Hi everyone, i'm looking for a little help. I have a 99 C30 with a 6.5 TD. It is a friend of mine and it came to me with an array of issues, fuel leaks, hard start, no start, heavy blue/white smoke on start up code for APP sensor. I installed new lift pump, fuel filter, APP sensor, ohmed wires from APP switch to PCM all good, repaired all grounds for engine harness to block and battery to engine block, bleed the fuel system. Connected my Snap on Modus and varified the APP function, Truck starts hard and runs real rough with smoke for about 15 to 30 seconds and in this time frame it has no throttle responce. Then in an instant, it snaps to life like it went into a "closed loop mode", smoke clears and has a throttle. I also have a new injector control module that I pluged in but no difference. Any thoughts???

 

I know this has been here awhile but I had very similar symptoms on my 2500 sub years ago and I literally replaced every fuel line, lift pump, sending unit, cleaned tank and new pmd and it ended up being what I feared and that was the injection pump. Mine would also start running just fine then start acting up again maybe 5 miles down rd and start bogging/no power and smoke.

 

6.5's are the 6.0's of the 90's.

its almost always the pump. DB4's are junk

 

I think you mean DS4 but they are not really

One cannot just throw parts at it. The said truth is the 6.5 lift is Not a pump - it is a solenoid driven spring operated pusher. If the spring weak - it buzzes but does pump pressure even though it will move fluid unrestricted

Without about 7 to 9 actual PSI at the IP at idle Or Wide Open Throttle. The OEM Pump does not work by its very design

I went through 12 OEM style all failing spec pressure in between in 1 week to a few months coming back as sluggishness or blown PMD's

Finally enough was enough and Sure Cure Lift Pump ended all the problems

The DS4 MUST HAVE ABOVE5 PSI Pressure to it at all times to work properly PERIOD

Having worked with a Stanadyne authorized rebuilder, repaired them and the test stand to check the rebuilds to spec, I an say that is the Stanadyne required minimum pressure but the DS4 is designed to take 15 PSI and has check and return valve to regulate internal pressure to 9 to 10 PSI and return excess so as to circulate fuel to cool the PMD and internally lubricate the IP as well as act as the first stage to multiply oil pressure for injection and to cause idle

The PMD is only meant to assist in throttle - not control it. When there is low inlet IP fuel pressure the PMD through PWM strains to hold the fuel solenoid (on the end in the center) more open for fuel that is not there. This blows PMD's and burns upen Fuel Solenoids. 90 percent of DS4 rebuilds need

So... it is not the DS4 that is junk it is the lift pump GM chose to use against Stanadyne's advice.

Of course the DB2 as a full mechanical IP won't suffer this Except while it may keep an engine running or start on low fuel pressure low inlet fuel runs it drier for internal lubrication and it just then dying a slow death.

So it is the lift pump and the pressure is critical if want the IP to last longer than not