This tool is a "must have" for anyone who works on Duramax fuel systems. When a Duramax will not start, the very first thing to check is fuel rail pressure. If the pressure is less than 2,500 PSI (LB7), one of the most likely causes is a defective injector that is returning excess fuel back to the tank. A return flow volume test will verify if excessive return flow is the problem. This cap is then used to identify the individual cylinder that is at fault. The injection line is removed for one cylinder at a time and the cap installed on the fuel rail in place of the injection line. When the cylinder with the offending injector is capped, the rail pressure will rise and the truck will start. In some cases it can be highly advantageous to have multiple caps, because several injectors that ret.
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