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Engine System
CYLINDER HEAD GASKET REPLACEMENT: Zeroing In On Pattern Failures
Edited from an article by Gary Goms, ImportCar Magazine, May 2002
Complete familiarity with the quirks and nuances of a cylinder head gasket replacement on a particular nameplate is a must for anybody who wants to write an estimate for a complete cylinder head gasket replacement service. If, for example, the cylinders have been saturated with ethylene glycol coolant, some nameplates like Honda might recommend that the piston rings be replaced to prevent the rings from sticking in the piston ring grooves. Related pattern failures, like broken exhaust manifold studs on 3.0-liter Nissan V-6 engines, are notorious for driving up the cost of the complete head gasket service.
In other pattern failure cases, the insulation on the knock sensor wiring harness on Toyota 3.0-liter V-6 engines may crack when the intake manifold is removed. After the engine is assembled, the cracked insulation can cause false knock codes.
While the above examples are far from a definitive list of potential pattern failures, they do illustrate the dangers of not advising the customer of pattern-failure or related service items that can double or even triple the head gasket R&R times. Familiarity with pattern and related failure items may also explain why more shops are pursuing nameplate specialization. Specialization, for what it’s worth, certainly helps predict and prevent the pitfalls of routine cylinder head gasket service.
DRY FAILURES
On a more general note, a cylinder head gasket may fail as a dry or wet burn-through. Each poses its own challenges, especially when estimating the complete cylinder head gasket replacement.
The dry burn-through is a situation in which the gasket fails without leaking coolant into the engine cylinder or onto the exterior engine block. Dry failures occur most commonly between two adjacent cylinders and result in an equal amount of compression loss for each cylinder.
Dry failures are more difficult to detect, either because spark plug access limits a technician’s ability to perform a compression test or because a distributorless ignition system limits his ability to perform a power balance test. On 1995 and later OBD II-compliant imports, however, the dry failure may set a generic random misfire code for all cylinders or, depending upon operating strategy, it may set a misfire code for the two adjacent cylinders.
The random misfire or P0300 code is set because the leaking compression from one cylinder flows back through the open intake valve on the adjacent cylinder and upsets fuel distribution in the intake manifold. On the other hand, if a misfire code occurs on two adjacent cylinders, the diagnosis can be narrowed down very quickly.
DRY FAILURE REPAIRS
Dry failure repairs present several difficult situations in communicating, writing and selling the complete head gasket service to the customer. If the engine has been operated for some distance with a dry burn-through on the cylinder head gasket, the hot combustion gases have more than likely eroded the soft aluminum cylinder head and/or the harder metal of the cast-iron cylinder block located between the adjacent cylinders.
If metal erosion has affected the cylinder head, the head either must be reconditioned or replaced, depending upon the application and the severity of the problem. The aftermarket is supplying an increasing number of newly manufactured replacement cylinder head assemblies to remedy the problems caused by machining head surfaces to eliminate sealing imperfections.
When the cylinder head gasket surface is machined to eliminate burn-through marks, the compression ratio of the engine is increased beyond design specifications. Again, depending upon application, this increased compression can cause another head gasket failure from engine-damaging detonation or it can cause the knock sensor to affect performance by excessively retarding the spark advance.
In the case of an overhead camshaft engine, the timing chain or belt also loses its tension and timing tolerances as the distance between the camshaft and crankshaft centerlines is reduced. Typically, the chain tension adjustment may be extended to its design limits, which reduces its ability to compensate for normal wear.
Worse still, the camshaft timing may be retarded as much as four to six degrees, which drastically reduces engine performance and fuel economy. In some cases, the retarded cam-shaft timing may adversely affect fuel trim, intake manifold vacuum, and other data stream parameters.
While installing a new OEM or aftermarket cylinder head eliminates this particular problem, aftermarket cylinder head gasket shims may also eliminate this problem by compensating for the metal removed from a cylinder block deck or cylinder head during the reconditioning process. The head gasket shim is a viable alternative to cylinder head replacement, providing that the thinner cylinder head casting will support a good gasket seal around the head gasket fire ring.
WET FAILURES
Wet failures also present their own set of difficulties in writing and selling the complete cylinder head gasket service. The wet failure occurs when coolant leaks into the engine’s cylinders. If the gasket failure is catastrophic, a large volume of coolant is swallowed by one or more cylinders and is ejected through the exhaust system into the atmosphere. The evidence of a catastrophic wet failure is a large plume of water vapor exiting the exhaust pipe, accompanied by the sickly sweet smell of burned ethylene glycol.
Catastrophic head gasket failures generally are easier to deal with than the slower, more occult, head gasket failures. First of all, catastrophic failures generally don’t result in burned block and head surfaces. Second, the driver usually notices the catastrophic failure and immediately seeks service. Last, the coolant generally doesn’t contaminate the engine oil long enough to cause rust and/or lubrication failure damage.
The occult, or slow failure, however, is more difficult to detect because the owner notices the failure only when he begins adding coolant at more frequent intervals. The slow failure may also show up as a boil-over or increased operating temperature during periods of high-load engine operating conditions. The slow failure may also begin to gradually contaminate the engine oil, oxygen sensors and catalytic converter with engine coolant, which results in some very expensive driveability and emissions-related failure situations.
WET FAILURE REPAIRS
Since it’s not unusual for an engine to experience a crankshaft bearing failure shortly after a cylinder head gasket replacement, it’s wise to recommend removing the engine’s camshaft cover to inspect for corrosion and the oil pan to inspect for crankshaft bearing scuffing or wear. If the oil pan accessibility is poor, it’s good insurance to thoroughly flush the engine with new oil and a brand-name flushing solvent to remove any traces of coolant. Once the flushing process is completed, it would also be a good practice to estimate bearing wear by performing an oil pressure test with the specified engine oil installed with the engine at operating temperature.
The last issue in dealing with a wet failure is the possibility of engine damage caused by severe overheating. The service writer must realize that the customer may have repeatedly overheated the engine and then overcooled it by adding fresh coolant. Smell is a key indicator of how bad the overheating may have been. A rank, burned, coolant smell emanating from an open radiator cap indicates severe overheating. A burned smell emanating from the crankcase may indicate that the engine oil has oxidized and overheated the engine bearings. In either case, the service writer should advise the customer that the complete cylinder head gasket service has suddenly become the complete engine replacement!
Last, service writers should always remember that a routine head gasket replacement is an ideal time to replace old coolant, heater hoses and intake manifold water hoses. It’s also an ideal time to replace, if necessary, the water pump, timing belt(s), timing belt tensioners, drive belts, noisy pulley bearings, spark plugs, air/fuel filters, and, of course, perform a crankcase flush and oil change.
In general, pre-ignition contributes heavily to premature head gasket failure. So, now is a good time to determine if incorrect spark plug heat range, over-advanced ignition timing or a faulty engine management system caused the initial cylinder head gasket failure. Accurately assessing cause and effect will go a long way to prevent future comebacks.
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