new here. I got an abandoned 03 es w/ 120k miles back in January. its obvious it was sitting for a while when I got it. it has an aftermarket fuel regulator, K&N air filter, headers with a straight pipe to the muffler. Im not sure if anything has been done to the internals. it had a rough idle and would sputter at a steady speed on the highway but I fixed that by cleaning the nox sensors. adjusted the fuel pressure to specs. it runs nice but the idle fluctuates 500-800 rpms at idle and when slowing down. no codes. not sure what to check next. im a diesel mechanic and this is a project car for my son when he gets his license.

 

UPDATE: took apart the intake and deep cleaned everything. sensors, manifold, vacuum lines. found a couple cracked lines and the intake manifold had a ton of blow by caked inside of it. no more idle fluctuations. cleared all codes and ran it to operating temp and when for a drive. a code did pop up for "system too lean", "evap emission system incorrect purge flow", and "02 sensor circuit bank 1 sensor 2". the 02 sensor fault has been persistent but idk if its cause there's no cat. its straight from the headers to the muffler. any pointers?