1. Reprinted with permission from Aircraft Maintenance Technology, March 1998
RECIP TECHNOLOGY
Y ou work in a shop that specializes in turbine equip-
ment. The few cabin class piston twins for which the
FBO is responsible get their maintenance done else-
where. From time to time, however, piston engine problems
do arise that demand your attention. And since everyone else
is busy, guess who the boss chooses to evaluate the problem?
If you don’t do this every day, it can be a little intimidating;
trying to rattle the rust out enough to remember the systems
and what to do first. So let’s examine some troubleshooting fun-
damentals for piston engines that will both simplify and speed
up the process.
CLARITY OF INFORMATION
This is the first and most important principle. A common
mistake is to rely on unclear or vague descriptions of the symp-
tom. Often, there are subtle events or symptoms which occur
that someone else may miss, especially if they are otherwise
occupied flying the aircraft. It is important to get information on
as many other parameters as possible, even ones that seem
unrelated. Many things are better sorted out later, in the shop. A
good situational awareness is a great asset in this phase of trou-
bleshooting.
The best way to get a good read on the symptom is to experi-
ence it yourself. Do your best to emulate the same conditions
and duplicate the problem. This way you will find the problem
and not create a different one; if you end up creating a different
one, the old one will come back to haunt you. Remember, the
same problem can give different symptoms in different condi-
tions.
Sometimes it will be necessary to rely on secondhand
descriptions of the symptons. It is then imperative that clear and
concise information is exchanged both ways. The person experi-
encing the symptom, usually the pilot, needs clear and definite
instructions to troubleshoot when the symptom occurs again
(provided safety of flight is not compromised).
Avoid wasteful speculation. This is different from hypothesiz-
ing. Speculation is where your mind sort of goes off without you
and causes you to start working on things before a test has con-
firmed that what you are doing will actually fix the problem.
Piston Engine Whereas, hypothesizing is based upon educated guesses that
presupposes that one is well educated. This brings up the next
important principle.
T r oubleshooting SYSTEMS KNOWLEDGE
Systems knowledge is somewhat similar to clarity of informa-
tion in that good system information is a must when diagnosing
By Thomas Ehresman a symptom. You don’t need to have mastery of all systems to be
effective at this. Accurate systems information can come from
any number of sources. Maintenance manuals, training materi-
als, experienced personnel, or manufacturer customer service

2. the no start problem is, however, rarely
encountered.
Fuel is a much more common culprit.
Over priming an engine will prevent it from
firing even once. The presence of a strong
fuel smell in the exhaust pipe indicates a
flooded engine. Moving the mixture to idle
cutoff and throttle to wide open while
cranking will solve this one whether you
“Flower pot” for checking magneto to engine have a carbureted or injected engine.
timing. Inaccurate timing can give excessive Letting the engine sit for awhile also helps
mag drops or high cylinder head temperatures. get rid of excessive fuel. Use wide open
This is also the preferred timing check method throttle and mixture idle cutoff while crank-
for Continental engines per TCM. ing.
Lack of fuel can be just as puzzling. A
are some good starting points. Caution: slight fuel smell in the exhaust pipe can
Don’t let unqualified sources lead you down usually tip you off that you need more
the wrong path. prime. A note on the Teledyne Continental A cold cylinder exhaust stack just after shut-
TSIO-360. These engines take a seemingly down indicates the malfunctioning plug or
DIAGNOSIS excessive amount of prime to start. Fuel injector. A small amount of water squirted on
At this point, sit down with your systems the stack will steam and possibly boil.
may be running out the induction drains
knowledge (whatever the source) and go but the engine often still does not have
through the system operation step by step. enough fuel to start. Best method: one, nied many times by the occasional firing of
Note which component and situation would three second shot of prime and small the engine when you let up on the start
cause the symptoms you’re experiencing. bursts of the same while cranking. switch but no firing when the start switch is
List these possible causes and move to the The last starting requirement can be a engaged. This is because of the grounding
next step. little more troublesome. The magneto on a of the right and left main breaker points by
piston engine is designed with a start fea- the start switch when in the start position.
TESTING ture that retards the timing to prevent “kick When the start switch is returned to the
Start with the most likely and simple back” and provides additional energy to the normal position, the mags are back in the
causes first and test to eliminate or confirm spark event. This additional energy is “both” position. (This is also true of Shower
them. Tests should be limited in effect to required because of the slow cranking of of Spark systems with separate mag switch-
the suspect component. Don’t apply a test the engine during starting. These require- es. The R & L main breaker points ground
that involves too many components or you ments are commonly achieved in two ways with the start switch engaged.) Since the
won’t narrow your field of suspects. Use as — the impulse coupling and the Bendix engine is still turning at this instant, a cylin-
specific of a test as possible; one that will “Shower of Sparks.” der at or slightly after normal firing position
eliminate possible causes. Accurate infor- The engine with an impulse coupling may fire and even allow the engine to start.
mation is essential throughout this process. will have a snapping sound while starting. If the Shower of Sparks unit is not buzzing,
Reevaluation may be needed after testing a The engine employing the Shower of the unit needs repair or replacement. If
suspected cause. Sparks system will have a buzzing sound in there’s “buzz” but no spark, check the con-
If no test indicates the source of the the cabin whenever the starter switch is tinuity and points in the retard system.
symptom then the dreaded shotgun may be activated. Know which system the aircraft Hot starts can be troublesome if you’re
pulled out. Again, limit the replacement of has before attempting to start the engine. not experienced with the procedures for
components to those which will affect your The absence of the clicking in an the particular engine fuel system. Space
symptom. Start with the most inexpensive impulse coupling engine denotes a problem doesn’t allow for the discussion of this prob-
ones first. If you have access to these com- with the impulse coupling, probably a bro- lem here. Consult the Pilots Operating
ponents without having to purchase them, or ken main or flyweight return spring. This Handbook or engine operators manual for
have the ability to return them if not needed, problem requires removal of the specifics.
the process will be much cheaper. This one, magneto(s) with the impulse coupling for Another common problem encountered
however, is a time eater. It may take a lot of repair. If the engine is very cold, stiff oil can is magneto timing. If the engine has just
shop time and shipping charges if it starts to also prevent the flyweights from returning shown up from the overhaul shop or main-
go very far. to “snap” the coupling. A thorough preheat tenance has just been performed on the
There are innumerable problems that solves this one. magneto, look for these symptoms and
can and do occur in piston engines. There The absence of buzzing in the shower of their source problems.
is, however, a group of problems that are sparks system, or the malfunction of the Kickback is caused by the start firing
generally the most common. What follows retard points in the magneto, is accompa- event happening too soon, before the piston
is an aid to help quickly troubleshoot six reaches top center. The usual causes here
frequently encountered problems. We’ll call are: improper retard point timing (Shower
them the top six. of Sparks system) or improper magneto to
engine timing (Shower of Sparks and
1. NO START/HARD STARTING: impulse couplings).
There are three requirements to get an If no firing by the engine is observed at
engine to start — fuel (in the right propor- any time, but you know the magneto is fir-
tion), oxygen (air), and fire (timed spark). ing (don’t use your tongue), the probable
If the engine won’t run, one of these things cause is the magneto-to-engine timing 180
is missing. off. When the “flower pot” method is used
Air is almost a given. Even with a signifi- it is not too difficult to set timing to before
cant intake obstruction there will be at least top center on the exhaust stroke instead of
enough air getting into the engine to get it the compression stroke.
to fire at an idle speed. One exception here
is a totally sealed off induction. This would 2. MISS/ROUGH RUNNING:
most likely occur after the engine has been “Bullet plug” used to set top center with the The two most common sources of this
removed from storage and someone forgot “flower pot.” symptom are ignition misfire and clogged
to remove the intake plug. This source of fuel injector. The misfire most usually
2 Aircraft Maintenance Technology/MARCH 1998

3. shows up on the preflight mag check. To difficult to troubleshoot on the ground is
find the offending plug, run the engine on the altitude miss. This is where a plug cir-
the magneto with the bad drop. If a four or cuit will misfire only when the aircraft is at
six point EGT/CHT system is installed, or above a certain altitude. If the aircraft
note the cold cylinder. Trace the ignition has the four or six point EGT/CHT, you
lead back to the magneto with the miss. can find this one in the air. If not, the “shot-
That plug circuit (plug, high tension lead, gun” method should be used on the igni-
or tower in the distributor block) has a tion system.
short or open. The most common cause The most common culprit is large plug
here is a dirty/bad spark plug or a chaffed gaps. Clean, gap, and test all the plugs.
ignition lead. Also, test the ignition high tension lead
A clogged injector will act very similar harness with a high tension lead tester.
to the misfire. The miss is always apparent Check the distributor block for cleanliness, External damage to a magneto caused by
if the injector is fully clogged (or nearly tracking, and cracks. chaffing screw and aggravated by internal arc-
ing and burning of the coil.
so). If the injector is partially clogged, the If none of these are the problem, other
miss will show up while leaning the engine but less common sources are things such
for cruise. (On the multiple point EGT as improper E-gap (magneto internal tim-
check for a very early peak and a higher ing) and weak rotating magnets or coils.
head temperature on one cylinder.) In For these internal mag problems, the mag-
either case, run the engine in the configu- neto(s) must be removed and troubleshot
ration with the noted miss (rich or leaned on a test bench. Note of caution: Do not
out) and note the cold cylinder the same attempt to do in-flight mag checks for high
as you would for a misfire. Hint: If the air- altitude miss diagnosis. Very often high
craft is not equipped with a four or six altitude misses turn into crossfires.
point EGT/CHT or some of the probes Crossfire is when the electrical energy
aren’t working, run the engine in the con- from a miss-fire finds a path of least resis-
figuration with the noted miss. While run- tance in one of the electrodes next to it. A
ning with the noted miss or immediately mag check while attempting to diagnose
after engine shutdown, use a squirt gun or which magneto is miss/crossfiring can lit- Cracked and burned coils, such as that pictured
bottle and squirt a small, fine stream of erally blow the exhaust system into pieces. here, will often show up only at altitude as alti-
water on each exhaust collector approxi- This is especially true of turbo-charged tude misses. Without an external damage indica-
mately one inch from the cylinder exhaust engines. Mag checks at even low cruise tion, these can usually be found best by disas-
port. The cylinder with the miss will show power settings will over temp most cylin- sembly or bench check of the mags.
up with a much colder exhaust collector at der and exhaust components.
this point. A note on bad mag checks for Cessna to engine timing. Advanced timing is gen-
Another common malfunction that is 180/182s. In very cold weather the O-470 erally characterized by smaller than nor-
will show bad mag checks (200-300 rpm mal mag drops and higher than normal
drop). This is because of the poor fuel cylinder head temperatures. A simple tim-
atomization in very cold air. Try pulling ing check will confirm or eliminate this
the carb heat on, releaning (if altitude one.
requires) and rechecking. Rough running
of these engines during cruise in cold 4. HIGH OIL TEMP:
weather can sometimes be cured by flying High cylinder head temperatures and
with carb heat on to bring induction air high oil temps often go together. In this
temperatures back up to normal. A carb air case look for the common causes of both:
temp gauge is perfect for this malady. bad or damaged baffling or its seals, cowl
flap rigging, mag timing, or cooling air
3. HIGH CYLINDER HEAD blockage.
TEMPERATURE: If just the oil temp is high and cylinder
You may have run into this with a par- head temps are normal, look for these
tially clogged injector. If not, try these common sources:
other sources. Baffling: Bad inter-cylinder Birds are culprits here, too. A favorite
or cylinder head baffling can cause local- spot for nests is the pockets formed above
ized cooling air loss. Check for stiff or or in front of the oil cooler. Rags or other
loose baffle seals. debris also lodge here easily. The oil cool-
The classic birds’ nest is often a com- er to baffling seal is also important. Air
mon culprit here too. A healthy starling going around instead of through the cooler
can build a very effective air block in 15 to cuts down on the volume and velocity of
30 minutes. the flow through the cooler as well as the
If a cylinder has recently been changed, cooling air pressure in front of it.
expect higher head temps on that cylinder Dirty or excessive paint on the fins will
for the next 30 to 50 hours of operation. slow heat conductivity and allow the tem-
If all the head temps are high look for a perature to rise more than normal also.
more generalized problem. Check for This one will be seen mostly on hot sum-
numerous gaps and holes in the baffling. A mer days where the cooler needs all the
small air leak goes a long way in dropping help it can get. A quick visual inspection
the cooling air pressure and causing tem- will cue you to this culprit.
peratures to rise. Also, check the cowl Another common source here is what’s
flaps for proper rigging. Some models of called the “Vernatherm” or temperature
aircraft have different cowl flap rigging for bypass valve. This unit is a thermostatic
A small amount of water squirted on firing different years and engines in the same valve that works much the same as the
cylinder’s exhaust stack will ball, model. coolant thermostat in your car or truck. As
sizzle, and immediately fall off stack. Another, but less common cause of the oil gets hotter, the valve closes harder,
high cylinder head temps, can be magneto forcing more oil through the cooler.
Aircraft Maintenance Technology/MARCH 1998 3

4. valve) or exhaust (exhaust valve) system. If
the leakage is not bad the valve may be
relapped without pulling the cylinder.
The valve may also be sticky and just
require some cleaning of the guide to
devolve deposits. This can be found by
Any debris caught between the plunger and seat of the oil pressure relief valve (TCM newer style checking the looseness of the valve in the
shown) will cause low oil pressure with hot oil but not with cold. When the oil is hot, pressures at idle guide when the valve rocker cover is
can even drop below redline. This is true for any abnormal/excessive internal oil leakage. If cleaning removed.
the plunger and seat doesn’t help, or if the debris is metal, further investigation is definitely warranted. Worn or sticky rings will cause a loss of
compression and power also. This is called
“blowby.” There is always a certain amount
Removal of the vernatherm is required to grade for ambient temperatures or high oil of blowby present in every engine. The
check. The culprits here are abnormal temperatures will often cause low oil pres- best way to determine if blowby is exces-
wear of the valve face and seat, or loss of sure. Oil can be damaged by excessive sive is to make sure the intake and exhaust
the expanding medium (some sort of wax). heat and/or excessive operating time valves aren’t leaking when the compression
An abnormal pattern for the valve face before oil change. All these factors will test is done. Remove the oil fill cap and lis-
on the unit is obvious contact or wear on tend to thin the oil and cause a pressure ten for escaping air past the rings.
one side and not the other. The face will be drop. Continental allows compressions much
worn on high time units but shouldn’t be Any excessive internal leakage in the lower than Lycoming but only under spe-
gouging into the seat. engine will also cause a drop in pressure cial conditions, namely no valve leakage
Loss of expanding medium is harder to even at normal operating temperatures. and using a special compression tester.
pin down. The easiest way to find out is The most common culprit here is debris Consult Continental service bulletins for
replacement with one that is known to under the oil pressure relief valve. This more details.
work properly. acts to hold the valve off it’s seat and bleed Excessive blowby is almost always
With either problem, the fix is replace- off oil, dropping the pressure. Remove the accompanied by higher oil consumption
ment of the whole unit. The valve seat may valve and clean the face and seat. Note the and slightly higher oil temperature than
also be in need of refacing. type of debris that was caught in the
In the winter the problem may be noth- valve. This may be an indication of
ing more than a congealed oil cooler. The larger problems as noted in the next
oil will get thick enough when bitterly cold section.
to obstruct flow through the cooler. This Excessive clearances from worn
will cause the oil temperature to rise and bearings, worn valve lifter bores, worn
the oil pressure to fall. When a congealed oil pump gears, bad prop oil transfer
oil cooler does free up a near instantaneous collars, or missing/loose internal oil
rise in oil pressure and fall in oil tempera- passage plugs (just back from the
ture will be noted. The fix here is the prop- engine shop) will cause a drop in oil
er grade engine oil for ambient tempera- pressure. An oil analysis and a look at
tures (consult the engine operators manu- the oil filter will usually cue you to
al) and winter baffling for the oil cooler or internal engine problems from worn
possibly the whole engine. parts as the cause of this symptom. Annealed rings, caused by piston overtemp, will give
Excessive piston ring blowby will also Oil pump volume is not something high oil consumption and, in advanced stages, low com-
elevate oil temperatures because of the hot that we can change in the field usual- pression. Notice the oil burned and blackened piston. If
gases escaping into the crankcase. See the ly, but is an aggravating factor on the top compression ring can be twisted to take a set at
section on low cylinder compression for some engines. Engines with standard or more than 90 degrees without breaking, it is generally
tips to troubleshoot this source. volume oil pumps but high flow considered annealed. The usual culprits of piston
demands are very sensitive to viscosi- overtemp are lean mixtures at high power settings (70
5. LOW OIL PRESSURE: ty and clearance/leakage factors. This percent or higher) or partially clogged fuel nozzles.
The three most important factors that is most common on naturally aspirat-
determine oil pressure in any given engine ed engines that have been turbo-normal- normal.
are: oil pump volume, engine internal clear- ized by someone other than the engine A not so common but serious cause of
ances/leakage, and oil viscosity. The two manufacturer. If a higher volume oil pump low compression is a bad crack in the
most common causes of this symptom are is not a part of the package, the oil pump cylinder. This one is a serious safety
oil viscosity and internal clearances/leak- may be hard pressed to deliver normal issue and deserves a little attention here.
age. pressure when the viscosity gets lower If hissing is heard outside the cylinder,
Oil viscosity is affected by oil grade, (higher temperatures, aggravated by hot make sure the source is pinpointed
temperature and oil damage. The wrong oil turbochargers). before you go any further. This kind of
Some aircraft have flow restrictor ori- crack sounds just like an air leak (it is). A
fices in the turbo supply line just to keep crack this bad is immediate cause for
This near new the oil pressure up. In these engines, any cylinder removal and replacement before
“Vernatherm” was change in flow demand, clearance, or leak- further flight.
the culprit in oil tem- age is directly reflected in the oil pressure Books have been written on this sub-
perature problems. when at normal operating temperature. ject, but my hope is this article has given
Upon removal and you a few pointers on making the piston
inspection there is 6. LOW CYLINDER COMPRESSION: engine troubleshooting experience a little
usually no external
This problem is usually found during a more hassle-free. When you get good at
indication of a prob-
lem unless the valve
scheduled engine inspection. However, it, it will be gratifying and even fun.
face is excessively excessively low compression on one cylin-
worn. A new ver- der is usually noticeable when running the
natherm solved the engine as well.
problem immediately. The three most common leakage points
are intake valve, exhaust valve, and rings. Kelly Aerospace Power Systems
If an intake or exhaust valve is leaking it Engineering Group
will be plainly audible in the intake (intake
4 Aircraft Maintenance Technology/MARCH 1998 KA021003