Fuel System Troubleshooting

Use the Q&A (questions and answers) below to help diagnose fuel system and related problems you may be having.
 
Q: I'm using a dead-head regulator on a carbureted 450 HP motor. What is the smallest fuel pump I should use?
A: Multiply horsepower by .23 (450 x .23 =104 GPH). Mallory's 110 and the Weldon #A2011-A are lower volume pumps, and therefore can be used with a dead-head regulator on this application. However, we would recommend the Weldon A2011-A, Mallory 140 GPH or compatible pump be used to offer some added insurance and a bit extra volume to compensate for unforseen circumstances.
 
Q: I'm using a return-style regulator on a carbureted 450 HP motor. What is the smallest fuel pump I should use?
A: Multiply horsepower by .17 (450 x .17 = 76 GPH). The next larger pump is a Mallory 110, but this pump is not designed to work on return style systems. It is best to use the Weldon A2005-A or the Mallory 140 GPH fuel pump.
 
Q: On a carbureted 450 HP engine and using a 140 GPH pump. What is the minimum fuel line size from the pump to the regulator?
A: 3/8" (-6 AN) is the minimum, but 1/2" (-8 AN) is recommended, especially if you plan on making more power in the near future.
 
Q: On a carbureted 450 HP engine using a 250 GPH pump. What is the minimum fuel line size from the pump to the regulator?
A: 1/2" or -8 AN  (Remember, fuel line size from the pump to the regulator is determined more efficiently by horsepower, not fuel pump size.)
 
Q: On a carbureted 450 HP engine, I have a 140 GPH pump and a return regulator. What is the minimum return line size?
A: 1/2" or -8 AN (Return line size is determined by fuel pump output). Remember, the return line demands zero or negative pressure for optimum performance and accuracy.
 
Q: On a carbureted 450 HP engine, I have a 250 GPH pump and a return regulator. What is the minimum return line size?
A: 1/2" (-8 AN) is bare minimum, but you may find that this application requires a 5/8" (-10 AN) line. (Again, return line size is determined by pump size, regardless of horsepower.) Remember, the return line demands zero or negative pressure for optimum performance and accuracy.
 
Q: I have a carbureted 450 HP street car and DO NOT want to purchase a fuel pressure regulator. What pump should you use?
A: This is really not a good option, but use the formula: 450 HP x .23 = 104 GPH. The next biggest pump is Mallory 110 GPH. The Mallory 110 GPH pump is a low pressure pump and can be used without a regulator. (Not my first choice for optimum power and safety!)
 
Q: I have decided to Drag Race my carbureted 450 HP street car with the 110 GPH pump and no regulator. At the track my fuel pressure drops from 7 PSI to 3 PSI during a pass. What can I do?
A: First, do not contact tech or warranty support and complain that the pump is defective! In general, ALL electric fuel pumps should be used with a separate pressure regulator when racing. The Mallory 110 GPH pump is a low pressure pump so you have (3) options:
  • Reduce the pressure drop by increasing fuel line size. This will help, but more than likely will not cure the problem.
  • Add a dead-head-style regulator and adjust the increase the pressure in 1psi or less increments and test. This is a sort of trial-and-error bandaid to a system that needs better, more compatible components. If the pressure is set too high it will blow out the needle and seat in the carburetor fuel bowl, possibly causing leaks and an over-rich condition. Finding a happy medium between adequate pressure that does not cause other problems and trying to fix the pressure drop issue might not occur.
  • Replace the 110 GPH pump with a higher pressure and volume Weldon #A2011-A or Mallory 140 GPH pump and a dead-head regulator. (A return style regulator is recommended)
 
Q: I have a 140 GPH pump and a dead-head regulator. When I jump on the throttle suddenly (or activate a Nitrous button), the fuel pressure drops 3-4 PSI and then comes back up in about a half-second. What's wrong? How can this problem be corrected?
A: The temporary drop in pressure is due to the recovery time of the regulator. Using a very large fuel pump along with a very high bypass pressure may help. However, the use of a return-style regulator is your best cure. Review the example provided in the Fuelish Tendencies article.
 
Q: I have a dead-head regulator and it will not adjust above 4 or 5 PSI. What's wrong?
A: There are (3) possible causes:
  • You are trying to adjust the pressure without the engine running.
  • The pump is only producing 4 or 5 PSI (the pump is a low pressure type or the bypass is malfunctioning).
  • The regulator is malfunctioning.
 
Q: I have a dead-head regulator and it will not stay at the pressure I set it to. The pressure slowly increases, especially at idle. What's wrong?
A: This response offers (4) possibilities:
  • You're trying to adjust the pressure without the engine running.
  • You are experiencing "pressure creep". This is common with dead-head regulators. If there is an O-ring on the plunger of the regulator, make sure it is not damaged.
  • The diaphragm in the regulator is leaking. Separate the regulator and check the diaphragm. It should be dry on top (or the opposite side of fuel delivery).
  • The bypass in the pump is stuck and the pump is producing excess pressure. Install a pressure gauge just before the regulator and verify pump pressure.
 
Q: I am using an electric fuel pump and a dead-head regulator. During full throttle acceleration, the pressure gauge shows a gradual drop before I lift off the throttle. What's wrong?
A: Here are some possibilities:
  • The fuel pump is too small ... (multiply HP x .23)
  • The fuel pump is not supplying the rated fuel volume. Connect a short hose to the outlet of the pump and fill a gallon fuel jug. For example, a 70 GPH pump should flow at least 1 GPM (gallon per minute); a 140 GPH pump should flow 2.1 GPM; a 250 GPH pump should flow 3.75 GPM; a 500 GPH pump should flow 7.5 GPM.
  • The fuel pump is not getting adequate power supply of at least 12-volts. Splice a voltmeter into the power wire (red wire on Mallory fuel pumps) near the pump. With the pump operating and the engine running at approximately 2000 RPM, there should be 12-14 volts at the pump. If not, use larger supply and ground wires, add a fuel pump relay, check vehicle grounds, or possibly upgrade to a better charging system.
  • The fuel pump bypass pressure is too low. Connect a pressure gauge just before the regulator. At idle, the pressure should be at least 3 PSI higher than the set pressure of the regulator.
  • There is too much restriction in the fuel lines. At full throttle, the pressure just before the regulator must stay higher than the set pressure. It it doesn't, there is too much restriction in the fuel lines and fittings, or the pump is too small.
  • Fuel filter(s) are too restrictive or clogged
  • Too much restriction between the fuel tank and the fuel pump.
  • The fuel pump is "sucking air". Check the pickup in the fuel tank and the fittings between the tank and the fuel pump. If fuel coming out of the pump looks "milky", there is air entering the system. Air can be drawn in from a fuel tank pickup that is too high, low fuel level in the tank, and around the fuel fittings (even though fuel may not be leaking out).
  • The regulator diaphragm is damaged. The top of the diaphragm (inspected upon disassembly) should be dry.
  • The fuel pump is "losing prime". Ideally you want to mount the fuel pump level with the bottom of the fuel tank, and as close to the tank as possible. Otherwise G-forces may cause the pump to lose prime. (NOTE: The #1 cause of "burning up" a fuel pump is due to its losing prime. The fuel running through the pump is actually keeping it cool.) Having a fuel cell or fuel tank with a sump area can cure most prime issues as long as the pump is not mounted too high.
 
Q: I am using an electric fuel pump and a return style regulator. During full throttle acceleration, the pressure gradually drops until I lift the throttle. 
A: Here are some possible causes:
  • The fuel pump is too small (multiply HP x .17)
  • The fuel pump is not supplying the rated volume. Connect a short hose to the outlet of the pump and fill a gallon bucket. A 70 GPH pump should flow at least 1 GPM (gallon per minute); a 140 GPH pump should flow 2.1 GPM; a 250 GPH pump should flow 3.75 GPM; a 500 GPH pump should flow 7.5 GPM.
  • The fuel pump bypass has not been plugged or disabled (NOTE: All Mallory return regulators include instructions on how to disable the bypass in a Mallory fuel pump. If you are using another brand you must contact the manufacturer.)
  • The fuel line between the tank and the fuel pump is too restrictive.
  • The fuel line between the pump and the regulator is too restrictive.
  • The fuel pump is not receiving adequate power supply of at least 12-volts. Splice a voltmeter into the power wire (red wire on Mallory fuel pumps) near the pump. With the pump operating and the engine running at approximately 2000 RPM, there should be 12-14 Volts at the pump. If not, use larger wires, a fuel pump relay, or a better charging system.
  • The fuel pump is "sucking air". Check the pickup in the fuel tank and the fittings between the tank and the fuel pump. If fuel coming out of the pump looks "milky", there is air entering the system. Air can be drawn in from a fuel tank pickup that is too high, low fuel level in the tank and around the fuel fittings (even though fuel will not be leaking out).
  • The fuel pump is "losing prime". Ideally you want to mount the fuel pump level with the bottom of the fuel tank, and as close to the tank as possible. Otherwise G-forces may cause the pump to lose prime. (NOTE: The #1 cause of "burning up" a fuel pump is do to it's losing it's prime. The fuel running through the pump is actually keeping it cool.) Having a fuel cell or sumped fuel tank can cure most prime issues as long as the pump is not mounted too high.
  • There is too much restriction in the return line. With the engine "off", activate the fuel pump and back the adjuster screw off. Pressure should now be less than 3 PSI.
 
Q: I'm running a Nitrous system. When the Nitrous system is activated, the fuel pressure drops to an unsafe level. What can I do?
A: It's best to run two separate fuel systems with a return style regulator on both systems, or at the very least on the Nitrous system. If you're going to use one large pump (rather than two smaller pumps), you may use two dead-head style regulators is parallel, or preferably a return and then two dead head regulators connected parallel. However, dead-head regulators are difficult to adjust correctly in a Nitrous application since the fuel is not flowing except when the Nitrous is activated. You will need pressure gauges that are visible during a pass down the race track to verify safe pressure levels and accuracy.
 
Also, dead-head regulators have a slower recovery time, which can lead to temporary pressure drops. If you are running only one pump and one regulator (not recommended), you should use a large return regulator. DO NOT use two dead-head regulators in series. NEVER run more than one regulator in a system with a return style regulator.
If after reading this information you still have questions, please create a support ticket on our Helpdesk.