33willys

iTrader: (1)

I can get only 50 pounds pressure at the fuel rail. Engine is a 2003 5.3 LM4 with the regulator at the fuel rails. I have replaced the regulator and in tank pump. The new pump is a Bosch unit. Fuel lines at first were 3/8" brake lines but changed them to 3/8" and 5/16" OEM nylon lines. My fuel pressure test gauge reads 58 LBS on my 2005 Silverado so I don't believe the test gauge to be reading wrong. Shortly after going into closed loop codes are set for lean condition on both banks. Long term fuel trips are maxed at 25% which also leads me to believe low fuel pressure. So, two new pumps, two regulators, new lines and new filter, what could be causing low fuel pressure?

Blue89Bird

Voltage.

ls1nova71

iTrader: (4)

Is it a flex fuel engine? They operate at lower pressure. They have a different pressure regulator on the rail, but you can't swap them out because the rails are different. Does the vacuum hose on your regulator go straight in the side, or straight up?

33willys

iTrader: (1)

The vacuum hose goes up on my regulator. If this is true that I have a flex fuel engine will I have to change out the rail and injectors to match the tune? Thanks Nova, you have given me the only solution that would explain my mystery.

Blue89Bird

pretty sure an LM4 is just the all aluminum version of the LM7 and isn't flex fuel.

33willys

iTrader: (1)

Darn the luck, that would have explained everything. Once again back to square one. What could possibly cause the pressure to be 50 pounds? My electrical system is healthy so the pump is getting what it needs I'm sure. Open to other ideas! Is it possible the fuel rail is partially clogged?

hookemdevils22

iTrader: (12)

did you actually test voltage at the pump?

DW SD

Run 12 to 14 gauge wiring to the pump with the hot site relay-energized. Measure it with a DVM (digital volt meter) @ the pump while running (should be 13V+). Make sure the grounding location is a good one. (I weld small studs or bolts to my frame for grounding points. Make sure the grounding point is clean and unpainted, not just the threads.

Doug

truckdoug

iTrader: (25)

the regulator could be bad, or not seated properly. the o-ring can leak or be pinched pretty easily.

did the injectors come with the rails and regulator? they should be ev6 type plugs for flex fuel and ev1 for regular truck and f-body

33willys

iTrader: (1)

Took the engine out of an '03 Trailblazer and put it directly into the '39. I did take the intake off and cleaned but everything went back together with original parts (new gaskets). Just got done testing the voltage at the pump. Gauge on dash rears 14.1 volts while running and voltage at the pump reads 13.9 volts. Long term fuel trips at about 20% and pressure is 48-50lbs. Going to get a third regulator today (used) and give it a try. Will have to inspect the fuel rail for junk but doubt that's the problem because after replacing everything the pressure stays the same. Real mystery and I'm missing something basic I'm sure. Keep the ideas coming. Thanks

truckdoug

iTrader: (25)

do you have the vacuum ref disconnected when you're testing?

ls1nova71

iTrader: (4)

That's the standard non flex fuel rail and regulator. What fuel filter are you using?

33willys

iTrader: (1)

Rainbird! First thing I did this morning was to remove the regulator from a friend's 6.0 that had been setting for several years. Somehow, I say SOMEHOW the round filter behind the regulator flipped out and directly into my mouth. Stale gas. Two jugs of lemonade later...still stale gas. OK, swapped the regulator into the 5.3 and...rainbird. Extra step on the 6.0 regulator and it leaked. Good news is the battery quit and stopped the pump and thereby limited the leak. Cleaned terminals, charged battery and drank Friday night supper. Started engine, 50 Lbs. Filter is from an '03 Colorado I5. Reason I picked that one is in one end and out the other and size is 3/8". Before that I used an '03 Trailblazer which had both inlet and outlet on the same side and required a 90 degree fitting that I thought would restrict flow. For the love of Pete what am I missing?

truckdoug

iTrader: (25)

if you got stale gas, could you have an obstruction in the pump?

33willys

iTrader: (1)

OK, still got only 50 lbs but when I rev it a bit the pressure drops a couple of pounds. I thought that when the throttle is popped there is a dip in vacuum and the fuel pressure should go up slightly. Under (somewhat) heavy acceleration vacuum drops which causes fuel pressure to rise and feed the heavier engine load. Is this correct?

ls1nova71

iTrader: (4)

Yes that is correct, odd that uours goes the opposite way. What does it go to if you unplug the vacuum hose, it should max the pressure out.

33willys

iTrader: (1)

Now that the NASCAR race is over I have time to do some testing. Pulled the vacuum hose from the regulator and the fuel pressure went to 56 pounds. Plug it back in and it goes to 48 pounds. Checked the long term fuel trims and without vacuum they read about 12% and connected they go up to about 20%. This is a new regulator but appears to me "sumthin ain't right"! First, the fuel trims indicate that without vacuum a 56 pound pressure reading is about right. Second, pressure should not drop that much with vacuum applied. Wrong regulator? Bad regulator?

DW SD

Are you certain of the calibration of the fuel pressure gauge? Seems like three regulators reading 50 psi might indicate your gauge is wrong?
Did I understand that right?

Any chance you have a vacuum leak or improperly connected hoses creating the lean condition?

Doug

33willys

iTrader: (1)

Doug, anything is possible but all indications are the regulator is bad. This is actually the second regulator, the used one was from a 6.0 and would not fit. I tried the gauge on my '05 Silverado and it read 58 Lbs. The long term fuel trim are high and seem to validate low fuel pressure. I did replace the intake gaskets and the engine seems to run fine so I really don't believe there is a vacuum leak. The tiny hose going to the regulator has plenty of vacuum so I've ruled that out also. According to research the pressure should jump up about 10 pounds if the vacuum hose is disconnected which mine does, it just starts low. I even tried blowing into the regulator to see if the pressure would come up. That didn't work but I did have a neat light headed trip! Before buying another $50 regulator I would like to fine a used one that is known to be good and try it.

not2oldchevy

I had the same problem with o3 5.3. That fpr for that and some more[wifes 2004 Tahoe] will just go to about 50 lbs. Some of the online parts suppliers will list the pressure they hold. The fpr for the 58 lb rails will not fit your rail. Before everyone screams flex fuel I know mine and wifes are not flex fuel.