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%! Here’s my notes from troubleshooting the fuel system.
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: E85 / Gasoline Pump
- Bosch 300LPH PN: 044 0580254044
- AEM 340LPH PN: AEM 50-1200
- 10mm ID fuel line from pump to housing
- Wiring: Minimum 12 Gauge GXL wire with 20 AMP fuse to supply power to pump
- Under hood Junction block terminal Pins: Metri-Pack 280 (other’s unknown)
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.
Too large a pump can boil your fuel
You can risk fuel boiling in the tank on a return style system if your pump is too large. What happens is the pump flows at it’s given rate while the regulator returns excess fuel from the injectors / fuel rail back to the tank. Say you only need 50LPH yet pump is 340 LPH then in theory 290LPH are going back to the tank.
It’s likely slightly more complex then my simple example. However, the principle applies, excess fuel goes back to the tank. If you flow too much fuel back to the tank, you’ll slowly heat the fuel to the temperature of the engine manifold where the injectors run. Over time you will risk the fuel boiling in the gas tank if it gets too hot.
Is my pump too large
Gasoline is designed through additives to resist boiling and instead vaporize. This makes the boiling point highly dependent on gas additive package along with altitude & temperature. Packages vary from station to station even where the station is located. Your gas could boil under right conditions say high altitude. When I ascend to 7500ft in heat my gas has boiled on it’s own. I cannot say with certainty due to additive packages what atmospheric pressure and temperature your gas will boil at. Long story short, if you touch your tank:
- Tank feels hot to touch, well over 100F
- It’s not hot outside, say 70F
- You’re not high in the mountains lets say under 5000ft
Then your pump could be returning fuel to tank so fast it’s heating the tank to temperature of fuel rail under the hood. In short, your pump maybe too big.
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
After I ordered the parts a friend recommended another pump to me! Instead of my AEM 50-1200 with 340LPH he recommended a Bosch 044 0580254044. It’s 300LPH E85 compatible pump and nearly a $100 cheaper! Such is how it goes, I’ve already ordered my AEM pump which to their credit is a very good quality and great tech support which maybe worth the extra $$ in and of itself.
Upgrading the wiring
First my thoughts on wiring. You want to be on the safe side. The circuit in the truck for the sending unit has a 20 AMP fuse. Calling AEM, and reading the pump instructions, said to make sure you have 12 gauge wiring or larger to the pump. Additionally, technical support at AEM told me my pump would draw 13.5 Amps at 55PSI and advised at least a 15 amp fuse.
I couldn’t easily determine what gauge the factory wiring was. It was clear it wasn’t 12 gauge. I could have cut it and spliced it back to count the strands, their dimeter, and metal type. Then calculate the exact gauge. It was however, signifantly thinner than the 12 gauge wire I ordered so I installed the 12 gauge.
Next, I MUCH prefer a mechanical connection as apposed to a soldered or crimped connection. Specifically terminal ends. These are what factory uses and in my opinion are the best way to go. If you want to be SUPER well connected, terminals and solder. However, I’ve had solder come apart before and break the connection so be warned.
The last think you want is wiring with a loose connection. It will create heat and possibly a fire. Combine that with fuel and you have a recipe sure to destroy you hard work and investment in your truck! After I completed everything I went back through voltage drop tests and felt for heat in the connectors.
I took apart the under-hood junction block. There underneath is a red connector. You’ll remove that with 7mm socket and a flat head screw driver. Loosen the bolt completely, then slide the flathead in the left and right sides (perpendicular to you). Wiggle the connector out. Once it’s out there’s two blue locking clips. Remove the one off the right hand side. It will slide right out with a little work from flate head screwdriver.
Using a connector pin tool, release the clip for Pin F1 (labels on rows and columns, see picture). This is the supply line to the pump, I believe it’s a female Metri-Pack 280 pin. I also believe the pins to the fuel sending unit are Metri-Pack 150 in the sending unit connector. You may like to order those to re-pin your new wire instead of soldering (don’t splice!). Next, there’s a ground on the frame / body mount between the front driver’s and passenger door. This is a dedicated ground to the pump. These two lines go directly to the fuel sending unit connector.
I originally considered running a fresh wire loom with just those two wires in parallel to your pump. However, when I manipulated the harness going to back of truck which contains the pump power supply line it disintegrated. I ended up taking apart, inspecting, re-taping, and putting new wire loom on the harness instead.
This is where things start to get really interesting. I never did figure out what pin terminals are on the harness from the pump itself to the sending unit, Metri-Pack 580, 680? This is partly because I got lucky! That cheap pump I bought had the correct connector for the pump and I simply depopulated the sending unit and repopulated it with the correct connector.
You’ll have to do some investigative work here to figure this out. I know people love those little crimp / butt splices. I HATE them. I’m a bit of a perfectionist and think working on your fuel system is the place to do that because the risk of loss or worse, injury is too great.
Had I not had that connector (labeled Old in photo above) I’d have searched till I found the correct pin terminal and populated the New harness from AEM with the correct pins. In short, I’d have done it right!
Dropping the tank
This is the easiest part of the whole deal! I cringe when I see people cutting holes in their trucks to get at the fuel sending unit! No need for that! Once you know how to get a tank off the GMT800 chassis you can do it as easy as removing a tire! It’s SUPER simple. So long as your tank is empty or only has maybe 5 gallons or less in it. Don’t ask me how I know that…
To make your life easier get your rear tires up on some ramps. You can optionally raise the front of the truck too but not really necessary.
Loosen the tank straps There’s two straps holding the tank on. They have 15 mm bolts. Loosen those slightly to get the tank down about an inch.
Disconnect the Evap system Hardest part is the bleeping quick disconnects for the evap system so tackle those first. Make sure to release the tabs (don’t break them) and pull the lines off the canister. You’ll want to unclip the vent line and any other lines for evap that didn’t come off with the vent canister. There’s two styles. I have the older style with evap canister on front inboard side of fuel tank. I only have to un clip the vent line.
Disconnect fuel filler and vent these are at rear of the tank, w/ 8mm ring clamps. Loosen the ends by the tank. Pull off lines from the tank if you can, if not leave be until you lower tank fully and they’ll come off.
Disconnect the wiring harness you’ll now take off the rear tank strap all the way (leave front on but can loosen more). Reach up (may have to do by feel). The tank pressure sensor is cliped on. Raise the tab and pull it off.
The pump supply line and level sensor harness is harder. It has a retaining clip. You need a mechanic’s pick in your hand to push down center of the clip and pull off. It is SUPER simple if you’ve seen it. However, you may need to unclip the harness retainer on the center support (metal bar above center tank). This will give you enough free play to lower the tank further and disconnect when you can see it.
Disconnect Fuel Lines front of the tank, above the slanted crossmember on the frame are the fuel supply and return lines. They are 3/8″ and have click tabs that connect the metal lines (to engine) to the PVC lines (on the tank). You won’t be able to see them but they are easy enough to unclip by hand. Disconnect both of them. Then push the PVC quick disconnect fittings off the metal line set.
Remove tank if you haven’t already remove the pump harness! Depending on how high the truck is, you can remove the front tank strap so long as the tank doesn’t hang by the pump harness. Now look at the retaining clip and remove it, and the harness. Your tank should be free! You can pull it out from under the truck.
Removing the sending unit
This part is self explanatory. If you can see it and you got this far you can remove the pump. A word to the wise though, clean the top of the tank off BEFORE you remove the sending unit. You don’t want to risk any debris falling into the tank where it’ll only clog up the sending unit filter or worse destroy the pump!
If this is your first time removing the tank, you can also clean up the inside of the tank. I used dish soap and LOTS of water. Then dried with clean rags as best I could; blew out with compressed air and let set to thoroughly dry over a couple days. Optionally, they sell detergents specific to cleaning tanks at your local parts store.
Getting pump in the sending unit
I had purchased a sacrificial $40 sending unit to study before I dropped the tank. This turned out to be a completely different basket design than the one I had in my tank THANK GOD. The cheapo sending unit would have needed lots of modification, as it turns out the sending unit in my tank required virtually none.
Here’s where you need to start getting creative. First you’ll have to cut the old fuel line out of the sending unit. I cut mine on the sending unit side. Then I de-pinned the original pump from the connector inside the sending unit. Next I disassembled the basket.
Thankfully I was able to use the pump harness and re-pin the sending unit with it from the cheapo sending unit I bought. You may have to get correct Metri-Pack terminals and re-pin accordingly. I just got lucky there.
I was also lucky in that the OEM sending unit style I had fit the AEM pump! I only had to do very minimal trimming with a rotary tool. The hardest part was getting the new fuel hose to fit in the sending unit. It wasn’t as flexible as the OEM hose so I had to cut to nearly the exact length. Additionally, MAKE SURE you install ring clamps on both ends of the hose to ensure it’s well mechanically attached to the pump. If that comes off it shouldn’t really hurt much but you’ll be stranded and repeating this process.
WARNING USE EAR PINCH CLAMPS HOSE CLAMPS for the fuel hose in sending unit! The screw TYPE clamps can loosen over time where as the pinch style will not! You will find yourself on side of the road if your sending unit hose comes loose in the tank.
Conclusions
I completed this process and my fuel pressure was unchanged! I suspect, the fuel pressure regulator I purchased was bad. I’ve had a run of bad luck with OEM parts lately. I theorize that once vehicles are old enough there’s no more warranty claims to worry about the manufacturer and their OEM suppliers focus on building new vehicles. The quality of OEM parts of legacy vehicles suffers as a result.
It’s good in this situation to understand which aftermarket suppliers are selling quality parts, have good technical support, and most importantly aren’t all snake-oil salesmen. Aftermarket industry is a minefield of low quality stuff and partly why most of us stick to OEM.
AEM was FANTASTIC. You cannot go wrong with their pump. However, if you want to try that Bosch pump for less money I hear good things. It’s also not going to require connector pin type guesswork as it’s got screw terminals. You can simply cut the factory harness, crimp ring terminals, and screw on. However, so far as I know Bosch doesn’t have a hotline you can call for sales support. I called AEM and they told me what to order for my setup as well as imparted lots of technical knowledge.
Additionally, you maybe able to get by with factory wiring. I didn’t want to roll the dice though so I upgraded. 12 gauge is way more than good enough for 15 amps. The factory harness has a 20 amp fuse to pump and I was told 14 gauge wire but haven’t been able to verify that part. Not sure what current draw the Bosch pump is at, likely less but don’t quote me. Factory spec pump draws as much as 6 amps. I chose to be on the safe side.
For reference my AEM pump shows 10.5 amp draw at 12.6V with engine off.
Tuning Notes
The pump was a needed upgraded, despite my fuel pressure is still out of specification. I logged data after the install and my fuel trims are all good. However, cold idle and cranking are not. Engine won’t turn over or stalls after it does. I suspect my idle airflow is now off and need to recalibrate that in my tune.
UPDATE resolved fuel pressure issue
My fuel pressure regulator was an out of wrong spec pressure regulator. I swapped in one advertised as 57PSI and I got 57 PSI. Turns out the parts catalog I ordered my regulator from when I replaced the pump originally had a flex fuel regulator in the list of gasoline engine parts. The flex fuel regulators are lower pressure than the gasoline regulators. Yet they never listed a pressure for the regulator at issue so I had no way of knowing other than through troubleshooting.
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.
