Hi! I have a 2004 Honda Fit. It's mostly been a great car. After purchasing used for around $5k, i had to have a wiring harness replacement & fuel pump. It was sitting on a property for a few months, so mice probably did that. It's also had 2 new alternators (1st was faulty), a new battery, new shocks & CV joints. The mileage is 190,500k. Recent oil change. SO! The issue is a rough idle in drive at stoplights. It's not very bad, but with my anxiety it's bad enough!! Why might this be happening? I addressed it with Seafoam into the fuel tank only, it seemed to help a little, but not enough. I just added the 2nd half of Seafoam bottle to the tank with about 2 gallons left, then filled with Chevron supreme. I'm really hoping i just had some dirty fuel on my last fill-up! Any suggestions besides the problem being the ignition coil or spark plugs? Oh yea it got new spark plugs last year; however! The mechanic who did the job is never being hired again, bc he doesn't do good work. I have a new GOOD mechanic now, an old man who own an honest shop & owns a Honda Fit. Thank you in advance!

 

Has the car been inspected and brought up to par?
When buying used, I would throw money at my regular mechanics to replace and pair anything without maintenance records.
You mentioned new plugs from a bad mechanic. What it truly done correctly with the correct or workable plugs? They might blow out later depending on the specifics. Your current mechanic know Honda's updated plug torque spec. It's a pain to get to them but they need to be correct.
Battery connections and corrosion, belt tension, and various bits and pieces can cause odd ghosts.

For troubleshooting,
assuming no OBD codes, once the engine is upto temperature
I'd monitor cylinder 1 advance, short and long term fuel trims, MAF, MAP, and PCM or another voltage in an OBD reader,
and work from there depending on the readings.