02 honda accord rough idle

#1

i have an 02 honda accord with 215,000 miles on it it idles at 750 rpms but sounds and feels like its only running on 4 cylinders! engine light codes r reading engine misfire in 4 cylinders! i have replaced the plugs and still runs the exact same way! has normal power above 30 mph and runs smooth at 60-70mph! any input would be greatly appreciated!

#2

First clarify whether or not you have the code interpretation right. Are you saying that you are getting 4 distinct misfire codes? Or that the misfire is being detected on cylinder 4? (Cylinder 4 would - I think - be P0304; a multiple or random cylinder misfire would be P0300).

If it is cylinder 4, then you have an advantage since you know where to look. You replaced the plugs. What of the wires? Get a spare plug (you can likely use one of your old ones if you have them - pick the one in the best shape), or buy a spark plug tester and find out if you actually do have spark on cylinder 4. If you don’t then you know to go from wires then back to the other ignition components.

If you can verify spark, then the problem is likely fuel delivery. You may have a dead injector or a problem in the injector power delivery. For a really quick check you can take a short piece of vacuum hose or something - stick one end in your ear and the other end on the injector. It should have a nice constant tick-tick-tick to it. You can also swap the #4 injector with another and see if the misfire moves to another cylinder.

If it is in more than one cylinder, then I would be starting with all of those things but for each cylinder.

If you somehow verify that you have fuel and spark to all cylinders then I would start with checking the compression.

#3

While true of any car and engine no matter the mileage, I agree with testing the compression considering it has over 200k miles on it.

It’s best to rule out a mechanical fault before replacing numerous parts that may accomplish nothing.

#4

A car that runs badly at idle but OK at higher load and RPM generally indicates a vacuum leak. Anything related to ignition will get worse at higher speed and load. Check all possible sources of vacuum leak, including the intake manifold gasket. Spraying a flammable liquid such as diesel starter ether is usually a good way to find a vacuum leak but be careful not to burn off your eyebrows,

If there is no vacuum leak, check all your intake air tubing for cracks that confuse the mass airflow sensor. Also check the throttle position sensor. That test will require an ohm meter and a manual.