There are many, many threads on air intrusion issues on this forum. It is, perhaps, one of the most frustrating, but generally the simplest problems that affect the idi's. It can be very discouraging when trying to resolve the air intrusion issue, as it seems like you check everything but the intrusion keeps coming back. IDIot has made a number of great suggestions, and you can pick up many more great suggestions form doing more thread research on here. In my own case, after I checked and replaced nearly everything I could think of, I found that one of the "new" return line caps form the new kit actually had a hairline crack in it. No visible fuel on the outside, but I found the crack on very careful visual inspection. Now, this would be a very rare case if you purchased a "good" return line kit. Also, much depends on your "technique" when installing the return line kit. You need to check all the injector caps over very carefully for "casting flash" on the plastic from when the caps came out of the mold. This extra "flash" has sharp edges and will cut the o-rings in a heartbeat. Clean up the flash with an exacto knife by scraping it off. Make sure the inner part of cap is SMOOTH! Use VITON o-rings and lube them with something when sliding them on. Many here suggest Vaseline, but I use a product called "Superlube". It is a synthetic grease that come in a gray tube and I get it at the local hardware store. Put the upper o-ring on the injector first and the slide the lower o-ring over the upper one and into the lower groove. Start with the o-rings, caps, hoses between injectors and rubber seals on the ends of the fuel lines (olives) from the the fuel pump to filter head and filter head to IP. Once you have all the engine fuels seals up to *****, and if you still have a problem, then start working your way back to the supply/return lines on the frame, the FSV (located in front of the front fuel tank and a PITA to get to). The fitting on the filter head can also leak air. The schraeder valve on mine leaked so I replaced it. Don't get too frustrated - just keep researching this well discussed subject in this forum and then work methodically checking/replacing sources of air intrusion and eventually you will find where the air is getting in.
