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.
Shortly 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 the 1980s.
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 miles.
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
The 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.
But 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
Because 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.
Because 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
Modern 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.
When 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 straightened.
SHORT-BLOCK ENGINE FAILURES
The lower 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 systems.
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 parties concerned.
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).