failure is always bad news! Besides taking away your Subie, it
forces you to make a painful financial decision. If the cost to repair,
overhaul or replace the engine is more than the resale value of your
car, the investment may not be worth it. But if your Subie is in good
condition otherwise, repairing or replacing the engine may be less
expense than trading for another used vehicle (always a gamble), or
taking on payments for a new Subaru.
Assuming you have gotten
past the initial trauma and has decided in favor of fixing the engine,
you have to figure out why the engine failed so the repaired engine (or
replacement engine) won't suffer the same fate.
A good place to start
your postmortem is to review the circumstances that preceded the
failure. Sometimes failures occur unexpectedly. One minute the engine
is running fine and your keeping up with traffic, and the next you're
sitting along side the road with the hood up wondering what happened.
In most instances, though, there is ample warning that something is
amiss long before the engine actually fails.
Unusual engine noises,
low oil pressure, engine overheating, loss of power, misfiring, hard
starting and similar drivability and performance complaints can all be
indications of problems that need attention. The underlying cause may
be something minor or major. There is no way to know unless somebody
checks it out. If a motorist ignores such warnings long enough, it can
be a very costly mistake because eventually the engine may succumb to
whatever is causing the problem, which is a classic example of the
famous preventive maintenance line, "You can pay me now or you can pay
CAUSES OF ENGINE FAILURE
The major causes of
engine failures can be lumped into four basic categories:
- Overheating (excessive heat)
- Lubrication (or the lack thereof)
- Detonation (Spark Knock)(Detonation)
- Misassembly (opps!)
Overheating can be
caused by any number of things. It is often the result of coolant loss
or a low coolant level, which is turn may be due to leaks in hoses, the
radiator or the engine itself. A weak radiator cap that leaks pressure
can allow coolant to escape from the system. Not getting the cooling
system completely filled after changing the antifreeze can allow steam
pockets to form that make the engine overheat or run hot. An electric
cooling fan that fails to come on due to a faulty thermostat, relay,
wiring or motor may be an overlooked cause of overheating. So too can a
slipping fan clutch. Even a missing fan shroud that reduces the fan's
effectiveness may be a contributing factor.
Another common cause of
overheating is a faulty thermostat. When most thermostats fail, they do
so in the closed position preventing the flow of coolant from the
engine to the radiator. Replacing the thermostat will obviously solve
the problem, but may not prevent the same thing from happening again at
some point in the future. So you might want to install a "fail-safe"
type of thermostat that still allows some coolant flow in the event of
Less obvious causes of
overheating can include a clogged radiator that is filled with sediment
as a result of coolant neglect, corrosion or using excessively hard
water. Incorrect ignition timing and/or a lean fuel mixture (which may
be due to air leaks, low fuel pressure, etc.) can also elevate normal
operating temperatures. An exhaust restriction (typically a clogged
catalytic converter) can also make the engine work harder causing it to
Too much heat in an
engine can cause serious problems because heat causes metal to expand.
The hotter the engine gets, the tighter clearances become until there
are no more clearances left. Overheating can cause valve stems to gall
and stick, and pistons to scuff and seize. So if you see either of
these conditions when you tear the engine down, it is a pretty good
clue that overheating caused the engine to fail. Excessive heat can
also cause cylinder heads to swell, warp and/or crack.
Aluminum heads are
especially vulnerable to warpage and cracking because aluminum has a
much higher coefficient of thermal expansion than cast iron.
Consequently, when a bimetal engine with an aluminum head gets too hot,
the head tends to swell up in the middle, causing it to warp and blow
the head gasket. If the engine has an overhead cam, the resulting
misalignment in the cam bores created by the warpage can gall or seize
the cam bearings, or even break the cam. Anytime you encounter a warped
or cracked aluminum head, or an OHC head with a seized cam, chances are
overheating caused the damage.
In some engines where
the center exhaust ports are Siamesed together, hot spots can develop
in the head between the exhaust ports causing the head to swell so much
it crushes the head gasket resulting in a blown head gasket. Replacing
the head gasket may temporarily solve the compression problem, but
unless the underlying cause of the elevated exhaust temperature is
diagnosed and corrected, the replacement gasket may eventually suffer
the same fate. Some aftermarket gasket manufacturers have gone so far
as to develop special reinforced replacement gaskets for engine
applications that have a history of crushing gaskets.
Every engine needs oil
between its moving parts not only to reduce friction but also to carry
away heat. Oil is the primary means by which the rod and main bearings
are cooled, as well as the pistons. So any reduction in oil flow may
cause these parts to run hot, gall and seize.
Low oil pressure is
often a contributing factor in engine failures. The underlying cause
may be a worn oil pump and/or excessive clearances in the main and rod
bearings as a result of high mileage wear or neglect (not changing the
oil and filter often enough).
starvation is almost always fatal to any engine, and is usually the result of a failed oil pump, a
plugged oil pickup screen inside the oil pan, or a low oil level.
Bearings that have been damaged as a result of insufficient lubrication
will be shiny and worn where the crankshaft journal wiped away the
Overhead cam engines are
even more vulnerable to oil starvation and low oil pressure problems
than pushrod engines because the cam and valve train are farther from
the pump. When an OHC engine is first started, it takes awhile for oil
pressure to reach the cam bearings. If the oil viscosity is too heavy
(especially during cold weather), it may delay the arrival of oil long
enough to starve and seize the cam. For this reason, most vehicle
manufacturers recommend using a 5W-30 oil in late model OHC engines
year round, but especially during cold weather. Refilling the crankcase
with the recommended viscosity oil can prevent a reoccurrence of this
type of failure.
If you suspect a low oil
level may have caused engine damage, check the dipstick to see how much
oil is in the pan. A low oil level may be the result of neglect, oil
leakage and/or oil burning.
Oil-fouled spark plugs
and a heavy buildup of black wet deposits on the backs of the intake
valves and in the combustion chambers would tell you the engine had
been using oil. Oil usually enters the combustion chamber past worn
valve guides and seals, and also past worn or broken piston rings and
worn cylinders. Worn valve guides can sometimes be knurled to reduce
clearances, but usually it requires installing new guides, guide liners
or valves with oversized stems.
Installing new valve
guide seals can often reduce oil burning dramatically. An engine that
sucks a quart of oil every few hundred miles may be able to go several
thousand miles without using any oil with a new set of valve guide
Any evidence of oil
leakage around the front or rear crankshaft seal, pan gasket, valve
cover gasket or other gaskets, would tell you new gaskets and seals are
needed. Most of these gaskets and seals will have to be replaced anyway
if you are opening up the engine.
Bearings ruined by dirty
oil will have foreign material embedded in the surface and/or be scored
by debris. Check for a plugged oil filter and/or a missing air filter
or oil filler or breather cap. The underlying cause here may be not
changing the oil often enough.
Detonation (Spark Knock)
is a form of abnormal combustion that results from too much heat and
pressure in the combustion chamber. The fuel ignites spontaneously
causing a sudden rise in cylinder pressure. The result is a sharp
hammer-like blow on the piston that produces a metallic knocking or
pinging noise. Light detonation is considered normal and should not
cause any damage, but heavy or prolonged detonation can crack rings,
pound out piston ring grooves, punch holes through the tops of pistons,
smash rod bearings and blow head gaskets.
Detonation is sometimes
confused with pre-ignition, which is altogether different. Pre-ignition
occurs when a hot spot inside the combustion chamber ignites the fuel
before the spark does. The hot spot may be an overheated exhaust valve,
a spark plug that is too hot or even a sharp edge in the combustion
chamber itself. Such hot spots can be caused by anything that makes the
engine run hotter than normal or inhibits normal cooling (such as a
buildup of carbon deposits). A hot exhaust valve may be the result of
insufficient valve lash, a weak valve spring, excessive wear of the
valve stem or guide, or retarded ignition timing. Pre-ignition can be a
contributing factor in detonation.
Detonation can have
numerous causes. One of the most common ones is loss of EGR. The
exhaust gas recirculation system dilutes the air/fuel mixture slightly
to lower combustion temperatures when the engine is under load. This
reduces the formation of oxides of nitrogen (NOX) and also helps
prevent detonation. So anytime you find evidence of detonation damage,
be sure to check the operation of the EGR valve and system.
Other causes of
detonation include excessive compression, elevated engine operating
temperature, pre-ignition, over-advanced ignition timing (spark knock),
lean fuel mixture, spark plugs that have too hot a heat range for the
application, low octane fuel, and even bad driving habits such as
lugging the engine excessively with a manual transmission.
If you find detonation
damage in an engine and discover a heavy accumulation of deposits in
the combustion chamber, it may be the result of a rich fuel mixture
and/or oil burning. Frequent short trip driving can also accelerate
deposit formation. Black, oily deposits in the combustion chambers and
on the backs of the intake valves would point to worn valve guides and
seals as the underlying cause. Black dry carbon deposits should lead
you to check for conditions that may be causing the fuel mixture to run
rich (a bad oxygen sensor, a defective coolant sensor that keeps the
computer in open loop, excessive fuel pressure in a fuel injected
You can probably ignore
misassembly as a factor in a high mileage engine failure. But in a
newly rebuilt engine or a low mileage failure, it should certainly be
considered as a possibility. Some common mistakes to look for:
- Incorrect engine bearing clearances. Galling or
seizure would tell you bearing clearances were too tight, while fatigue
failure would point to excessive clearances.
- Incorrect torque on critical fasteners like head
bolts, rod and main bearing cap bolts, etc. Head bolts should always be
lightly lubricated with 30W engine coil prior to installation, and then
torque in the proper sequence to the vehicle manufacturer
specifications. Too much torque can crush the head gasket while too
little torque or uneven torque can lead to leaks. Too much torque on
rod and main bearing cap bolts may crush or deform bearings, or cause
the bolts to fail.
- Misalignment. If the center main bearings show much
greater wear than the end bearings, the crankshaft may be bent or the
main bores may be misaligned. The underlying condition must be
corrected by straightening or replacing the crank and/or align boring
the block. The same applies to camshafts and cam bearings (pushrod
- Failure to clean parts properly during engine
assembly (not scrubbing out the cylinder bores with soap and water to
remove debris and honing residue after they have been bored or honed,
for example). Any junk that is left in the engine can scour bearings
and wear surfaces.
- Failure to lubricate parts properly
during engine assembly. Camshaft lobes require a high pressure engine
assembly lube that will stay put until the engine is started and oil
reaches the cam. Bearings and cylinders also need to be coated with oil
or assembly lube to prevent a dry start.