A common complaint on the 5.2 Diesel is: “After sitting overnight, the engine does not start unless I pump the primer or spray some WD-40 at it.” While a number of issues can be present, one of the more common failures is a leak in the low side fuel supply to the fuel injector pump.
This problem can best be summed up with an example.
Remember the last soda or cocktail you played with. You stuck the straw in the glass, put your finger over the top of the straw, then lifted the straw up. The liquid stayed in the straw until you slid your finger off the top of the straw. Puff, all the liquid drained back into the glass. The same thing is happening with the low side fuel system. A small leak allows the fuel to drain back to the filter or further. The result is a crank, no start – at least without priming it.
Here are some good numbers to have handy. Typically, cranking vacuum on this engine without any problems or issues will be 3” to 5” Hg. Once the engine is started and running, the vacuum will drop to 2” to 3” Hg. Accelerating the engine to 2000 to 2500 rpm without a load, the vacuum will rise to maybe 5” Hg. The other magic number is when the engine is shut down, the system will hold 0.5” to 2” Hg almost indefinitely.
Start with a vacuum gauge being tee’d into the hose leaving the fuel filter housing headed for the fuel injection pump. Bleed the line out using the primer. Then start the engine. While the engine is idling, block off the fuel line between the tee and the filter housing. You will notice the vacuum rising. On a good system the injector pump will make at least 18” to 20” Hg before the engine will stall. That’s what you are going to look for. If the pump only makes 10”, 14” or 16” Hg before it stalls, there’s an air leak. Inspect the hose for rot and cracking. Remember, air will leak in before fuel will leak out.
Let’s say the pump made 21” Hg and the engine continued to run. Great! Release the blocked hose and allow the vacuum to stabilize. Now move to the hose going into the fuel filter inlet. This time you will block the inlet hose. It will take a little longer to make the same vacuum; you will see the primer plunger actually get pulled down. The numbers are the same, 18” to 20” Hg before the stalls. If it cannot make the vacuum, there is a leak in the fuel filter housing assembly. Possible components include the housing itself, the fuel filter or the water in filter (WIF) sensor.
If you continue to see a good vacuum being developed, just continue back toward the fuel tank until you get a “failed” reading. At that point you just went by the leak or problem. Don’t forget O-rings are a good possibility, also where the hoses and line meet steel. In the case of plastic lines, you do not want to crush them, so you will need to make up a block-off tool to continue testing there.