Engine Diagnostics

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As the trend toward the integration of ignition, fuel, and emission systems progresses, diagnostic test equipment must also keep up with these changes.

New tools and techniques are constantly being developed to diagnose electronic engine control systems. However, not all engine performance problems are related to electronic control systems; there fore, technicians still need to understand basic engine tests. These tests are an important part of modern engine diagnosis.

Compression Test:

Internal combustion engines depend on compression of the air-fuel mixture to maximize the power produced by the engine. The upward movement of the piston on the compression stroke compresses the air fuel mixture within the combustion chamber. The air fuel mixture gets hotter as it’s compressed. The hot mixture is easier to ignite, and when ignited it generates much more power than the same mixture at a lower temperature.

If the combustion chamber leaks, some of the air fuel mixture will escape when it’s compressed, resulting in a loss of power and a waste of fuel. The leaks can be caused by burned valves, a blown head gasket, worn rings, slipped timing belt or chain, worn valve seats, a cracked head, and more.

An engine with poor compression (lower compression pressure due to leaks in the cylinder) won’t run correctly. If a symptom suggests that the cause of a problem may be poor compression, a compression test is performed.

A compression gauge is used to check cylinder compression. The dial face on the typical compression gauge indicates pressure in both pounds per square inch (psi) and metric kilopascals (kPa). Most compression gauges have a vent valve that holds the highest pressure reading on its meter. Opening the valve releases the pressure when the test is complete.

The steps for conducting a cylinder compression test are shown.

Ford, Toyota, and other hybrids use Atkinson cycle engines. These engines delay the closing of the intake valve, which means that the overall compression ratio and displacement of the engine are reduced. Therefore, when conducting a compression test on these engines, expect a slightly lower reading than what you would expect from a conventional engine.

To conduct a compression test on a Toyota Yaris, you must use a scan tool and the one from Toyota is preferred. The scan tool allows you to enter into the engine cranking diagnostic mode. This mode allows the engine to crank with the fuel injection system disabled. It also makes sure that the starter motor/ generator is not activated (except for activating the starter motor to crank the engine), which not only is good for safety purposes, it’s also good because the load of the generator cannot affect the test results because it’s not energized. Always follow the sequence as stated in the service manual. Failure to do so will result in bad readings.

Wet Compression Test-- Because many things can cause low compression, it’s advisable to conduct a wet compression test on the low cylinders. This test allows you to identify if it’s caused by worn or dam aged piston rings. To conduct this test, add two squirts of oil into the low cylinders. Then measure the compression of that cylinder. If the readings are higher, it’s very likely that the piston rings are the cause of the problem. The oil temporarily seals the piston to the cylinder walls, which is why the readings increased. If the readings don’t increase, or increase only slightly, the cause of the low readings is probably the valves.

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Conducting a Cylinder Compression Test:

1 Before conducting a compression test, disable the ignition and the fuel injection system. Most manufacturers recommend that the engine be warm when testing.

2 Prop the throttle plate into a wide-open position to allow an unrestricted amount of air to enter the cylinders during the test.

3 Remove all of the engine's spark plugs.

6 Carefully install the gauge into the spark plug hole of the first cylinder.

5 Many types of compression gauges are available. The screw-in type tends to be the most accurate and easiest to use.

4 Connect a remote starter button to the starter system.

7 Connect a battery charger to the car to allow the engine to crank at consistent and normal speeds needed for accurate test results.

8 Depress the remote starter button and observe the gauge's reading after the first engine revolution.

9 Allow the engine to turn through four revolutions, and observe the reading after the fourth. The reading should increase with each revolution.

12 Each cylinder should be tested in the same way.

11 Before removing the gauge from the cylinder, release the pressure from it using the release valve on the gauge.

10 Readings observed should be recorded. After all cylinders have been tested, a comparison of cylinders can be made.

13 After completing the test on all cylinders, compare them. If one or more cylinders is much lower than the others, continue testing those cylinders with the wet test.

14 Squirt a small amount of oil into the weak cylinder(s).

15 Reinstall the compression gauge into that cylinder and conduct the test.

16 If the reading increases with the presence of oil in the cylinder, the most likely cause of the original low readings was poor piston ring sealing. Using oil during a compression test is normally referred to as a wet test.

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PROCEDURE:

1. Make sure the engine is at operating condition.

2. Remove the radiator cap, oil filler cap, dipstick tube, air filter cover, and all spark plugs.

3. Rotate the crankshaft with a remote starter button so that the piston of the tested cylinder is at TDC on its compression stroke ( +++37). This ensures that the valves of that cylinder are closed.

4. Insert the threaded adapter on the end of the tester's air pressure hose into the spark plug hole.

5. Allow the compressed air to enter the cylinder.

6. Observe the gauge reading ( +++38).

7. Listen and feel to identify the source of any escaping air.

CAUTION! Always follow the precautions given by the manufacturer when conducting a compression test or other engine-related tests, especially when doing this on a hybrid vehicle. In most hybrids, the engine is cranked by a high-voltage motor. Because this motor is required to run the test, the high-voltage system cannot be isolated. Therefore, extreme care must be taken and all appropriate safety precautions must be followed.

+++Rotate the engine so that the piston of the cylinder that will be tested is at TDC before checking leakage.

+++ The reading on the tester is the percentage of air that leaked out during the test.

+++Cylinder leakage test results.

Measured Leakage Conclusion Less than 10% Good Between 10 and 20% Acceptable Between 20 and 30% Worn engine Above 30% Definite problem 100% Serious problem

+++Sources of cylinder leakage and the probable causes.

Source of Leakage Probable Cause Radiator Faulty head gasket

Cracked cylinder head; Cracked engine block Throttle body Damaged intake valve Tailpipe Damaged exhaust valve Oil filler or dipstick tube Worn piston rings Adjacent spark plug hole Faulty head gasket

Cracked cylinder head--Cylinder Leakage Test

If a compression test shows that any of the cylinders are leaking, a cylinder leakage test can be performed to measure the percentage of compression lost and to help locate the source of leakage. A cylinder leakage tester applies compressed air to a cylinder through the spark plug hole. The source of the compressed air is normally the shop's compressed air system. The tester's pressure regulator controls the pressure applied to the cylinder. A gauge registers the percent age of air pressure lost when the compressed air is applied to the cylinder. The scale on the gauge typically reads 0% to 100%. The amount and location of the air that escapes give a good idea of the engine's condition and can pinpoint where compression is lost.

A zero reading means there is no leakage in the cylinder. Readings of 100% indicate that the cylinder won’t hold any pressure. Any reading that is more than 0% indicates there is some leakage. Most engines, even new ones, experience some leakage around the rings. Up to 20% is considered acceptable.

When the engine is running, the rings will seal much better and the actual leakage will be lower.

SHOP TIPS - Some leakage testers read in the opposite way; a reading of 100% may indicate a totally sealed cylinder, whereas 0% indicates a very serious leak. Always refer to the manufacturer's literature before using test equipment.

The location of the compression leak can be found by listening and feeling around various parts of the engine.

WARNING! On some computer-controlled engines, certain components must be disconnected before attempting the power balance test. Because of the wide variations from manufacturer to manufacturer, always check the appropriate service manual. On all vehicles with an electric cooling fan, override the controls by using jumper wires to make the fan run constantly. If the fan control cannot be bypassed, disconnect the fan. Be careful not to run the engine with a disabled cylinder for more than 15 seconds. The unburned fuel in the exhaust can build up in the catalytic converter and create an unsafe situation.

Also run the engine for at least 10 seconds between testing individual cylinders.

Cylinder Power Balance Test:

The cylinder power balance test is used to check if all of the engine's cylinders are producing the same amount of power. Ideally, all cylinders will produce the same amount. To check an engine's power balance, each cylinder is disabled, one at a time, and the change in engine speed is recorded. If all of the cylinders are producing the same amount of power, engine speed will drop the same amount as each cylinder is disabled. Unequal cylinder power balance can be caused by the following problems:

¦ Defective ignition coil

¦ Defective spark plug wire

¦ Defective or worn spark plug

¦ Damaged head gasket

¦ Worn piston rings

¦ Damaged piston

¦ Damaged or burned valves

¦ Broken valve spring

¦ Worn camshaft

¦ Defective lifters, pushrods, and/or rocker arms

¦ Leaking intake manifold

¦ Faulty fuel injector

A power balance test is performed quickly and easily using an engine analyzer, because the ? ring of the spark plugs can be automatically controlled or manually controlled by pushing a button. Some vehicles have a power balance test built into the engine control computer. This test is either part of a routine self-diagnostic mode or must be activated by the technician.

+++The vacuum gauge is connected to the intake manifold where it reads engine vacuum.

Connect the engine analyzer's leads according to the manufacturer's instructions. Turn the engine on and allow it to reach normal operating temperature.

Set the engine speed at 1,000 rpm and connect a vacuum gauge to the intake manifold. As each cylinder is shorted, note and record the rpm drop and the change in vacuum.

As each cylinder is shorted, a noticeable drop in engine speed should be noted. Little or no decrease in speed indicates a weak cylinder. If all of the readings are fairly close to each other, the engine is in good condition. If the readings from one or more cylinders differ from the rest, there is a problem. Further testing may be required to identify the exact cause of the problem.

Late ignition timing; Manifold Leak; Weak valve spring; Leaking head gasket

Carburetor or injector adjustment; Burnt or leaking valves; Sticking valves; Restricted catalytic converter or muffler

+++ Vacuum gauge readings and the engine condition indicated by each.

Vacuum Tests:

Measuring intake manifold vacuum is another way to diagnose the condition of an engine. Vacuum is formed by the downward movement of the pistons during their intake stroke. If the cylinder is sealed, a maximum amount will be formed.

Manifold vacuum is tested with a vacuum gauge.

The gauge's hose is connected to a vacuum fitting on the intake manifold. Normally a "tee" fitting and short piece of vacuum hose are used to connect the gauge.

Vacuum gauge readings can be interpreted to identify many engine conditions, including the ability of the cylinder to seal, the timing of the opening and closing of the engine's valves, and ignition timing.

Ideally each cylinder of an engine will produce the same amount of vacuum; therefore, the vacuum gauge reading should be steady and give a reading of at least 17 inches of mercury (in. Hg). If one or more cylinders produce more or less vacuum than the others, the needle of the gauge will fluctuate. The intensity of the fluctuation indicates the severity of the problem. For example, if the reading on the vacuum gauge fluctuates between 10 and 17 in. Hg we should look at the rhythm of the needle. If the needle seems to stay at 17 most of the time but drops to 10 and quickly rises, we know that the reading is probably caused by a problem in one cylinder. Fluctuating or low readings can indicate many different problems.

For example, a low, steady reading might be caused by retarded ignition timing or incorrect valve timing.

A sharp vacuum drop at regular intervals might be caused by a burned intake valve. Other conditions that can be revealed by vacuum readings follow:

¦ Stuck or burned valves

¦ Improper valve or ignition timing

¦ Weak valve springs

¦ Faulty PCV, EGR, or other emission-related system

¦ Uneven compression

¦ Worn rings or cylinder walls

¦ Leaking head gaskets

¦ Vacuum leaks

¦ Restricted exhaust system

¦ Ignition defects

Oil Pressure Testing:

An oil pressure test is used to determine the wear of an engine's parts. The oil pressure test is performed with an oil pressure gauge, which measures the pres sure of the oil as it circulates through the engine. Basically, the pressure of the oil depends on the efficiency of the oil pump and the clearances through which the oil flows. Excessive clearances, most often caused by wear between a shaft and its bearings, will cause a decrease in oil pressure.

Loss of performance, excessive engine noise, and poor starting can be caused by abnormal oil pressure.

When the engine's oil pressure is too low, premature wear of its parts will result.

An oil pressure tester is a gauge with a high pressure hose attached to it. The scale of the gauge typically reads from 0 to 100 psi (0 to 690 kPa). Using the correct fittings and adapters, the hose is connected to an oil passage in the engine block. The test normally includes the following steps:

1. Remove the oil pressure sensor ( +++43) and tighten the threaded end of the gauge's hose into that bore.

2. Run the engine until it reaches normal operating temperature.

3. Observe the gauge reading while the engine is running at about 1,000 rpm and at 2,500 rpm (or the specified engine speed).

4. Compare the readings to the manufacturer's specifications.

Excessive bearing clearances are not the only possible causes for low oil pressure readings; others are oil pump-related problems, a plugged oil pickup screen, weak or broken oil pressure relief valve, low oil level, contaminated oil, or low oil viscosity.

Higher than normal readings can be caused by too much oil, cold oil, high oil viscosity, restricted oil pas sages, and a faulty pressure regulator.

Oil Pressure Warning Light--The instrument panel of most vehicles has an oil pressure warning lamp that lights when the oil pressure drops below a particular amount. This lamp should turn on when the ignition key is initially turned to the on position and the engine is not running. Once the engine starts, the lamp should go out. If the lamp fails to turn off, there may be an oil pressure problem or a fault in the warning lamp electrical circuit. To determine if the problem is the engine, conduct an oil pressure test. If there is normal oil pressure, the cause of the lamp staying on is an electrical problem.

+++43 The oil pressure gauge is installed into the oil pressure sending unit's bore in the engine block.

+++44 Oil leaking from around the oil pan gasket.

Description Probable Source Honey or dark greasy fluid Engine oil Honey or dark thick fluid Gear oil with a chestnut smell Green, sticky fluid Engine coolant Slippery clear or Brake fluid yellowish fluid Slippery red fluid Transmission or power-steering fluid Bluish watery fluid Washer fluid +++45 Identification of fluid leaks.

---Engine Type:

Gasoline

Diesel

Gasoline and Diesel

Gasoline Diesel

---Visible Sign:

Gray or black smoke

Blue smoke

White smoke

----Diagnosis Incomplete combustion or excessively rich A/F mixture incomplete combustion

Burning engine oil

Coolant/water is burning in the combustion chamber

Fuel is not burning

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Probable Causes:

¦ Clogged air filter

¦ Faulty fuel injection system

¦ Faulty emission control system

¦ Ignition problem

¦ Restricted intake manifold

¦ Clogged air filter

¦ Faulty fuel injection system

¦ Faulty emission control system

¦ Wrong grade of fuel

¦ Engine overheating

¦ Oil leaking into combustion chamber

¦ Worn piston rings, cylinder walls, valve guides, or valve stem seals

¦ Oil level too high

¦ Leaking head gasket

¦ Cracked cylinder head or block

¦ Faulty injection system

¦ Engine overheating

+++46 Exhaust analysis.

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Prev.: Engine Identification



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