Fuel Pump Troubleshooting - Part 1

I can't count how many times over the years that I get questions about troubleshooting a fuel pump both mechanical and electric. And some of these are very expensive. And guessing wrong is both annoying and frustrating. Because of this I have included a collection of tech references straight from Carter Fuel Pumps site to try to shed some light on this problem and attempt to help get it right the first time. A little front end info can be invaluable in making the right decision.

Carter website ref page

Mechanical Fuel Pumps Basics

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Mechanical Fuel Pump Failure - Some of those failures include:

• Diaphragm ruptures or leaks due to excessive mileage or heat
• Check valve failure
• Diaphragm spring becomes weak or breaks
• Lever arm wears due to excessive mileage

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Mechanical Fuel Pump Driveability problems

Recently, more hard starting and driveability complaints are being blamed on the fuel pump when in fact the gasoline is at fault. The current blends of unleaded fuels, such as E85 have shown excessive volatility, alcohol content and particle contamination. All of these contribute to hard starting and driveability problems. To correctly diagnose driveability problems, you need to know what is causing the complaint:

• Vapor Lock – Normally occurs after a short dead engine stop or a prolonged idle period, and shows up as a sag on accelerations, resulting in a dead engine. Cold engine driveability is not affected. If vapor lock is suspected, look for:
  • No black smoke out exhaust pipe
  • No fuel pressure (fuel pressure gauge)
  • Dry carburetor air horn
  • No accelerator pump discharge
• Fuel Foaming – Often occurs after a fill-up of fresh fuel, when cold fuel hits a hot carburetor fuel bowl. Fuel foaming shows up as a sag or a series of short jerks on acceleration, resulting in a dead engine. If fuel foaming is suspected, look for:
  • Black smoke out the exhaust pipe
  • Normal fuel pressure (fuel pressure gauge)
  • Wet carburetor air horn
  • Normal accelerator pump discharge
  • Engine starts after a long wide open throttle crank
• Alcohol Mixtures or Octane Boosters – Affect the volatility of the fuel and show up as hard starting or driveability problems. Most alcohols cause fuel system corrosion, soften plastic and rubber parts, and dislodge rust and foreign particles in the storage tanks, which can cause fuel filter plugging and air/fuel mixture changes.

What can be done to solve these problems

• Install an electric fuel pump – Fuel turns to vapor in the fuel lines more readily when it is subjected to a vacuum as in mechanical pump installations. An electric pump installed near the tank pressurizes the fuel lines and prevents most vapor-lock complaints. Also, an electric pump supplies fuel to the carburetor as soon as the key is turned. This usually solves many hard starting complaints caused by evaporation of the fuel in the carburetor bowl.
• Insulate or shield fuel lines from heat sources, especially those lines subjected to a vacuum.
• Eliminate kinks and sharp bends in the fuel supply lines.
• Install a vapor separator filter and return line back to the fuel tank.
• Install an in-line filter at the carburetor inlet to trap contamination and prevent flooding.
• Choose your gasoline station based on your experience with their products.
• Avoid alcohol and octane boosters if possible.

Basic electric pump general instructions

This doc covers a variety of general good practice notes on installing both in-tank and in-line pumps.

PDF doc [754K] link

Carter Fuel Pumps website