Okay so #GTAIV #GTA4 has a bug called the Taxi bug where the game will spawn nearly exclusively Taxi's if it runs out of VRAM (Default 40MB) to spawn vehicles. Modders increased this to allow for modded cars.
This fixes that bug, hooray. However this also causes an issue where engine sounds stop playing after a while.
This is caused by the game keeping around more cars from different areas of the map, because it relied on Memory pressure to clear them out. Once you have > 18 vehicle types in the game, the new ones lose audio.
Therefore I have 2 solutions to propose:
Increase the number of engine audios allowed to play at once; This enables more car diversity, but it does mean that cars can spawn in the wrong areas. I think this is fine though.
Patch the Traffic spawn script to be more active in swapping out the loaded traffic set for the one for the area; Instead of waiting for memory pressure, check every 1 second to see if it should be updated. Shouldn't cause any noticible latency, keeps cars spawning where they should be, and fixes the audio bug by keeping everything where it should be.
Or really, option 3: Do both.
Both both fixes the cause (too many sounds to play) and the cause (not enough slots to play them).