another lift pump question

[Showgood1]

On the '93 6.5 I had the oil pressure swich went bad so I wired around it to keep the pump going untill I could get it fixed. The guy doing my waranty work said that the pump is not supposed to get the full 12v but instead is stepped down to 10v or so somewhere else in the system. I know the voltages are nominal 13.5 to 16 but the point was that it was stepped down. It wasn't long after that trip to the shop that I had to take it in again and have it replaced. He thought that my temporary solution may have shortened the life of the pump and that the higher voltage ment slightley higher pressure (going into the injection pump) wich contributed to the poorer milage I was getting. I know the system is completely different on the dmax's but can the higher input pressure at the pump hurt performance buy makeing it harder for the system to controll the pressure in the fuel rail? Or is the pressure just so high in the fuel rail already that it doesn't matter?

[DmaxMaverick]

HOGWASH!!

Higher pressure is not going to cause injector pump damage, lift pump damage or poor mileage. If anything, it will help all the above.

The current is not reduced to the pump by design. The voltage may drop, but it is due only to GM's poor wiring to the pump, which includes the weak OPS.

On/before startup, the pump is powered by a relay controlled by the PCM. After start, the OPS supplies voltage to the pump.

I suggest you find a REAL mechanic. Not someone that makes excuses and rationalizes poor design.

The rail pressure on the Duramax ranges from about 4000 to 23000 PSI. A lift pump with a few PSI's won't have any effect on that side of the system. The system also has a low pressure side, inside the same pump. It's job is to supply the high pressure circuit with a constant pressure fuel supply. Adding a lift pump between the tank and fuel pump will not damage anything. It will convert the current system from vacuum (tank to pump) to pressure. This has benefits and drawbacks which have been discussed before, so I won't go into them now.

IMO, a lift pump will have no damaging effects on the Duramax fuel system. At some time in the near future, mine will have one.

[ 02-01-2004, 12:48 PM: Message edited by: DmaxMaverick ]

[jbplock]

Originally posted by Showgood1:
...I know the system is completely different on the dmax's but can the higher input pressure at the pump hurt performance buy makeing it harder for the system to controll the pressure in the fuel rail? Or is the pressure just so high in the fuel rail already that it doesn't matter? Showgood1,

As Dmaxmaverick said the Dmax has two OEM pumps - a low pressure pump that supplies the high pressure pump. My understanding is that pressure on the rails is regulated on the high-pressure side. So a slight pressure increase from an added lift pump at the input to the OEM low-pressure pump should not adversely affect operation of the high-pressure side. The lift pump may actually help maintain fuel flow under high demand situations like towing a hill with a performance box. Some of us (myself included) have a bypass regulator on our lift pumps to limit pressure to approx 0psi while others are running without regulation. As far as I can recall, no one has reported problems with either of these arrangements.

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