Dealing with return line caps that won't seal

We know leaking caps are frustrating, and if you are installing new caps for the first time, we appreciate your belief that this is a crappy design.  Unfortunately, we have to agree.  Not because they don't work, but because they rely so heavily on the skill of the mechanic.  Putting them on is more like an art then a science, but once on properly, they are highly reliable and will last 100,000 miles or a decade or longer.  That said, know that once they have been through enough thermal cycles, they tend to become somewhat petrified on the injector.  Disturbing them can cause a leak, and only a new O-ring can correct it at that point.

First, understand how the cap works.  There is a groove in the top that is responsible for sealing the top of the cap to the injector, as well as holding the cap on.  There is no second groove, and the lower O-ring seals by virtue of the slight taper to the cap body.  Interestingly they seldom leak at the bottom, and almost always leak at the top.  Caps, CAN be defective, but are seldom that way.  We at Moose Diesel, inspect each one to make sure they were molded properly, and the defect is always when the grove inside intersects the drain hole in some way.  When installing a cap, the best practice is to make sure it has a thin coating of lube on the inside of the cap.  Too much lube can clog the grove and prevent proper seating.  Rotate it as you press it on, and avoid side-loading the cap.  Press down evenly with fingers 180 degrees apart.  It should seat with a detectable "click" and sit far enough down on the injector so that the injection line nut never touches it.  

Injectors can also be the reason a cap leaks.  We've seen some injector bodies get deformed from over-tightening of injection lines, or debris can get in the groove from people using dirty sockets to screw them in, or from poor handling.  Always clean off the injector with a solvent and some shop air before installing the new O-rings.  New O-rings are almost never the problem.  We would say never, but have learned not to speak in absolutes, as we have seen many crazy things over the years.  We know it is tough working in an engine bay, so us a mirror to inspect the injector on the back side you can't easily see.  

When confronted with a cap that will not seal, it is necessary to see if it is the cap or the injector.  An easy way to do this is swap caps between an injector that is not having a problem, with the one that is.  We know that idea sounds awful, since getting one to seal right feels like a win, but this is science and not chance.  If it sealed right once, it will seal right again.  The O-rings when new, are very durable and will survive numerous installs and uninstalls.  If the problem follows the cap, try a different one.  Even an old one is worth a shot.  If it is the injector,  well.. not much can be done other than making sure the groove is clean.  It may have a barely perceptible nick in it, so removing it and trying a wire wheel around the cap seating area is about all the shade tree mechanic can do.  

Return line pressure can play a factor.  There should never really be any kind of pressure in the return line to the tank.  But a partially obstructed, kinked or badly flowing return line can increase pressure to the point of a leak.  The drain into the tank has a small rubber tip on it that has been known on rare occasions to glue itself shut.  Shooting some compressed air into the return line, and having someone verify they hear the air through the fuel filler neck is a worthwhile test.  If there is a question on this, you can try bypassing the problem injector, and just supply that cap with one or two lengths of hose as needed, temporarily, to see if the leak persists.  Just let the hoses collect fuel in a jug and run the engine up to temp.

Lastly, we do sell an aid for all this trouble shooting and you will find a link to it here.  This is the same rig we use to test our caps in-house.  Obtaining one can be your best bet against pulling your hair out or driving yourself insane.  If you are in need of individual parts, just give us a call.