Have you been chasing fuel problems? Bad gas mileage on your Chevy Truck? Then you are in the right place. I built up a 2001 Tahoe with a LQ9 (6.0L LS). Initially Fuel mileage was BETTER than the smaller 5.3L LM7 I replaced. However, somewhere along the line my gas mileage dropped by nearly 25%!
Backstory
My around 240,000 miles I started replacing original parts on my 2001 Chevy Tahoe as part of preventative maintenance. This included the fuel pump which I replaced with a new OEM pump. I did a bunch of tuning and other changes around the same time. However, I noticed in my fuel log that my mileage dropped around time I replaced the pump which lead me to troubleshoot the pump.
I discovered my fuel pressures were roughly 20% lower than factory spec! This can easily be tuned around. Even the closed loop system / fuel trims can account for it. However, lower fuel pressures in theory can lead to less atomization of fuel and thus less complete fuel burn, in short lower fuel mileage. Hence me running through multiple pumps and lots of troubleshooting some of which I’ve saved below:
| Specification | Measured | |
| OEM Pump Pressure | 62PSI | 75 PSI (deadheading pump) |
| Rail Pressure (KOEO) | 55 to 58 PSI | 45 PSI |
| Rail Pressure Engine Running | 45 to 48 PSI | 42 PSI at idle |
| Average Pump Current Draw | 6 AMPS | 4 AMPS @14Volts |
| Pump Flow LM7 | 140LPH | Didn’t measure |
Regardless of my engine build and injectors Key On Engine Off (KOEO) pressure should be factory spec 55 PSI minimum, not 45 as measured. I got a warranty replacement pump from Delphi and had same measurements, despite pump is supposed to be factory spec!
Factory Troubleshooting Routine
I utilized the factory service manual through All Data DIY as a basis to test the system. I tried two fuel pressure regulators, my original which had worked at spec pressure in past and a new one. I checked all lines for obstructions, tried two brand new pumps, checked injectors for leaks, dead headed pump, did leak down tests on rail. Couldn’t find source of issue. Then I reached out to Delphi who makes the pump.
Delphi Troubleshooting Routine
Delphi tech support was quite helpful. They provided me troubleshooting routine that went beyond the factory service manual’s procedure. Everything worked per their troubleshooting guides (shared below) with one exception. Current draw at idle was 4 AMPS, two amps lower than their average spec of 6 AMPS. I didn’t have any means to test that driving the truck. Regardless this lead me to think perhaps the pump is underrated.
Time to upgrading Fuel System pump
My truck has a naturally aspirated LQ9 with a custom built valvetrain by Roger at Vinci Performance. My goal in the build is to produce higher torque curve than the factory 6.0L across the RPM range with as much torque towards lower RPM as possible. No clue what the engine makes for power, although it’s certainly higher than the 345HP LQ9 rating, probably higher than the 400HP LS2 rating. I think much of the extra power in the LS2 is due to tuning. Most of engines are same outside of material blocks are made from. Cam specs are also VERY similar. The LS intakes compromise more than the Truck’s but they are very similar in flow.
Why not an adjustable pressure regulator?
My first though was to put in an adjustable fuel pressure regulator and calibrate the pressure according to factory spec. However, nobody makes a drop in replacement regulator as such for a factory truck fuel rail. Either way, bit of a mute point as my engine likely demands more fuel than the factory 140LPH pump can supply (140LPH @ 48PSI is 400HP). I presume I’m making over 400HP and if I get the additional upgrades I’d like in future I want to hit around 500HP which demands 175LPH of gasoline.
Figures calculated with this link: https://raceworks.com.au/calculators/fuel-pump-calculator/
Additionally, nobody makes a direct fitment adjustable pressure regulator for the trucks. I’d have to redesign the fuel lines to put one in as well as the rail and I just didn’t want to do that.
Pump fitment
This was the hardest part. I didn’t want to take my pump out of the tank just to measure the pump hanger. I eventually purchased a cheap fuel pump off eBay for $30. Rough pump hanger dimensions below:
| Measurement | Dimension |
| Pump Housing Diameter | 1.5″ ID |
| Pump Housing Height | 3.5″ |
| Pump Inlet Diameter | 11MM OD |
| Pump Outlet Diameter | 9 ~ 10MM OD |
Note: for my application it doesn’t seem to be of concern but the factory fuel supply line is 0.344″ ID 0.375″ OD. This is 3/8″. The lines are rated to 75 PSI and AN6 size. I think if I were to respec my build for over 500HP then I might need a larger fuel supply line but factory is good for what I’m doing.
Choosing a Pump
I wanted to keep factory fitment and basket. The after market pump hangers are hundreds of dollars. They also require custom plumbing adding to the expense and time to install. Since I am keeping the factory fuel tank the obvious solution is to use the factory pump assembly just swap the pumps out.
There were numerous generic pump solutions out there. Specs on those pumps were hard to come by though. I have a friend that used an AEM pump so I chose to go with them. Calling their tech support they helped guide me through what I will need. Per below:
- Pump: 340LPH fee flow (E85 / Gasoline Pump) PN: AEM 50-1200
- 10mm ID fuel line from pump to housing
- Minimum 12 Gauge wire with 20 AMP fuse to supply power to pump
I have upgraded to Flex Fuel injectors (separate topic) but don’t have any of the alcohol sensors for E85. As configured my truck is gasoline only. I could buy the cheaper gasoline only fuel pump but figured the few extra dollars maybe worth it for the E85 pump incase I add a alcohol sensor in the future. Additionally, they did the math for me saying my peak current draw would be 13 amps at 48PSI. He recommended a 20 AMP fuse with 12 gauge wire to provide a bit of headroom.
Note: Appears factory has a 20 AMP fuse for the fuel pump already. Need to verify gauge of wire. My friend who installed this fuel pump on a 2006 Silverado uses the factory wiring.
Pump Installation
You’ve caught up with me! I’ve yet to order the parts. I’ve just speced them out thus far. Will update this post as project progresses.
Related platforms
This project isn’t limited to my LQ9 build in my 2001 Chevy Tahoe. The same and or similar specs would relate to your project if you’re working on any of the following:
Gen 3 Small Block Chevy SBC: LM7, LQ9, LQ4, L59, 4.8L, 5.3L, 6.0L, 6.2L Vortec Engines
Silverado, Suburban, Tahoe, Sierra, Denali, Yukon and other GMT800 truck chassis in both 1500 and 2500 configurations
Years 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006
Note: around 2003 General Motors went to a return less style fuel system. This doesn’t utilize a fuel pressure regulator rather the power train control module (PCM) varies pump duty cycle to regulator pressure. The same should apply minus the pressure regulator.
It may apply to additional builds as well. Feel free to message me with questions.
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