Fuel Pressue

[GARY PAGE]

Lift Pump
I had an incident today and the truck died but re-started right away after a pull up to 2600 RPM. I had just filled the tank prior to this incident. The lift pump has been replaced with a carter pump and so has the OPSU. Being it was getting a little cool and damp I pulled apart all the connectors and cleaned them and then applied dielectric grease to them thinking it could be connector issue, no help it did it again. I then changed the fuel filter and drove it. After I started out in the first 2 miles it missed a bit but then caught. I assume it may have sime residual air after that small miss it was fixed it, yet a code 35 was registered; but no SES that I saw. I have been changing fuel filter a lot lately almost every oil change, so I decided to do some diagnostic investigation. I applied a fuel pressure gauge to the drain line and opened it prior to changing the old filter. The gauge would go to 4.5 psi and then quiver back to 4 PSI at idle. After I changed the filter, the gauge held steady at 4.5 PSI at Idle. Being curious I extended the line and clipped the gauge under the wiper blade and drove a bit. What I found was informative. When I pull hard the pressure drops to almost nothing. The pump is new but may not be worth a tinkers D. My take on the hole lift pump is it is a marginal design to begin with and any down stream issues cause your problems. I thing a larger pump with and external regulator would be the way to go, it's not keeping up or atleast mine is not. Does any one have advice on this before I start to re-invent the wheel?

[Turbine Doc]

Gary,
I had almost exact symptoms a month back culprit was dirty fuel filter, I connected gage same way as you with gage connected to drain on the bottom of the bowl

[GARY PAGE]

After the filter change as a said the it held steady at idle 4.5 PSI. Yet under a pull it drops toalmost nothing? I think this could put air in the system after a pull?

[GARY PAGE]

Tomorrow I am going to run a direct connect battery and ground to the pump and report readings using 12 GA wire. This will take the OPSU contacts and wiring out of the equation.

[whatnot]

COuld it have picked up something in the tank and be partially blocking the fuel pickup? It might be worth a try to blow compressed air back down the line and see if it makes a difference.

[Turbine Doc]

Gary missed the already changed filter part; my apologies, with a clean filter mine drops from 4.5 to 2 psi on a hard romp of the accelerator or heavy tow load until I hit desired cruising speed, then it settles back to 4psi if I'm running 75mph empty. I'll be adding second lift pump in parallel as I've been told by others that under right conditions delivery as designed by the General just meets demand.

[BuffaloGuy]

I'm like you Gary. I bought a fuel pressure gauge and will be installing it soon because I suspect GM might have engineered this a little too close. But if they did then it brings up:

What is the optimum fuel pressure? What is too much? I would think that even 1 or 2 psi, on the bottom end, is enough under a heavy load since the lift pump is only needed to keep a supply ready and waiting for the IP.

I am thinking of adding a new in the box carter fuel pump i have from another project but I am concerned it might create too much pressure.

Anyone know what the upper limit is?

[ucdavis]

Prior posts suggest double digits is too much, though I can't figure out why. Seems to me all the components should be capable to 10+ psi w/possible exception of the filter housing which isn't bomb-proof.
Also, prior posts indicate near zero @ WOT or a bit above or below, so that's a fairly standard finding. I will hazard a guess that a minimum of 2 @ wot would be just peachy as the IP wouldn't starve or have to expend itself to suck the fuel.

[tom.mcinerney]

Ken, see thread this forum initiated by RJWest ,9/15/'03 , topic = " 2 Lift Pumps " (now at page#5), for advice re pressure.
Also relevant:
In Ask Dr Lee, topic="pumps in series or pumps in parallel" begun by 'tbogemirep', 10/8/'03.
In 6.5L forum, topic= " Holley Blue (or Red) for Fuel Lift Pump" begun by 'slagona', on 10/16/'03.

[BuffaloGuy]

Thanks Tom Mac. I read all the posts you mentioned. WHat I gathered is that it is a concern to have too much fuel becuase of the foaming tendancy of diesel when the return fule reenters the main tank.

Dr. Lee basiaclly said that if you have any pressure at the IP that is good enough. Even 1 lb of pressure means that the IP has all the fuel it can use.

So the start to this project is a gauge (as usual). If you don't measure it how can you diagnosis it.

I've got the gauge so taht's where I will start. Thanks for the help guys!
Ken

[GARY PAGE]

Somehow my post did not take. As I said I would run a test with direct wiring to the pump. The test results indicated an in-significant increase in pressure across the band. Yet some modest improvement in the range of .25 LB. I think the relay idea to the OPS contacts feeding the coil and the relay contact feeding the lift pump are great, just make sure it has got a fly back diode (reverse diode built in the relay coil) or an external diode in reverse bias to catch the I/DT transit from frying the OPS contact due over voltage to releasing the contact coil (Spark Coil). One article suggested applying a capacitor across the OPS contacts. Don