It is very important that you diagnose the cause of a problem before performing a repair. It is not
unusual for an inexperienced technician to spend many hours of work only to discover that the repair
was unnecessary.
Five major diagnosis areas are covered:
• Possible reasons for oil consumption
• Causes of rough running or a loss of engine power
• Engine noises
• Oil pressure problems
• Cooling system problems
There are many causes of engine problems. Some are the result of normal wear and tear or a lack of maintenance. Engine problems also might be due to previous work on the engine. Problems that appear to be engine-related can also be caused by other automotive specialty areas, such as the transmission or emission controls. Sometimes a problem with a system causes an engine to fail. If the problem is not taken care of, the failure will recur. This chapter should serve as a reference for future problems. The descriptions of various problems are listed in the index at the back of the book. More in-depth training in engine diagnosis comes under the overlapping specialty area of engine performance.
DIAGNOSING PROBLEMS BEFORE A REPAIR
An engine should be correctly diagnosed before disassembly for two reasons. It should be determined
that an overhaul is really necessary. The damaged starter motor drive frame shown in Figure 3.1
resulted in a diagnosis of catastrophic engine failure. Failed belt-driven accessories can also lead you to
believe that there is a seized engine. The exact location of a problem should be determined
while the engine is running. A thorough discussion of the problem with the owner of the
vehicle is also helpful. Sometimes an owner’s driving habits or maintenance procedures can be the
cause of the problem.
OIL CONSUMPTION
Piston rings are usually the first thing a customer suspects when a car starts to use oil, even though oil can be lost through a variety of other conditions. Oil loss can be due to either external leakage or internal oil consumption. Excessive internal oil consumption can sometimes be spotted as an oily coating on the inside of the exhaust pipe. Black soot at the exhaust pipe often indicates an overly rich air-fuel mixture, not oil consumption.
The rate of normal oil consumption depends on such things as the size of the engine, the weight and shape of the vehicle, the viscosity and service rating of the oil, engine rpm and load during use, engine temperature, and the amount of oxidation and dilution of the oil. Information about oil is covered in detail in Chapter 14.
From time to time an owner will complain of an occasional rapid oil loss. This might be a normal condition that sometimes occurs after 1000 or more miles of city driving followed by a highway trip. City driving can result in extra fuel and water dilution in the oil. Before leaving on a long vacation trip, the customer checks the oil and the dipstick registers “full.” But when the diluted oil becomes thoroughly heated, evaporation of the pollutants gives the appearance of rapid oil consumption as the oil level drops a quart in a few hundred miles.
Bad Valve Guides or Seals
The cause of internal oil consumption is often
worn valve guides or defective valve guide seals. There might be exhaust smoke during deceleration
because of oil leaking into the combustion chamber through the intake valve guides. Deceleration
causes very high engine vacuum, which pulls oil into the combustion chamber.
A spark plug that is oil fouled on only one side
indicates leaking valve guide seals. Carbon deposits on the necks of the intake valves are another indication . Look for carbon deposits when disassembling the cylinder head.
Different types of valve guide seals are described . Valve guide seals should always be replaced during a valve job while the heads are disassembled. includes a procedure for replacing valve guide seals without
removing the heads from the engine. Oil Consumption from Piston Rings When oil is consumed past piston rings, one common cause on high mileage engines is lack of ring tension resulting from cylinder wall and ring wear. A record of poor engine oil maintenance contributes greatly to an increased wear factor. An engine that suffers from a lack of regular oil changes will often have plugged oil control rings (Figure 3.3).
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| FIGURE 3.3 A Plugged oil control ring. |
The piston’s oil-control rings need to be able to scrape oil from the cylinder walls and return it to the crankcase
through the underside of the piston (Figure 3.4).
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FIGURE 3.4 Oil ring drainback. Slots (a) or holes (b) in the oilgroove return oil to the crankcase as the piston moves down. |
Spark plugs provide a window to conditions in the cylinder because they extend into the combustion chamber. Figure 3.5 shows abnormal spark plug conditions associated with oil consumption.
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| FIGURE 3.5 Examples of oil-fouled spark plugs. |
consumption might be caused by worn or stuck piston rings
Excessive Rod Bearing Clearance
A high mileage engine will probably have worn crankshaft bearings. Excessive bearing-tocrankshaft journal oil clearance can result in low oil pressure at idle. Oil consumption can increase at higher engine rpm as oil leaks out between the connecting rod journal and the rod bearing. At higher engine speeds, this can result in too much oil being thrown onto the cylinder walls, overwhelming the oil rings, which cannot return all of the oil to the crankcase (Figure 3.6).
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| FIGURE 3.6 Increased oil clearance causes more oil to be thrown onto cylinder walls |
Incorrect Engine Oil Dipstick
It is especially important to make sure that the correct oil dipstick is used after an engine change or short block installation. Manufacturers often install the same engine in different vehicle models. Depending on the vehicle, the engine can be equipped with a different oil pan, which sometimes requires a different length oil dipstick. Excessive oil consumption can result from too short a dipstick. Every time the owner mistakenly adds a quart of oil to the crankcase, the crankshaft throws the oil on the cylinder walls and the overfull engine burns off the excess.
Plugged Cylinder Head Drainback Holes
When engine oil has not been changed often enough, thick sludge can plug the oil return holes in the cylinder head. These drainback holes allow rocker arm oil to return to the crankcase (Figure 3.7). The problem can be temporarily solved by cleaning out the holes, but it is a symptom of a poorly maintained
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| FIGURE 3.7 Plugged oil drainback holes will cause exhaust smoke |
to the crankcase; it floods the valve guide, making the valve stem seal ineffective.
Leaking V-Type Intake Manifold Gasket
Intake manifold vacuum can draw oil into the intake ports from the lifter-valley area under some intake manifolds (Figure 3.8). This is a tough problem to find. A smoke test is a good way of finding an intake manifold leak. A cranking vacuum is another way to test for internal air leaks before the
engine is disassembled. These procedures are covered later in this chapter. When removing an intake
manifold, always visually inspect for the possibility of previous intake gasket leakage.
V-type engines equipped with an exhaust gas recirculation (EGR) valve on the intake manifold often experience oil-fouling of the spark plugs that are closest to the EGR valve. This is caused when the intake manifold warps or the manifold gasket fails. Replace the gasket with one designed for high temperature applications.
Crankcase Pressure
Normally, there is a slight vacuum in the crankcase. One possible reason for excessive oil leakage is a positive crankcase ventilation (PCV) valve that
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| FIGURE 3.8 Oil can be drawn into the intake manifold past a faulty manifold gasket |
• Pinch the line that leads to it, or cover the end of the PCV valve with your thumb.
• With computer idle speed adjust disabled, if the PCV valve is good, idle speed should drop.
Blocking the flow of air to the PCV valve enriches the air-fuel mixture.
Be sure to check for a restricted filter or a kink in the breather line from the valve cover to the air cleaner. This can result in oil leakage caused by crankcase pressure.
If the PCV system is to be effective, the entire crankcase must remain sealed. A leaking or misplaced gasket can cause enough air leakage to result in failure of the PCV system. This includes the timing cover, oil pan, valve cover gaskets, and crankshaft seals. If the engine is not airtight, suction from the PCV valve
will not create sufficient vacuum in the crankcase. An oily air cleaner, or oil in the hose to the air cleaner,
often points to a crankcase pressure problem.
A ring seal tester can be used to test an engine’s amount of blowby. It measures airflow out of the crankcase in cubic feet per minute (cfm). Normal airflow is about 5–8 cfm. Above 8 cfm indicates that the rings are not sealing properly. Unfiltered air allows dirt to enter the engine, causing engine wear. This can result from leaking vacuum hoses, vacuum control units, vacuum accessories, or manifold leaks. Crankcase pressure
can also cause oil to migrate up the distributor shaft and into the distributor. To locate a leak in the PCV system:
• Seal the breather and PCV valve.
• Use a smoke tester (covered later) or blow (lightly) into the dipstick tube with a rubbertipped blowgun (regulated to no more than 2–3 psi). Listen for leaks, using a piece of hose or a stethoscope with the metal end pulled off.
A leak is often not readily apparent, especially at the top side of a valve cover gasket or where the
intake manifold meets the block at the front or back. Oil might not leak out because of gravity and suction
from the crankcase vacuum of the PCV system.
Oil that leaks through gaskets and seals is a common cause of oil consumption.
Rear Crankshaft Seal Leaks
A rear main bearing seal leak can be identified when oil is found on the engine side of the flywheel
or torque converter. Oil on the opposite side indicates front transmission seal leakage. Oil that has been sprayed in a circular pattern is also indicative of a crankshaft seal leak. Most crankshaft seal leaks are caused by excessive crankcase pressure.
Oil leaks streaking down the block can be due to a leaking oil gallery plug, cam plug, or seal retainer block. The block could also be porous or cracked.
Black Light Testing. When black light testing, a 1-ounce bottle of fluorescent dye is added to engine oil to help locate leaks. When a black light is used, the source of the leak will be highlighted in bright yellow-green streaks. A mirror can be used to bounce the black light into hard-to-see areas. Washing the engine first is helpful but not necessary.
There are two types of ultraviolet (UV)-fluorescent lights available. The traditional black light type uses a vapor bulb powered by 110 volts. It requires a warm-up period of about 10 minutes. With the 110-volt lamp, yellow-green dye is easily visible in the leaking oil; oil without dye appears purple. Use care when handling the lamp. The bulb is fragile and is expensive to replace. A different type of light system uses a high output 12-volt UV/blue lamp that comes on instantly and is used with yellow glasses (Figure 3.9). UV
light can be damaging to your eyes, so use caution. With the 12-volt lamp and glasses, oil is yellow and oil with dye in it is a brighter yellow-green.
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| FIGURE 3.9 A UV light powered by the vehicle battery is used with yellow UV glasses, which prevent eye damage |
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