Bought a used Galant. Cheap. Owner knew it had problems and suspect they knew much more than they let on. Slight exhaust leak and bad catalytic converter. Replaced CC. Midas wanted 120 to "check it out" but suggested the codes: P0507 and P0431 meant there was a vacuum leak below intake.

After extensive study, I elected to remove / clean throttle body and IAC. Missed the MAF. Improved things but still not resolved. Replaced IAC and engine immediately went from 2500 rpm idle to 1000. However it constantly surged at idle. Check engine light went out and engine codes disappeared. Replaced Throttle Position Sensor and it immediately went from 1000 to 3000 rpm. Reset TPS (without machine) till I had 1000 rpm idle. Still surging idle. Discovered the hose from top of the black tube running from MAF to Throttle body was hard, cracked and broken off the inlet tube behind throttle body. Installed a new hose. Still idles with continuous surge (up and down quickly). Removed air filter and it was pretty clean, blew it out with compressed air and cleaned the MAF honeycomb with carb cleaner, taking care to avoid anything else. Dried it and put back on. Same problem. Reset the computer and went to AutoZone and checked engine codes and now get P0102 Mass or Volume Air Flow Circuit Low Input, P0107 Manifold Absolute Pressure/Barometric Pressure Circuit Low Inputand P0113 Intake Air Temperature Circuit High Input.

I'm not sure what or how to identify the real issue at this point. And I certainly don't want to endlessly buy parts in a trouble-shooting guessing game.

Any qualified help would be greatly appreciated.

 

check out your vacuum hoses on the top of the plenum as they might be reversed. Check out the hose routing.. Let us know.