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I can certainly imagine one of the symptoms could potentially be engine fuel starvation. If the fuel flow to the engine isn't totally blocked, how can one recognize a clogged fuel filter?

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  • $\begingroup$ It sounds like you're asking in the context of a gasoline piston engine, but could you confirm? $\endgroup$ Mar 17 at 0:02
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The effect would almost certainly be similar to running lean - ie, if not enough fuel is making it to combustion your fuel to air ratio will be that of a lean mixture

Effects of lean mixture include

  • Overheating
  • Knocking sound when running, perhaps even popping/backfiring from exhaust
  • The RPM will "hunt", meaning it will increase/decrease attempting to find a stable RPM
  • Slow to pickup, ie when increasing throttle it will not change quickly RPM as it should
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  • $\begingroup$ I had all these symptoms except for backfiring on my automotive engine when the filter was clogged. $\endgroup$
    – Efe Ballı
    Mar 18 at 13:47
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It's pretty simple. There will be no symptoms until the flow demand of the engine is more than the flow that can make it through the filter.

On a carbureted engine, as fuel in the bowl is consumed, the float valve will have to open more and more to supply enough fuel volume to keep the level in the bowl above the metering orifice or jet. Once the float valve is fully open and the flow is still insufficient, the level in the bowl will drop until the fuel metering jet starts to un-port; then the mixture leans out to the point where the engine starts to lose power from an over-lean mixture, then misfires and stumbles. If you reduce throttle, it'll recover once the fuel flow from the system is more than what the engine is trying to use, and float bowl refills enough for the metering system to work again.

A similar thing happens when you restrict the flow to an injected engine, except that because the fuel distribution is fairly even, you tend to see just a power reduction from a lean mixture, then instead of stumbling and rough running, it just quits (like when you lean the mixture too much yourself).

It'll also happen sooner (less clogging) on an injected engine, since the injection system's metering equipment requires significant inlet pressure to work properly. A carburetor needs less than 1 psi of fuel pressure at the carb to be able to keep the float bowl full (which is why gravity feed works fine with wing tanks, but if you have injection you have to have fuel pumps even with the fuel up in the wings) and so it can take a lot more flow restriction before the head of pressure at the carb drops to a critical point.

So the main symptom will come on when fuel demand is highest, on takeoff, and will show up as an unexplained power drop, possibly followed by stumbling and rough running, or just quitting, but when you ease up on the power, at some point it should come back.

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  • $\begingroup$ Thanks! This was very helpful. So say I flew for 15 minutes and with no issue at a steady 1.5 gal/min (demanded and supplied), then throttled up a bit and my fuel flow demand of the engine was now 2.0 gal/min. If I throttled back down to a point where the engine required 1.5 gal/min, I then shouldn't be experiencing the same issues? $\endgroup$
    – synchh
    Mar 16 at 15:45
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    $\begingroup$ If there was a fuel flow restriction it should've shown up during the full throttle part during takeoff. In any case, yes if you experience something, that goes away when you reduce power, there is likely a fuel flow issue and an obstruction could be a cause. It could be other things, like an intake restriction; you never know. A clogged filter is just a possibility, one of a bunch a things to be checked during troubleshooting. $\endgroup$
    – John K
    Mar 16 at 16:12
  • $\begingroup$ The restriction would show up during take-off run if already there. If there is some debris in the tanks that clogs the filter during flight, it will show up during en-route climb—or at the worst possible moment if you need to go around. $\endgroup$
    – Jan Hudec
    Mar 16 at 20:06
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Most significant symptom of a clogged fuel filter, is reduced power at high power settings. High power already taxes the filter's ability to flow fuel, with a higher flow. Then demanding high flow will result in limiting fuel to flow through a lower limited capacity filter. Sub par climb performance would be a good indicator.

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    $\begingroup$ Not many diesel aircraft engines -- not none, but your "such as accelerating while climbing a hill" makes me think you're not talking mainly about aviation... $\endgroup$
    – Zeiss Ikon
    Mar 15 at 18:54
  • $\begingroup$ @ZeissIkon good spotting - seems to be fixed now. It is easy to temporarily forget which SE site you're on sometimes, mechanics/aviation does have areas of crossover. $\endgroup$
    – Criggie
    Mar 16 at 4:37
  • $\begingroup$ Yep, piston engines have evolved along with both ground/water transport and aircraft. Slower in aircraft, due to certification requirements, except during the major wars. $\endgroup$
    – Zeiss Ikon
    Mar 16 at 11:08
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Without a cockpit indication (bypass light, annunciator, etc), then one has no way of knowing if a fuel filter is the problem, or if there is some other cause. If one has insufficient fuel flow, then one has insufficient fuel flow; one can't fix the filter issue in flight, or address it other than perhaps to switch tanks, depending on the system, filter, and location. If the immediate problem is stoppage of fuel flow or reduction to the point of partial power or loss of engine power, or surging, then whether it's a filter, pump, line leakage, fuel flow divider, etc, it doesn't really matter. You can't trouble shoot those problems in flight. Follow your procedures. These may include using fuel boost, switching tanks, shutting down an engine, etc.

There are numerous problems that could replicate what one might see with a blocked fuel filter. Unless a filter is entirely clogged with a contaminant, then reduced flow is the norm. In some cases, flow improves, or engine performance improves, if power is reduced; debris drawn against filter media or the filter inlet may move or allow fuel passage with a reduced fuel draw. Outside of that, changing fuel flow manually, follow the procedures and don't guess as to the cause.

On the ground, troubleshooting is another matter, and fuel flow may be measured across the filter, at injectors, at the carburetor inlet, etc. Tanks may be inspected, filters opened or flushed or back flushed, etc, to check for contamination.

Look closely at what's recently happened. Did you just switch fuel tanks? Perhaps you have contaminated fuel. Perhaps debris was behind the fuel selector, and admitted downstream by moving the selector. Perhaps the selector has failed in a partial position. I've seen all those happen, as well as ice crystals in fuel, algae, and debris in tanks. Leaking fuel fittings, failed fuel flow divider diaphrams, incorrect fittings, old fuel lines in certain cases, and other factors have lead to problems which could just as easily be interpreted in flight as a filter problem...but you have no way of knowing without a cockpit indication. Likewise, an obstructed fuel tank vent might lead to similar problems and might manifest itself similar to what might occur with a plugged or obstructed filter, including smoother engine operation at reduced power. In any case, it's a good time to be on the ground wishing you were in the air, than the other way around.

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