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Engine Failure 101

An engine 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 me later."


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 failure.

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 run hot.

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).

Oil 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 bearing material.

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 seals.

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 engine, etc.).


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 & OHC).
  • 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.

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