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First of all, I want to warn everyone that I am not mechanically inclined...I purchased my first car last summer, a 2001 Honda Civic LX coupe. I'm trying to get to know my car better mechanically and I do most of the maintenance while learning at the same time.

Anyways, about two or three months ago, a friend and I went out of town for the day, and coming back, my check engine light came on, and I lost a lot of power. Luckily this was just outside of the city, so I went to the nearest auto parts store to see if they could tell me what was wrong.

They scanned it with their computer, and it was giving the P0304 (cylinder 4 misfire) error code. Looking on here, I wasn't able to find an answer (and to be honest I didn't really look that hard..) He told me that, since a month prior I changed my spark plugs, it couldn't be that, but it could be the cylinder boot (?) or something like that. So I buy the $130 replacement part, and it worked fine until yesterday.

My car, recently, has been running not so great. In heavy stop-and-go traffic, it putters, and sputters badly, occasionally (about once or twice a day). It idles at really low RPMs (all the time), and sometimes it feels as if it's going to stall (sometimes it nearly does).

Yesterday, the CEL came on again. I go back to the same store, and it's giving off the P0304 error code again. I told the guy that I just dropped $130 on a part that they said would fix it, and that I just had the spark plugs replaced recently. He said it could be anything from fuel injectors, to a number of other things that I didn't pay attention to.

Basically I need someone to tell me what the possible issues are, and if they could link me to another thread that could walk me, step-by-step, in pictures, without a lot of technical jargon, to a resolution. I'm planning to take my car on an extensive one month road-trip across the US in September and I need it to be running in tip-top shape, I don't really wanna be in the middle of the New Mexico desert or somewhere else stranded

Also...another issue, when I first start-up my car and drive, it makes a weird noise, something like, let's say when you lightly put a stick over a bike rim that's spinning (if you can get the image..) whenever I press the throttle. It doesn't affect anything, but it sounds junky and is annoying. After a while, it goes away and it sounds fine. Any ideas?

Thank you greatly in advance!

 
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UPDATE:

Now, my car is running terribly. Idles at 100rpms, complete loss of power, cannot go faster than 60mph.

Putters terribly, I'm dying here! I'm going to go to a mechanic tomorrow and seeing if there is anything they can do. Still, if anyone has any pointers, it'd be amazing. Thank you.