General Modified EFI Fuel System Diagnostics

General Modified EFI Fuel System Diagnostics: For most mechanics, diagnosing fuel system failures is a relatively simple task. OEM Fuel systems have already been designed and tested in its entirety with the vehicle. Once the Fuel System has been modified, all bets are off. Having a modified fuel system on a vehicle complicates diagnostics dramatically, as many assumptions made during diagnostics on an OEM fuel system are no longer true. No longer is it assumed that built up heat can escape, or that the fuel line size used is adequate. Here are a few tips regarding diagnostics on a modified Fuel System. The topics discussed are not related to seeing if there is a blown fuse or an empty fuel tank. We assume that you would look at these more obvious failures anyway. These are some "not so obvious" problems that can occur.

Most Common Symptoms:

  • Cause: Cavitation. Symptoms include:
    • Inconsistent fuel pressure. Occasional or repeated Pressure Drop-offs.
    • Fuel system failure occurs after operating for a few to several minutes before symptoms are noticed.
    • Symptoms worsen over time. Each time the vehicle is operated, symptoms occur sooner and sooner.
    • As the vehicle warms up or in hot weather, symptoms worsen.
    • Pump operating sound becomes loud or has a "grinding" or "ratcheting" characteristics.
    • Pump is very warm or hot to the touch.
  • Cause: General pump failure or inlet plumbing failure. Symptoms include:
    • Operating pressure is lower than regulator set pressure, or possibly no pressure.
    • Operating pressure may be inconsistent.
  • Cause: Insufficient capacity. Symptoms include:
    • Sharp fuel pressure "drop-offs" during high engine load (normal operation during low power operation).
    • Typically symptoms occur in "higher gears" during a drag race run.

If you are experiencing any of the above issues, the problem with the fuel system is not the regulator, but likely the fuel pump or its installation. Restrictive, dirty or blocked fuel filter(s) can also create any of these symptoms as well. Replacement of filters is the typical "first point" of component replacement. Filter replacement is typically helpful only with fuel systems that have many tens if not hundreds of hours of operating time. In very rare instances (particularly considering the use of E85 or Methanol), filters can become excessively dirty after very little time of operation (cases of extremely "dirty" fuel).

Cavitation Failure: If the Fuel System is new or newly installed, the problem is likely an improperly designed (or modified) fuel system. While pump failure could be simply a bad pump, more likely the cause is improper plumbing due to having too small of an inlet fuel line size.

Other causes can include too much heat build-up or having the mounting location too high or too far from the fuel tank (external mounted pumps). One special note about cavitation: A pump that has experienced cavitation, even for a very short duration, is most likely damaged requiring repair or replacement. As people who have experienced this failure always note that the symptoms get worse and sooner, each time the system is operating. Cavitation can also occur when fuel temperatures at the pump are high. As a diagnostic rule: If the pump is too hot to comfortably keep your hand on it (above 120°F), then heat may be building up too much, causing the failure.

Another under considered possible source of failure can be tank venting. If the venting system is not modified or is allowing a vacuum to be produced, then cavitation can certainly be the result.

General pump or plumbing failure: If a pump is damaged or not operating efficiently, then it may not be capable of producing enough pressure. The role of the regulator is to restrict or prevent return flow back to the fuel tank (the regulator allows fuel to return to the fuel tank, once the fuel pressure is at the set pressure). In rare instances, debris is capable of getting lodged in the main seat thus preventing proper sealing (allowing fuel to leak though the seat, returning back to the fuel tank).

There is one quick way to diagnose if the failure is debris in the regulator: Step 1: Plumb the return line from the regulator into a fuel safe container. Step 2: Operate fuel system while observing the return line. If fuel is exiting the line, while the pressure is insufficient, then the failure is debris in the regulator. If fuel does not exit the return line, then likely the failure is something else).

In some cases (rare), a properly operating pump can experience these symptoms by pumping air. Air can enter into the fuel system through fuel line leaks, or air being introduced in the fuel tank (such as a return line that is "mixing in air". An example of this last issue is shown in this Fuelab video:

Insufficient Capacity failure: Insufficient capacity is fairly easy to understand. If the pump's flow capacity under pressure is not enough, then pressure drops off under heavy engine load. While this failure seems very obvious, it may have other causes instead of a failed pump or even too small of a pump. Plumbing issues such as discussed in the previous section can cause this.

Another suspect worth investigating is voltage. Just because a pump is turned on and is operating does not mean that the voltage is sufficient. The best way to check this is while the pump is operating: check the voltage AT THE PUMP. Checking the voltage at other location can give improper results.

While failed components on themselves can be an issue, the biggest cause of these described failures is the implementation or operating conditions. Just as one should not blame a failed engine component on faulty design or a production flaw, if there was no oil in the oil pan. This same analogy can be applied to many issues regarding fuel system failure. Proper diagnostics of fuel system failures will help prevent needless component replacement and frustration.

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