Before World War II, auto
manufacturers built engines that featured splash lubrication systems,
poured-in-place crankshaft bearings and L-head valve arrangements. In
short, these engines were configured exactly like the engine you might
have in your power lawn mower today. Due to their lack of technical
sophistication, most needed to be taken apart at 20,000-mile intervals
for routine repairs like valve grinding and carbon removal.
after World War II, most auto manufacturers began producing a new
generation of engines that featured pressure-lubricated, replaceable
crankshaft bearings, overhead valve cylinder heads, oil filters and air
filters. These features were popularized on engines like the post-war
Oldsmobile “Rocket” V8s and became a staple of engine design through
Today’s engines have gone a step further by including
overhead camshaft, multi-valve aluminum cylinder heads, variable valve
timing, electronic fuel injection, electronic ignition, tuned air
intake systems, turbocharging, and supercharging. In short, we’ve
progressed from engines that routinely failed in less than 20,000 miles
to engines that, with good care, still run well at 200,000 or more
But, as the engines changed, so have their failure patterns.
When I started in the trade, knocking connecting rod bearings and
burned valves were the norm. Today, the most common failures are timing
belts, intake gaskets, cylinder head gaskets and engine damage caused
by neglected maintenance schedules. Because customers depend upon their
parts professionals for guidance in repairing modern engines, let’s
take a look at what parts and services you should recommend in dealing
with each of these relatively expensive repairs.
TIMING BELT FAILURES
function of the timing belt is to open and close the intake and exhaust
valves at precise intervals in relation to crankshaft rotation. When a
timing belt fails on a “free-wheeling” or “non-interference” engine,
little damage is done because the manufacturer has built the engine to
allow the fast-moving piston to clear the now-stationary valve.
not all engines can be built with valve-to-piston clearance. If a
timing belt fails on an “interference” engine, the pistons bend the
valve stems when they collide with the valves. The bent valves create a
loss of cylinder compression, which either causes the engine not to
start or to run roughly with a lack of power.
A basic timing belt
replacement on a non-interference engine should include the replacement
of the timing and balance shaft belts if so equipped. In addition, many
mechanics prefer to replace all camshaft and crankshaft seals,
tensioner pulleys and the water pump as preventive maintenance. Because
some state emissions requirements have extended timing belt replacement
intervals from 60,000 to 100,000 miles, the rationale of these
mechanics is that most of these components won’t last through another
timing belt replacement interval.
If the vehicle has an
interference engine, the valves are likely bent when the timing belt
fails. In many cases, a mechanic will perform a cylinder leakage test
on the engine to determine if the valves are leaking before replacing
the timing belt. If the valves are leaking, the cylinder head must be
removed so that the valves can be replaced and the valve guides and
seats serviced. If a skilled automotive machine shop isn’t available,
installing a remanufactured cylinder head is often the quickest method
of restoring the engine to service.
Keep in mind also that, when the
cylinder head is replaced, it’s also a good time to replace the spark
plugs, spark plug wires, drive belts, and filters. Replacing these
parts at this time is a bargain for the customer because no extra labor
is required. For the dealer and jobber, the job becomes more profitable
because the part content of the repair is increased.
INTAKE GASKET FAILURES
the hotter-running cylinder head tends to expand more than the
cool-running intake manifold, a scrubbing action takes place at the
intake manifold gasket. Eventually, the friction between the cylinder
head and the intake manifold wears out the gaskets. Because intake
manifolds incorporate passages for coolant, most failing intake gasket
drip coolant from the corners of the intake manifold onto the garage
floor. A small volume of coolant might also leak into the engine and,
in extreme cases, cause a lubrication failure in the engine block.
removing the intake manifold allows greater access to critical
components, it’s a good time to inspect and replace worn fuel injection
and ignition parts. If the intake surfaces and gaskets show signs of
rust or corrosion, it’s also time to prevent a repeat failure by
replacing the engine coolant.
CYLINDER HEAD GASKETS
engines are normally built with cast-iron cylinder blocks and aluminum
cylinder heads and intake manifolds. The upside of the “bi-metal”
engine is the huge weight savings created by using aluminum. The
downside is that aluminum expands 1.5 times more than cast iron during
any specific temperature change.
Here again, the disparity between
the expansion coefficients of cast iron and aluminum causes the
cylinder head gasket to wear out. Most of the head gasket wear takes
place at the stainless steel “fire ring” bordering the cylinder.
Eventually, the fire ring begins to leak coolant into the cylinder.
Because most of the coolant exits with the exhaust gases, the leaking
coolant contaminates the oxygen sensors and catalytic converters.
the engine is turned off, cooling system pressure might force enough
coolant into the cylinder to contaminate the engine oil and rust the
steel and iron parts of the engine. When rusting takes place, the
crankcase begins to accumulate a heavy coating of sludge. If the engine
oil is contaminated with enough coolant, the oil loses its lubricating
value. In this case, the internal parts of the engine can be severely
damaged or seize together.
If the engine suffers a severe
overheating condition, the aluminum head will warp or lift away from
the gasket at the center of the cylinder bank. When the cylinder head
warps more than a few thousandths of an inch, it must be replaced or
SHORT-BLOCK ENGINE FAILURES
part or “short-block” of an overhead-camshaft engine block assembly
consists of the cylinder block, crankshaft, pistons and connecting
rods. The short block assembly of a pushrod-style engine also includes
the camshaft and timing chain assembly.
To prevent oil consumption
from ruining catalytic converters, modern engines require precise fits
between the pistons, piston rings and cylinder blocks. Crankshaft
bearing clearances have also been reduced to control the amount of
air-borne oil in the engine. In years past, mechanics would attempt to
“rebuild” a worn short block assembly by installing new crankshaft main
bearings, connecting rod bearings, and piston rings. In modern
applications, however, in-field rebuilding will not reduce oil
consumption enough to meet required standards for modern emissions
Because modern vehicles are complicated and have limited
component accessibility, it’s not unusual for labor times to range
between 20 to 40 hours for a short-block replacement. If a short block
has been ruined due to lack of lubrication, overheating or preventive
maintenance, it’s always best to prevent an expensive comeback by
recommending a new or remanufactured short block replacement assembly.
The key in any modern engine repair is to restore levels of oil
consumption to original standards. If an engine repair is done
correctly, it will become a profitable undertaking for jobber and
installer alike. If not, it will become an enduring headache for all
Gary Goms is a former educator and shop owner
who remains active in the aftermarket service industry. Gary is an
ASE-certified Master Automobile Technician (CMAT) and has earned the L1
advanced engine performance certification. He is also a graduate of
Colorado State University and belongs to the Automotive Service
Association (ASA) and the Society of Automotive Engineers (SAE).