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Hi everyone.

So I've got a 2002 Honda Civic EX that's having a misfire problem. It only misfires when its under load (and it also has a little hesitation when at idle but I'm not positive if that's a misfire or not), typically between 1000-3000 RPM's in 2-5th (it's also a manual). It only does it when it's warmed up, won't do it with a cold engine. So far I've replaced both O2 sensors which didn't help at all, and I also cleaned some part of the EGR system although I can't remember exactly what it was called. It was mounted on the back side of the engine, had to take of the intake manifold, throttle body, fuel injectors + rail, and one other piece that also escapes me. There was a lot of carbon build up in that, most of which i cleaned out, and that made it drive a lot smoother but still didn't help the misfires. I was thinking it would be the O2 sensors because it only did it when it was warmed up, so once it switches over to the O2 sensors for fuel ratio, but as I said before those didn't help. It still gets phenomal gas mileage (34+ every tank) but obviously the misfires can cause more serious problems. Also, when I turn on the AC at idle it hesitates/misfires like hell, and in general if you're driving and have your foot on the gas, the second you turn on the AC you can feel the engine cut way back. Seems like the AC is too much of a load on the engine even though it shouldn't be. I'm decent at working on cars so I can try most things suggested, albeit if you could give a little bit of an explanation/link to a place that explains how to try something that'd be awesome. Thanks in advance for any suggestions.

 
Rep Power: 134

Do you have a CEL on with any codes?
What O2 sensors did you put in the car?
Did you try a new gas station - possibly bad fuel?
When were spark plugs replaced and what brand?
Check fuel pressure, compression, a functioning EGR system and a clean air filter.
When I had a bad EGR valve and clogged port on my Odyssey, it stuttered like driving over a washboard road. I did not have a CEL and code too.
This will get you started anyway...

 
Rep Power: 519

Disconnect the wiring from the EGR valve and then drive it under the same conditions, see if the miss while driving at steady speed is gone.

 
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I put in a denso and a bosch, I was going to get 2 denso's but they didn't have the upstream denso in stock.
It was driven up the east coast at countless gas stations, different gas/premium gas made no difference.
I just texted the previous owner and he doesn't know the last time they were changed/brand, going to give those a shot this weekend.
How do i check fuel pressure and compression? Air filter is fine.
CEL is on with all 4 cylinder misfire codes in addition to random misfire code

 
Rep Power: 0

I did this before when I found it on a different forum and it didn't make a difference. The misfires don't really happen when cruising at a constant speed, more when you accelerate.

 
Rep Power: 134

I you have another code other than the P03XX misfire codes, troubleshoot that one first.
Otherwise, I would consider replacing the spark plugs with OEM NGKs. No BOSCH plugs. While in there, you can check for a faulty coil too. At idle, pull the electrical connector at each coil separately and if working properly they should cause the engine to stumble or make a difference in the idle for the worse. If no change, you have a faulty coil.
Try these first before plan B which would be checking the injectors, short in the circuit/open wire, poor ground, loose connectors.

 
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You're talking about the connectors on the top of the engine going to the spark plugs right? Unplug that while at idle and see if it stumbles?

 
Rep Power: 223

yep those ones, careful with the clips, sometimes they get brittle

 
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Replace all 4 spark plugs and the misfires seem to be gone. Drove about 260 miles yesterday, ran the ac at idle, etc etc and it was as smooth as ever. So I understand that if they have a weak spark because they're old it could cause a misfire, but what are the possibilities of all 4 plugs going bad at once? That was why I initially ruled it out, it didn't seem very likely that all 4 plugs went bad together.

 
Rep Power: 223

How old are the plugs? Were they the original ones? Do the old plugs look discolored? If they were all original or were all replaced at the same time, and you went way past the replacement interval it wouldn't be that unusual for all 4 plugs to be giving weak spark.

 
Rep Power: 0

Sorry... late to the party!