Purging fuel filter after filter change !

[sooty]

Last year GM offered a free fuel filter (the new dual pleat) for my 2001 Dmax. I went to the dealer and had it installed free of charge. The service tech changed the filter and pumped the prime pump and never opened the small vent screw on the top of the filter housing. When I asked him about not opening the vent, he said it is not necessary to do that and it only makes a mess by letting diesel out when you are pumping. I have since replaced the filter and did not open the vent valve, just pumped the primer about 20 times until I felt some resistance. OK, so where does all of the air go under this scenario and why do they put the vent valve on if it is not needed. Jim

[mark45678]

the dealer is more worried about making a mess... the air when it get in the high perssure side causes poor injector spray pattern due to low pressure at the injector tip. I cant say for sure if this is still true with the duramax / bosch injection system, But I do know it will hurt fuel economy with older trucks with rotory pumps. I open the bleader and purge the air on my truck , never fill up with the truck running . the Duramax does have a return fuel line back to the tank , I would hope thats where the air is going.......

[DmaxMaverick]

When you "turn off" the engine, the FPR dumps rail pressure to the return. Pumping with engine off forces fuel/air through the pump to the FPR return. If you try priming with the key on, it shouldn't work. The FPR will be holding out for desired fuel pressure, which you can't reach with the primer. If you have a scanner able to read desired/actual fuel pressures, you can see a slight actual fluctuation as you pump the primer. Not much, but the desired for start/idle is ~4,000 PSI.



[edit]
To answer your question....
There's nothing wrong with priming with the bleeder closed. The primer will (should) draw fuel from the tank and push it through the system. We only need to prime the HP pump, and it will take care of the rest as long as there's minimal air in the supply line. The HPCR system is very different than previous/other systems, and handles air very well. Most of the air will get dumped to the return via the regulator. At an operating pressure of 4-26K PSI, any remaining air is negligible and will be pushed out of the system in short order.