adrav

I've owned my civic since 1999. Has 230K on the engine. Just did major tune up and took in for professional valve adjustment. Replaced radiator and hoses, new oil pan, and both O2 sensors. Plugs and rotor distributor cap replaced 7 months ago. I've been trying to track down the mysterious Honda engine tap issue. Worse in the cold or at start up. I thought the valve adjustment would cure the problem but it didn't. It's also not as responsive as I'm used to between 2000-3000 RPM and I'm losing about 60 miles per tank of gas. I'm thinking the tapping could be barrings?? Could the mileage be a vacuum problem? I had a car in the early 90's with a vacuum advance issue. Maybe?? I did look and listen for any vacuum leaks but couldn't find any. The idle is also a little rough. Not terrible, but annoying for me. I want my perfect car back which I know might be impossible at 230K. Ideas?

ezone

How bad is bad?
Some engine noises are average at that sort of mileage.

But if the noises are severe enough that the knock sensor is retarding ignition timing, that might explain mileage drop. That sort of noise is probably a good indication that something bad is going on.
Wiped out rod bearings would do this.

Common stuff:
Cold aluminum pistons can make noise until they reach their operating temperature and expand to fit their holes. That engine can do it. "Normal."

Piston slap can remain after warmup if skirt/wall clearance is too large. "Not normal", but would take a complete rebuild to fix.

Carbon knock can mimic both of the above, but can go away if cleaned off. "Average." Try some carbon buildup cleaning procedures. Use top tier gas.

Main bearing rattle is common on those engines at high mileage. Mostly noticed after engine is all warmed up. "Average", but would need rebuild to fix.