2. Diagnosing Engine Problems
Brake Systems- level: 2
Engine Construction
Topics covered in this presentation:
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compression test
Leakage and vacuum tests
Smoke test
Emission analyzer test
Brake Systems- level: 2
2. Diagnosing Engine Problems
Technician A says that the Otto-cycle gasoline engine has four strokes per
cycle. Technician B says that the crankshaft makes four revolutions
a. Technician A only
b. Technician B only
c. Both A and B
d. Neither A nor B
Brake Systems- level: 2
2. Diagnosing Engine Problems
Technician A says that the purpose of the flywheel is to control the twisting of
the crankshaft. Technician B says that the purpose of the vibration damper is
to help provide a continuous flow of power. Who is right?
a. Technician A only
b. Technician B only
c. Both A and B
d. Neither A nor B
Brake Systems- level: 2
2. Diagnosing Engine Problems
Technician A says that the crankshaft turns 720 during one four-stroke cycle.
Technician B says that the camshaft turns 360 during one four-stroke cycle.
Who is right?
a. Technician A only
b. Technician B only
c. Both A and B
d. Neither A nor B
Brake Systems- level: 2
2. Diagnosing Engine Problems
Technician A says that an in-line six-cylinder engine is usually longer and heavier
than a V8 engine of the same displacement. Technician B says that there are
twice as many teeth on the crankshaft sprocket as there are on the camshaft
sprocket. Who is right?
a. Technician A only
b. Technician B only
c. Both A and B
d. Neither A nor B
Brake Systems- level: 2
2. Diagnosing Engine Problems
Technician A says that “blowby” consists of gases that leak past the valves.
Technician B says that blowby reduces the pressure in the crankcase. Who is
right?
a. Technician A only
b. Technician B only
c. Both A and B
d. Neither A nor B
2. Diagnosing Engine Problems
Brake Systems- level: 2
Testing for Compression Loss
For an engine to run, it must have three things:
• Sufficient compression
• Fuel (in a flammable ratio to air)
• Ignition (timed at the appropriate instant)
One of the most common and least expensive pieces of test equipment is the
compression tester. A compression tester is simply a pressure gauge that is
inserted into a spark plug hole.
2. Diagnosing Engine Problems
Brake Systems- level: 2
Compression Test Procedure
Keep spark plug cables, or coil-over plugs, in order for the correct reassembly.
Use caution when removing spark plug cables. First, twist the rubber boots to
loosen them from the spark plugs.
Clean around all the spark plugs with compressed air. Then remove all spark
plugs, so that the starter can crank the engine easily.
Insert the compression gauge into a spark plug hole.
Crank the engine through at least four compression strokes.
Check and record each pressure reading.
2. Diagnosing Engine Problems
are performing equally and engine
performance is acceptable, the engine passes the test.
Brake Systems- level: 2
Interpreting Compression Test Results. If all cylinders
When compression test specifications are available, they are only an estimate.
If specifications are not available, locate the compression ratio in the
specification manual and use the following formula:
Variations in compression among cylinders should be no more than 20%.
2. Diagnosing Engine Problems
Brake Systems- level: 2
When two cylinders next to each other have low compression, a blown head
gasket is usually indicated.
Wet Compression Test. When
cylinders show poor results, perform a wet
compression test.
Squirt about a tablespoon of oil into each low cylinder.
The oil makes a seal around worn rings, boosting the compression reading.
When cylinder readings are low but increase to normal during a wet test, a
piston ring problem is indicated.
2. Diagnosing Engine Problems
Brake Systems- level: 2
Cylinder Leakage Test (CLT)
The cylinder leakage test can accurately pinpoint causes of leakage. Regulated
compressed air is introduced into the cylinder through its spark plug hole. The
piston is positioned at TDC on the compression stroke, ensuring that both valves
are completely closed.
An acceptable leakage reading on
the tester’s gauge is usually less
than 10% to 15%, although
vehicles with up to 30% leakage
might still be performing to the
owner’s satisfaction.
2. Diagnosing Engine Problems
Brake Systems- level: 2
If a cylinder shows high leakage, listen to locate the sound of the leaking air.
The following lists cylinder leakage test results
Here are some possible locations for leaking air and the likely causes:
Oil filler = leaking rings or piston
Manifold intake = leaking intake valve
Exhaust pipe = leaking exhaust valve
Bubbles in the radiator = blown head gasket, or a crack in the head or block,
which allows the regulated air to enter the cooling system
Brake Systems- level: 2
2. Diagnosing Engine Problems
2. Diagnosing Engine Problems
Brake Systems- level: 2
Engine Vacuum
Intake manifold vacuum can be very useful in determining engine problems.
Vacuum readings compare pressure in the intake manifold to atmospheric
pressure. Vacuum is measured in either inches of mercury (in. Hg) or
millimeters of mercury (mm Hg) or kilopascals (kPa). The vacuum gauge is
connected to a manifold vacuum source.
2. Diagnosing Engine Problems
Brake Systems- level: 2
Throttle plate opening controls the amount of air
that can enter the engine. Movement of the
accelerator pedal opens or closes the throttle plate,
changing engine vacuum. vacuum should Range
from 16 to 22 in. Hg at idle
At idle, with the throttle plate nearly closed,
engine vacuum will be high.
At medium throttle opening, when cruising,
engine vacuum will be close to 8–10 inches.
At WOT, engine vacuum is zero.
2. Diagnosing Engine Problems
Brake Systems- level: 2
To test for weak piston rings:
Raise the engine speed to about 2,000 rpm.
Snap the throttle closed and watch for an increase of 2–6 inches of vacuum
above normal.
Worn rings will not increase vacuum sufficiently during deceleration.
Generally, the higher the rise, the better the condition of the rings.
Brake Systems- level: 2
2. Diagnosing Engine Problems
Sticky valves are indicated when
the needle drops quickly or drifts.
It could be because valves are
hanging up in their guides rather
than closing freely
The most common cause is oxidized gasoline. A lack of proper lubrication during
cylinder head assembly can also result in sticky valves.
Leaking valves are indicated when
the needle drops at regular
intervals.
Brake Systems- level: 2
2. Diagnosing Engine Problems
2. Diagnosing Engine Problems
Brake Systems- level: 2
Smoke Testing
Smoke testers are popular diagnostic devices for finding vacuum, oil, cooling,
and exhaust leaks
A smoke machine generates thick, white smoke at a pressure of 1–1.5 psi.
2. Diagnosing Engine Problems
Brake Systems- level: 2
Emission Analyzer Test
Shops that do engine performance and emission control work have an infrared
exhaust gas analyzer that can be used to check for a collapsed piston skirt or
piston ring or cylinder damage.
With the engine idling, individually short out
each cylinder and record the HC reading.
The HC reading should increase as each cylinder
is shorted out, but a significantly higher reading
indicates a problem cylinder.
2. Diagnosing Engine Problems
Brake Systems- level: 2
ENGINE NOISES
Noises can be located by listening through a stethoscope. The end of a large
screwdriver, a piece of hose, and a long wooden dowel are other listening
tools to help pinpoint noises. An amplified stethoscope is very effective as
well
Brake Systems- level: 2
2. Diagnosing Engine Problems
It is important to try to determine the location of noises before disassembling
the engine. There have been cases where engines have been disassembled,
inspected, and rebuilt, and when reinstalled in the car, still had the same
problem
Noises are often transmitted from their origins to other locations and can be
difficult to locate.
Belts are a common source of noise. If you suspect a belt of making noise,
disconnect the belt and run the engine for a short time, or spray soapy water on
the belt while the engine runs to see if the noise changes.
Brake Systems- level: 2
2. Diagnosing Engine Problems
2. Diagnosing Engine Problems
Brake Systems- level: 2
Crankshaft Noises
Crankshaft noises can be caused by a variety of things. They are generally deeper
in pitch than other engine noises. It is important to isolate the source of the noise
so that an accurate diagnosis can be made
• Front Main Bearing Knock. Excessive front main bearing clearance results
in a heavy knock when the engine warms up. The knock is generally most
pronounced at 1,500–2,500 rpm. Loosening accessory drive belts will
often reduce the intensity of the knock.
• Thrust Bearing Knock. End thrust is movement of the crankshaft in a
forward and backward direction. The crankshaft has a machined surface
that controls end thrust. It can be worn, allowing the crank to move back
and forth. Excessive end thrust will cause a clunk to happen when the
vehicle leaves a stop sign.
• EGR Valve Knock. When there is a low-speed knock, check the EGR valve
by manually opening the valve and accelerating the engine to about 2,000
rpm to see if the noise disappears. If it does, replace the EGR valve.
2. Diagnosing Engine Problems
Brake Systems- level: 2
Exhaust Gas in the Coolant.
The tester samples air in the filler neck of the radiator.
The combustion leak tester is used with the engine running.
Carbon monoxide (CO) exhaust gas in the radiator will change the color of the
liquid in the tester.
Brake Systems- level: 2
2. Diagnosing Engine Problems
If compression is too low or if coolant entering the cylinder causes the plug not
to fire, the tester will not give a reading.
The block check tester can sometimes give a false reading with coolant. It works
best on pure water.
To perform this test, lower the coolant level until it is at least 2 inches below the
radiator filler neck.
The bulb of the tester is squeezed repeatedly for about 1 minute to force gases
in the radiator through the fluid in the tester.
If the color of the blue-green fluid changes to yellow, a combustion leak is
indicated. If the fluid remains blue-green, a leak is not present at that moment.
When test results are positive on a V-type engine, disable all spark plugs to one
bank of cylinders.
2. Diagnosing Engine Problems
Brake Systems- level: 2
Allow the engine to idle for 5 minutes to purge all gases that might remain in the
cooling system from the first test and then retest.
If the test is negative, the leak is from the disabled bank of cylinders and that
head can be removed for repair.
If the test is positive, be sure to repeat the test on the other bank of cylinders
before removing the head. Both heads are independent of each other and could
have leaks.
Brake Systems- level: 2
2. Diagnosing Engine Problems
Brake Systems- level: 2
2. Diagnosing Engine Problems
An infrared exhaust analyzer can also be used to check for exhaust gas in the
coolant
Hold the probe over the radiator filler neck to check for HCs in the coolant. Be
sure not to suck coolant into the tester probe.
If there is CO in the cooling system, exhaust gas is leaking in during combustion.
2. Diagnosing Engine Problems
Brake Systems- level: 2
Cooling System Pressure Test
The system is pressurized with the engine off.
The pressure should remain stable
A pressure tester can also pinpoint the location of a leaking core plug. Use a
flashlight and an inspection mirror to pinpoint the leak. Experienced technicians
will not begin a repair until they are positive of the locations of leaks.
2. Diagnosing Engine Problems
Brake Systems- level: 2
OIL PRESSURE PROBLEMS
Low oil pressure can ruin an otherwise good engine in a short time. A faulty
oil pressure sending unit is sometimes the cause of a low oil pressure reading
on an electric dash gauge or light.
Pressure can be tested by temporarily installing a gauge in place of the
sending unit. Use an oil sending unit socket to remove the sender because
pliers can damage it.
Brake Systems- level: 2
2. Diagnosing Engine Problems
Thank you
Ahmad Mustafa Aboul-Yazid