2001 Nissan Frontier 2.4L Engine P0400 Code: No/Reduced EGR Flow Diagnosis and Fixes

Recently purchased a 2001 Nissan Frontier with a 2.4L engine. The truck has been sitting idle for approximately two years, which may have contributed to some performance issues. The vehicle is currently displaying a P0400 diagnostic trouble code, indicating no or reduced EGR flow. The owner previously replaced the EGR valve, though the old one was functioning properly. I performed a thorough cleaning of the throttle body (TB), removing significant carbon buildup from the four clean-out ports located halfway up the intake manifold. I also tested the exhaust temperature sensor — resistance decreased as engine temperature increased, confirming it is working correctly. Despite these efforts, the P0400 code persists. An interesting observation: the vacuum line from the throttle body to the fuel pressure regulator has strong vacuum, but the line just above that, which connects to the EGR solenoid, shows no vacuum at all. According to my understanding, this line should have constant vacuum and route either to the EGR valve under certain conditions or to a tube leading to the air cleaner when EGR is not active. Could there be a blockage in the throttle body between the working vacuum nipple and the one that's not receiving vacuum? Additionally, after reassembling the throttle body with three new Nissan gaskets following carbon cleaning, the engine runs smoothly as long as the air filter cover remains open. When the cover is installed and tightened, the engine begins to bog down significantly and becomes undrivable. I suspect either a dirty or damaged Mass Airflow (MAF) sensor — though I did not remove it for cleaning. Could this be the root cause? Any insights or troubleshooting steps would be greatly appreciated.