Power-Deficient Engines

A look at a very common complaint and suggested cures; many of which only require a visual inspection.


A frequent reader question we receive is: Why does my engine seem to lack the power it once had? We can’t tell you exactly what’s wrong, but we can get you pointed in the right direction, and that begins with a systematic approach.

We can also tell you that one of the most common and hard to troubleshoot engine complaints is that of low power or low static rpm. Complaints on this subject were once so numerous on the Lycoming O‑235 series that Lycoming published an entire service instruction on the subject (S.I. 1388C).

Plane owners of all types will occasionally complain that their “engine isn’t making good power like it used to.” In most cases, the engine runs smoothly and there is no immediately obvious cause of the power deficit.

Static rpm isn’t always a good indication of power deficit, since so many factors (density altitude, outside air temp, mixture etc.) go into determining what the engine’s full‑power rpm is on the ground. A better way to know if you’ve truly got a power‑poor engine is to load the plane to gross and attempt to replicate book performance figures, especially takeoff distance with wind factored in and rate-of-climb.

(Maximum level‑flight speed is not a foolproof measure, since aircraft rigging can affect it.) If your rate of climb at full gross weight is only half what the book says it should be, you’ve probably got a power‑poor engine.

Three things that will be helpful to the maintenance technician who tries to troubleshoot the predicament are: (1) How long the problem has existed, and (2) Is it occasional, or continuous and (3) How accurate are the power instruments? Did the power deficit come on suddenly? If so, the cause might be a mechanical failure (broken valve spring) or other sudden event, such as internal blockage of the muffler or the intake.

If the power loss has come about slowly, entirely different causes may be to blame (e.g., timing shift, cam wear).

Likewise, it’s important for the troubleshooter to know if the power loss comes and goes, or is permanent. If it comes and goes—it could be a blocked muffler (loose flame tubes or baffles are bouncing around, covering up the exhaust outlet and then exposing it again) or a carburetor heat door that is fully shut one day and cracked open the next. A chronic, power deficit will probably be harder to troubleshoot.

In either case, if the power loss began immediately after annual inspection or other “hands-on” maintenance, begin checking into what was done at the maintenance check that could have caused the power deficit.

Sanity Checks

The very first thing to do when trying to check out a power‑poor engine is rule out obvious power‑killers (things that don’t require engine disassembly to get at or fix). These would be such items as:

The manifold pressure gauge and tachometer: Check for substantial errors in the “power poor” direction. The manifold pressure gauge should indicate field barometric pressure (corrected for elevation, of course) with the engine stopped. Tachs are best checked electronically, either with a hand-held digital model available through aviation catalogs.

A hand-held unit can be used either inside or outside the plane, and no tape is required. The commonly available $40 model has alleged accuracy to a maximum of 10 rpm, but we like the two other choices much better (either the TruTach II, about $190 or the long popular Proptach-3 at $270, www.proptach.com). You will find a readout to one rpm, greater accuracy, repeatability, stability and stabilization circuitry. You get what you pay for.

Next on the list is the induction air system: Check for shop rags, clogged air filter, alternate air door not closing, collapsed scat tubing, birds’ nests, animal remains or animal damage.

Carburetor heat: Check for proper rigging of door (door must close fully before knob hits panel in the cockpit).

Throttle and mixture controls: Check rigging. Make sure full control travel is occurring (stops are hit) at the throttle or injector. Primer: Closed, locked, and not leaking. The O-rings can wear and leak. Repair is quite easy.

Additional Sanity Items

Once you’ve exhausted the truly obvious things, it’s time to move on to less obvious items, such as:

The exhaust system: Check visually for blowouts, cracks, leaks. Rap on the mufflers with a rubber mallet and listen for loose baffles or debris. Turbocharged engines: gain access to the compressor and check for bent or damaged blades.

If blades have been rubbing (bent over at the outer periphery), something probably went through the exhaust turbine—begin FOD (foreign object damage) inspection. You can also use soapy water and pressurize first the induction and then the exhaust systems with a shop vac (using filtered air).

Ignition system: Check mag-to-engine timing with the “flower pot” type setup. It’s one of the most accurate methods. Check rpm drop (max 175 per mag, 50 rpm difference verify with against your specific POH).

Remove spark plugs, clean, and check under high pressure in a bomb‑test machine. Consult the aircraft records to determine when was the last time the magnetos were removed for anything more than cursory inspection. Weak rotors, weak magnets, faulty coils, have a way of escaping notice for hundreds of hours. Mags should be opened and inspected every 500 hours (or 400 hours for some mags per service bulletin). Weak mags are often the culprit.

Manifold pressure at idle: Check to see if MP is high (18 inches) at idle rpm. This can indicate a serious induction air leak or bad ring wear (poor compression in one or more cylinders).

Differential compression check: It generally takes compression substantially worse than 60/80 to affect horsepower. Also, if only a few cylinders are bad, the engine may shake.

With a Continental engine, much lower compression is OK as long as any leakage is not past the valves, which can be heard. Be sure to use a tester with the calibrated orifice and follow the CM bulletin exactly (CM bulletin 03-3).

Rich Man, Lean Man

An engine that is set up too rich or too lean will (if the fuel flow is off far enough in either direction) act power-poor. On an engine with constant‑speed prop, a quick check can be made for EGT rise at full power. With a carbureted engine, there should be some EGT rise (the actual amount depending on many factors) when the mixture is retarded.

This applies in cruise, too, of course. If you fail to see any EGT rise when leaning the engine, it means one or more cylinders were already on the lean side of peak EGT when you started leaning. That’s too lean.

Instructions for setting up Continental fuel‑injector systems (on the aircraft) can be found in Service Bulletin No. M97-3C. On Lycoming engines with Bendix or Simmonds fuel injectors, the servo bodies have to be flow‑checked on a special flow bench. I.e., you have to send the injector out.

Before sending anything out, of course, you’ll want to clean all injector nozzles (see Lycoming S.I. 1275) and be sure the correct series of nozzle is installed (Continental only). CM nozzles come in a bewildering variety of flow ranges, and it’s possible your engine has the wrong ones.

Bendix late‑style nozzles have a removable restrictor orifice which, if it’s left out, can cause problems. Be sure nozzles are correct and functioning properly.

The Valve Train

Sad to say, if you’ve already checked all of the foregoing items and come up empty‑handed, about all that’s left is the valve train. If you find anything wrong here, it’s probably going to be expensive to fix.

A check of dry tappet clearance is often worthwhile, especially if any cylinders were recently removed. (This is now a required part of Lycoming S.B. 388C, interestingly.) Power isn’t affected unless something is very seriously amiss in this department. For example, you may find a mushroomed or bent pushrod, or a rotator cap (Lycoming only) may have fallen off a valve.

Springs occasionally break (especially after an engine over-speed event) and valve guides sometimes pull loose from cylinder heads. Either of these can cause power loss, although there will generally be accompanying roughness.

The most likely source of any serious power deficit in a Lycoming engine, unfortunately, is a badly worn camshaft (one or more lobes scuffed flat). This is more likely in certain models (such as O‑320‑H, O‑360‑E, and TIO-541) than in others, but it does occur, sporadically, in just about all Lycoming models. (Continentals, too, although it is definitely rarer in a Continental.)

How do you determine whether you’ve got a badly worn cam, without taking the engine apart? Basically, you remove all rocker covers and put a dial indicator, one by one, on each rocker, so that you can swing the prop and note the rocker or valve travel for each valve.

The total throw should be very nearly identical for each valve. A flat lobe will be immediately obvious, because you’ll have a rocker that barely moves.

Tip: We’ve noticed that many Lycoming cams start scuffing on the intake lobes first, and generally it starts with the front of the cam. Therefore, when doing the dial‑indicator trick, start with cylinder No. 1 (which, on a Lycoming, will usually be the front-most cylinder on the right or starboard engine side).

If you notice a major “split” in travel values for the intake and exhaust lobes, you can stop right there. Otherwise, do cylinder No. 3 next; then do just the exhaust lifters on cylinders No. 2 and 4. (The same intake lobes work the intake valves for cylinders 1 and 3, and also one lobe works intake valve 2 and 4. Exhaust lobes are fully dedicated.)

You can also find a bad cam lobe, on engines that use barrel‑type lifters (most Continentals, Lycoming O‑320‑H, O‑360‑E, TIO‑541), by pulling the lifters out of their crankcase bosses. But the dial‑indicator trick is easier and quicker.

Another tip: If you own a high‑time (1,500 hours or more) Lycoming engine that has developed a power deficit gradually, over a period of time, and you have done checks of mag timing, carb heat, and tachometer accuracy (and the engine is running smoothly at all rpms), skip straight to the dial‑indicator check. (Particularly if the engine is flown less than 100 hours a year.) You want to rule out a bad cam right away, rather than spend needless hours (and dollars) looking for more esoteric, unlikely causes of power loss.

Here are some additional considerations from the Lycoming troubleshooting guide for power-poor engines, and they certainly can apply to CM engines as well:

Excessively dirty air filter. Sometimes even new filters may have an excessive air drop through them. If this condition is suspected, remove filter and run engine to full throttle without filter installed to observe whether the engine performs better. (This test should be performed in a dust-free area and on a hard surface.)

Carburetor heat door not rigged properly. Even though door is going from full open to full closed position when aircraft is shut down, when aircraft engine is operating, vibrations and airflow may cause door to open slightly. If this condition is suspected, tape or wire the door shut for test purposes. If this solves the problem, adjust and replace parts as necessary.

Incorrect magneto-to-engine timing. Use the “flower pot” timing tool rather than the “eyeballing” methods mechanics use on Lycomings. And if you have to change the external timing, chances are the internal E-gap needs work as well.

Fouled spark plugs. You need clean plugs. If the fouling is constant, lean in taxi, use TCP or check for hotter authorized plugs in the latest engine maker approved spark plug service bulletin.

Leaks in induction system and exhaust system (turbocharged particularly). Be sure any pressurized air tests are kept at low pressure and filtered air is used for any test. You don’t want to blow dirty air into an engine.

Improper fuel flow. Remove screens and flush out dirt. Disconnect gauge and install master checker to determine the accuracy of the aircraft instrument. Check for any restriction in the air inlet or manifold. Use of improper fuel can certainly cause both short and long-term issues. Lycoming has a service bulletin on this.

Controllers out of adjustment (turbocharged). Damaged turbocharger impeller, binding or tight turbocharger wheels (turbocharged).

Excessive dirt build-up in the compressor housing or on the compressor wheel (turbocharged). Kinked or restricted oil lines from engine to actuator, and actuator to controller.

Wastegate out of adjustment (turbocharged). Inlet orifice in actuator plugged (turbocharged). Wastegate stuck open (turbocharged). Piston seal in wastegate actuator leaking. Noted by excess oil coming out of drain (turbocharged).

Oil pressure too low to close wastegate (turbocharged). Injector and controller linkage not adjusted properly (541 series engine). Butterfly in wastegate is warped. Do we see a pattern here with turbos being potentially more troublesome? They certainly are and they need to be properly maintained. Turbos are much less forgiving of deferred maintenance.

Broken baffles in muffler (normally aspirated engines). Poor combustion—top the cylinders required if compression is low. Crankshaft to camshaft timing incorrect. This condition may be checked by first disconnecting starter, remove top spark plugs and rocker box cover on #2 cylinder.

Turn engine to T.D.C. on the compression stroke on #1 cylinder, observe that when piston in #1 cylinder goes over T.D.C. on compression the intake valve in #2 cylinder is just starting to open and the exhaust valve is just closing.

If this condition does not exist, the crankshaft-to-camshaft timing is off. NOTE: On engines with fixed-pitch propellers the engine probably will not turn static rpm. On engines with constant-speed propellers the engine will probably turn up static rpm. but manifold pressure will be a little low.


Most of the problems on a power deficient engine center around failing to do simple maintenance that allows the engine to both have proper spark intensity at the right times and for the spark plugs to be in good shape.

Make sure the engine can breathe properly and doesn’t have intake leaks. All odd sounds should be traced to their origin. And it’s always a good idea to start with the last maintenance area that was worked on if the problem starts after maintenance. Things get left loose or improperly reinstalled.

This article originally appeared in the August 2014 issue of Light Plane Maintenance.

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