I have a 1999 Accord EX with the 3.0L V6 and 136k mi. Here's my situation: Pulled codes P0300, P0302, and P0303. I've just replaced the cap & rotor, confirmed good spark on all plug wires. Some carbon deposits on all spark plugs (Denso KJ16CR-L11, 12k miles on plugs)

Compression
Approximately 180psi on cylinders 1-4 and 6
80psi on cylinder 5

From searching, most of the people with misfire issues replace the cap, rotor, wires, and plugs to start. I think mine are in good shape. I'd appreciate some diagnositic advice before I do any more tinkering or spend any more money. Thanks.

 

I think it's rather obvious you have a compression problem on cylinder #5. You're going to want to buy a Helms or take it to a Honda dealer/good mechanic if you can't do this stuff yourself.

 

I know that a compression problem with 5 is obvious. But why would I get a P0302 and P0303 instead of a P0305?

 

Which cylinder mis-fires are those from...I dont feel like looking it up.

 

P0300 Random Misfire
P0301 Cylinder 1 Misfire
P0302 Cylinder 2 Misfire
...
P0306 Cylinder 6 Misfire

 

Basic troubleshooting: Fix the obvious problems, then see what codes come up afterwards.

 

Try NGK plugs and see what happens.....That #5 Cylinder is prolly throwin your shit out of whack.

 

Denso plugs are fine. I'm interested as to what is causing the low compression...I haven't heard of a case of really bad compression like that on the J series.

 

could the head gasket cause it to leak ?

 

That's typically the most common cause but unless the heads have been tampered with I dont possibly see why you'd get a heads gasket leak in 136k miles.