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- Pre and Post Pump Fuel Filter Considerations
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Pre and Post Pump Fuel Filter Considerations
Pre and Post Pump Fuel Filter Considerations: Most mechanics know that almost every fuel system has two fuel filters. These two filters serve two very different purposes and therefore have very different attributes. While everyone thinks about "Post – Pump" filters, many do not consider the "Pre – Pump" filters. Often called "Straining Filters", these coarse filters are responsible for preventing debris in the fuel from being able to lock-up or damage the fuel pump. The filters are typically very coarse in nature, particularly in high flow applications. This filter is coarse, not just to reduce pressure drop, but to ensure that fine debris will not begin to block flow over time. Fuelab uses a 75 micron rating for its straining filters (including the in-tank filter Model 83801). Fuelab also recommends the filter rating be between 75 and 150 microns for the "Pre – Pump" filter, when using their fuel pumps. While it may be tempting to use a finer (lower micron rating) filter upstream from the fuel pump, it can lead to pump failure. Not only are finer filters more restrictive, but they can increase in pressure drop over time. If the pressure drop across this filter becomes enough, the fuel pump can cavitate (boiling or vaporizing of the fuel). The same amount of pressure drop after the pump, however would not cause this failure. The "Post – Pump" filter is typically the filter that is replaced on vehicles as part of normal maintenance, while straining filters are typically installed inside the fuel tank itself (and not a part of normal maintenance). Considerations for this filter include: • Fuel Type. Filter material or media can have compatibility issues. Use of fuels like E-85 can causes problems with normal "paper" filters. Be sure that the filter you use is compatible with the fuel you intend to use. • Filtration. The micron rating relates to the size of the particle that is blocked (or stopped, depending on the type of rating). The finer (smaller the value) the filter is, the smaller the particles of debris that get filtered. • Capacity. Filters can have two ratings with respect to capacity. Sometimes they are given a general flow rating, or simply demonstrate the total filter surface area. It may be obvious, but worth noting: In general the larger the filter is, the larger the capacity. Having larger filters can help reduce maintenance frequency as well as improve flow capacity. More information available at link: http://fuelab.com/forums/topic/fine-filters/ TOPIC 2 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: o Inconsistent fuel pressure. Occasional or repeated Pressure Drop-offs. o Fuel system failure occurs after operating for a few to several minutes before symptoms are noticed. o Symptoms worsen over time. Each time the vehicle is operated, symptoms occur sooner and sooner. o As the vehicle warms up or in hot weather, symptoms worsen. o Pump operating sound becomes loud or has a "grinding" or "ratcheting" characteristics. o Pump is very warm or hot to the touch. • Cause: General pump failure or inlet plumbing failure. Symptoms include: o Operating pressure is lower than regulator set pressure, or possibly no pressure. o Operating pressure may be inconsistent. • Cause: Insufficient capacity. Symptoms include: o Sharp fuel pressure "drop-offs" during high engine load (normal operation during low power operation). o 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. More information is available through link: http://fuelab.com/forums/topic/avoiding-cavitation/ 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 the Fuelab video at link: https://www.youtube.com/watch?v=dP6Pogm6Ts0. 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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