +14% for example, it would be adding fuel to deal with a lean mixture and get the air/fuel ratio back to 14.7:1.
Without doing a fuel injector balance test, you wont know if there were injectors flowing more or less than the others.
What was the original issue that made you replace the injectors?

I purchased OEM Honda accord parts at Honda dealer. replaced problematic two old injectors. and I detected resistances on all four injectors that possessed exactly same value. After error codes p0302 and p0303 appeared, then, I check injectors wire and looked at bases of each injectors I found #2 and #3 injector bases were not tightly inserted to the base, and the bottom of injectors was very dirty covered with dirty debris and oils fixed around of injector bottom as images attached. Sb had touched the injectors. The injector leak and contamination as well as congested injectors may be the error sources.
I will check again for the injectors if after inspection the injectors were changed value, and if the original OEM injectors on the sites. What should I do now to increase the fuel trim value? before inspection the both value of ST and LT were zero.

The fuel trim value will come back down towards zero again when the issue that is causing the rich condition is taken care of. Think the of fuel trims as the ECM telling you what it is doing right now. So it is telling you that it sees a rich mixture (too much fuel) and it is taking fuel away to deal with that problem. I do see all the dirt around the bottom of the injector seals, I would clean all that as well as the holes where the injectors sit, use a vacuum at first if possible so you don't get anything failing into the intake manifold, and make sure those seals are cleaned as well.
Why did you replace two injectors to begin with if the fuel trims were close to zero? At this point since cylinders 2 and 3 are setting misfire codes, I would replace the seals, clean it all up and see if the fuel trims come back towards zero. You have to make sure all the injectors and the fuel rail are seated correctly, be sure they are all down. It can be difficult with dry injector seals, some manufacturers will tell you to use a very small amount of mineral oil on the seals to help them seat down fully.,
here are some guides to help with testing and replacing them if necessary, but I think this is more of a self inflicted type of scenario where the codes are setting because they weren't seated down correctly. It sounds like the middle of the fuel rail was not down all the way when cylinder 1 and 4 were fully seated.
But if the codes come back after getting them seated correctly, I would follow the testing guide below first, you need to be sure this isn't also possibly an issue with loose pins in the injector harness connectors or damaged wiring.
And an injector balance test is where you would have a fuel pressure gauge on the fuel rail to monitor fuel pressure, and each injector is pulsed for a specific amount of time and the pressure drop of each one is measured and compared to the others. The pressure should drop almost exactly the same for each injector. The test can be explained more if needed, but clean everything up and see if the codes come back.

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

https://www.2carpros.com/articles/how-to-replace-a-fuel-injector

Injector and wire related several tests and install were done without problems. when I drove to dealler the fuel trim, ST and LT were all zero. Befor drove out from dealler I detected total values with code reader and found the values were both down to nagative value they said it would down to normal after drive several days.

The Long Term is more of a learned value that will keep adjusting according to the Short Term. So their comment about it adjusting down is not incorrect, how long it takes depends on how the ECM is programmed to do so. Some manufacturers will be different than others, if its back below 5% either +/- then youre good.

Right now the values of Short Term and Long Term were +/- 2.3% on idle driving, and 0% when unded driving conditions, and driving conditions were normal. But problem was found, when parking to the ascending location foward, the start up engine was normal, but parking to the descending location, the start up engine was failed. After foot accell several times to increase fuel meter to highest level before start engine, then could be able start engine. What was the mechanism of this phenomenon? The fuel pressure was low? There was no leak.

Those fuel trims are just about perfect. You will have to check fuel pressure and see what it is when first starting. Its possible the fuel pump is weak, and has a leak of fuel volume, but can be semi ok once started. I would see what the fuel pressure is during cranking.

Now I am driving on the car, still a problem just when I parked front on the down slop direction, the car was difficult to start, when parked front on uphill direction. the car easily started. Now the only way to resolve the problem was that put car key on the position II before starting engine, foot gas peddle strongly three to five times until see the gas meter reached highest position, and foot brake several time so that brake peddle became firm. I still could not figure out what was the problem. Like your commented that possibly the fuel pump is weak, or has a leak of fuel volume, but nowhere found gas leak. starter may be a problem. what should I finally make conclusion and fix the issue completely?

No the Starter is not the issue if the engine will turn over. From what you are describing, with the vehicle starting or not starting depending on the direction the vehicle is facing, up hill or down hill points to still a fuel problem. You mentioned there were no leaks found, but what exactly is the fuel pressure doing during these fault periods?
Another clue is having to prime the fuel pump by turning the key to the "On" position twice and then it will start, this also points to either low fuel pressure or not enough fuel volume. You need to put a fuel pressure gauge on the fuel rail and see what the readings are.
Im not sure what you mean by pushing the pedal down 5 times to see the gas meter reach it's highest point? Do you mean the actual gas level gauge on the dash board instrument cluster?

You are right ! I thought was that when parking to down hill, the fuel supply was not enough, in order to increase gas supply to the chamber, I foot pedal several times then fuel meter in the dash board jumped to its Highest level, then engine was easily ingot started. I will measure fueling line pressure. Thanks!

AL514 is one of our best!! Please let us know what happens.

Ok if you are having to press down on the accelerator pedal a bunch of times to get your fuel gauge to read correctly, or at least full is a very odd problem, With the vehicle being a port fuel injected engine, the ECM (engine computer) is what is going to decide the fuel input according to various sensor readings. On a cold engine the ECM is mostly looking at the coolant temperature reading to determine how much fuel to add. On more modern vehicles, holding down the gas pedal will actually cause the ECM to cut/stop fuel altogether, this is called "Clear Flood Mode" and can clear out a flooded engine by only allowing spark.

I think you are dealing with more than one issue here. But start with getting a fuel pressure reading, first at just 'Key On', so you should get a 2 second prime of fuel pressure. Then crank the engine over while watching the fuel pressure gauge, and see if pressure drops off when you are cranking, if so, you have some better direction now.
Remember too, the fuel gauge reading may not be correct. Though the fuel level gauge should read semi accurately whenever you park, gas is going to move towards the front of the gas tank when facing downhill, its another odd fault here but this might all be related to your fuel tank, filter and pump assembly.

You would be surprised at how many vehicles come into a shop for a no start condition, and the gas tank is empty but the gauge is still reading full. That is a faulty fuel level sending unit, a wiring issue, or false data/information being fed into the cluster. Here is a guide to check out if you havent already. Let us know what else you find.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Where is fuel pressure test port location on honda accord 2009?
I got pressure tester from Autozone but could not fine the fuel pressure test port location. May you provide the information to me please?
Thanks a lot!

There may not be a test port. You may have to T into the fuel line to get a reading. You should be ok with a couple of small hose clamps since this is not a high pressure fuel system.

Thanks!
I will stop more working on the fuel line pressure test.
Currently no more problem starting engine now, just using my method to foot gas pedal 4 times to increase gas meter to its top level than start engine.

This starting up methods are reliable on my car though a little complicated than normal start up method.
Thanks!

Good to hear, please use 2CarPros anytime we are here to help.

The issue with the gas pedal being pumped up like that is very odd, unless Im misunderstanding what you are describing. Are you able to still see good fuel trim numbers? as close to 0% as possible.