Cylinder head gasket failures are
one of the most common mechanical issues facing today’s modern import
technician. One reason head gasket failures are common is because the cylinder
head gasket itself can become a wearing part. Since the aluminum cylinder head
expands at a slightly higher rate than the cast-iron cylinder block, the
cylinder head actually slides a few thousandths of an inch across the gasket
surface as the engine warms up to operating temperature.
After thousands of thermal
expansion events, the fire rings crimped onto the cylinder head gaskets begin
leaking combustion gases into the cooling system. All too often, this
combustion gas leakage causes the coolant to be forced out when the engine
thermostat is closed. When this happens, combustion gas builds up under the
closed thermostat and pushes coolant out of the radiator into the coolant
overflow reservoir.
In other instances, coolant will
be drawn through the worn cylinder head gasket fire ring and be lost into the
exhaust system. The net result in either case will be that the engine runs low
on coolant, which will cause a major overheating complaint. See Photo 1.
Photo 1: The presence of a witness mark between the cylinders indicates that the cylinder head gasket is sealing the combustion chamber.
ANALYZING DAMAGE FROM OVERHEATING
Because the damage seldom stops at
the cylinder head gasket, it’s important to assess damage to the engine and
cooling system before delivering a repair estimate. In severe cases, a “check
engine light” complaint might result because oxygen sensors and catalytic
converters become contaminated with coolant. In other cases, the rotating
assembly in the engine block can be severely damaged by coolant in the oil.
This condition normally requires a complete engine repair or replacement.
While exceedingly rare on modern
engines, combustion detonation will also aggravate head gasket wear problems
and will cause a premature gasket failure. Detonation can be caused by a faulty
exhaust gas recirculation (EGR) system, bad fuel or over-advanced ignition
timing.
The first step in assessing engine
condition is to check the engine oil level. If the oil level is overfull and
has an opaque, viscous appearance, it is likely contaminated with coolant. If
the oil appears darkened and has a burned smell, it’s very likely that the
metal overlays on the engine bearings have softened and peeled away.
A severely overheated engine will
also usually emit a burned odor from the radiator or coolant reservoir. If the
engine has been severely overheated, it’s also likely that sediment from the
water jackets has boiled into the coolant stream and deposited in the coolant
reservoir or radiator core tubes. If this appears to be the case, the radiator
must be cleaned or replaced to ensure against a continued overheating
condition.
If the engine starts, but has a
clicking noise similar to a valve train problem, the pistons could be scuffed
due to lubrication failure or excessive piston expansion. Excessive heat
expands conventional pistons at the corners of their skirts, causing a
“four-point” scoring pattern easily recognized by experienced engine
rebuilders. In contrast, a lubrication failure usually scores a piston at the
center of its skirt thrust area.
CYLINDER HEAD DAMAGE
In addition to the indication mentioned
above, a sheared timing belt is also indicative of severe cylinder head
overheating. As the engine overheats, fore-and-aft cylinder head expansion is
limited by the cylinder head bolts screwed into the cast-iron block. The
cylinder head then begins to arch upward at the center, seizing the camshaft in
its bores and stripping the cogs on the timing belt.
Because an overheated valve can
stick in its guide, intake and exhaust valves might also bend in
non-interference engines. See Photo 2.
Photo 2: The absence of light shining underneath this precision-round straight edge indicates that the cylinder head is straight. Although warping of 0.002” per cylinder is the maximum generally allowed, the tolerance on some modern engines is much less.
If the cylinder head is warped
beyond specification, the cylinder head must be straightened or replaced. In
addition, an experienced cylinder head shop will check for cracks in the
cylinder heads that are indicative of a severe overheating condition.
CYLINDER HEAD RECONDITIONING
In my experience, choosing between
reconditioning and replacing a cylinder head depends upon the individual
application. Some cylinder head castings are so thin that they can’t be
reliably repaired. In other applications, a fully equipped cylinder head repair
shop can weld cracks and straighten aluminum cylinder heads to specification.
A shop should also be able to
replace hard valve seats, restore valve guides and resurface cylinder heads to
OEM specifications. Keep in mind that rough mill cuts will quickly ruin most
modern cylinder head gaskets.
In contrast, late-model cylinder
heads require a mirror finish to prevent scuffing the cylinder head gasket
during warm-up. See Photo 3.
Photo 3: Quality is in the details. The edges of this combustion chamber have been chamfered to eliminate the possibility of a sharp casting edge causing destructive preignition.
When stock is removed from the
cylinder head gasket surface, camshaft timing will be retarded because the
center-to-center dimension between camshaft and crankshaft is reduced. In some
cases, thicker head gaskets or head gasket shims are available to compensate
for stock removal from the cylinder head. If no method of compensating for
stock removal is available, the cylinder head should be replaced. On the other
hand, many cylinder head shops like to remove a small amount of stock to remove
corrosion pitting and wear damage from the head gasket surface. In most cases,
the amount of change in camshaft timing is negligible and causes no noticeable
change in engine performance. See Photo 4.
Photo 4: An adjustable aftermarket camshaft sprocket will restore correct camshaft timing on this Toyota 22R engine.
CYLINDER HEAD CLEANING
Perfectly clean gasket surfaces on
the cylinder head and block are essential to a successful cylinder head gasket
replacement. Because gaskets tend to bake onto metal surfaces, gasket removal
can be somewhat challenging.
Never use sharp metal instruments
that can scratch a gasket surface or use rotating abrasive-fiber pads that can
gouge irregularities into precision-machined surfaces. Abrasive pads can also
shed abrasive fibers into the engine lubrication system. During cold start-up,
high oil pressure often pushes these fibers through the oil filter bypass valve
and allows them to embed into soft engine bearings. See Photo 5.
Photo 5: Cleanliness is extremely important in making a reliable gasket repair. The surface on this Toyota 22R engine block has been resurfaced to remove pitting and wear marks.
The most preferable method for
cleaning aluminum surfaces is using a chemical gasket remover and a plastic
scraper available from the paint section of your local hardware store. I’ve
also used hardwood clothes pins as well as brass windshield scrapers to remove
hardened residue from machined aluminum surfaces. In some cases, hardened
gaskets can be scraped away with single-edge razor blades, but such a procedure
requires care and patience to avoid damaging the cylinder head.
CYLINDER HEAD GASKETS AND BOLTS
To accommodate cylinder head
expansion, modern cylinder head gaskets are usually either of the
graphite-composite or the multi-layer steel (MLS) types, neither of which
require re-torquing. Because it’s hard to make general statements about modern
head gasket installations, it’s extremely important to read the enclosed
instructions.
Although some applications do
require a dab of sealant on cylinder head bolt threads and at various
metal-to-metal joints, many MLS gaskets are already coated with a sealant at
the factory. In most cases, applying additional sealants to a new head gasket
can cause a loss of bolt torque retention as the engine warms up.
In addition, many modern cylinder
heads require torque-to-yield (TTY) bolts that should be replaced with the new
head gasket. Because these bolts are stretched beyond their limits of
elasticity to achieve maximum clamping force, bolt torque is rated in degrees
of rotation rather than by the conventional foot-pound or Newton-meter values.
In addition, many new cylinder head bolts are pre-lubed out of the box.
If conventional bolts are being
re-used, it’s important to lubricate them according to manufacturer’s
instructions or with a light coat of 30w non-detergent motor oil. Oils
containing friction modifiers will not provide correct torque values on
cylinder head bolts.
Last, it’s important to prevent
engine block damage by blowing any liquid residue or debris out of the bolt
holes with low-pressure compressed air. The block threads on aluminum engines
must also be inspected for pulling, especially if the engine was overheated.
Engine block thread repair kits are generally available through OE and
aftermarket sources.
ENGINE WARM-UP
Once assembled, it’s important to
bleed air from the cooling system according to recommended procedures.
First, make sure that the engine
is filled to its rated capacity with the correct coolant. Next, allow the
engine to go through at least one warm-up and cooling cycle to allow additional
coolant to be drawn into the cooling system and to check for leaks.
If coolant is circulating through
the system, the upper radiator hose and radiator will feel hot to the touch. If
the radiator feels cool after the engine reaches operating temperature, air
might be trapped under the thermostat. If in doubt, run the engine through
another heating and cooling cycle to draw in more coolant.
Last, if the coolant remains at
the recommended level in the coolant reservoir, the vehicle is ready to release
to the customer.