Writing a prescription for a sick engine
Even if you perform regular preventive maintenance on your equipment, you occasionally will run into an engine that just won't start when you expect it to. You can save time in these situations by following a logical step-by-step procedure to determine the cause of the "illness" before performing any service repair.
Make a general diagnosis While an engine occasionally may develop serious troubles, most defects are minor. You usually can remedy them at minimum cost. In many instances, in fact, correcting minor trouble prevents--or eliminates--a major failure at a later date. Therefore, it's wise to develop a habit of following a planned sequence in the diagnosis and elimination of trouble. The following should help locate problems. * Know as much as you can about the system you're inspecting. Repair manuals for specific engines often include troubleshooting charts and hints to help isolate problems on that particular model. * Investigate the specific problem. Ask yourself--or the operator--what the equipment was doing when the trouble occurred. Was the trouble intermittent or constant? Did you or the operator have any warning? (Often an operator's passing comment provides a clue to locating the cause.) If you weren't present when the problem occurred, inspect the system personally. Often, problems are obvious after a quick visual inspection. Loose or missing parts, for example, can cause problems, and their correction is evident. Similarly, many electrical malfunctions result from broken or shorted wires, loose or corroded connections, blown fuses or similar problems that you can easily correct.
Do not hesitate to use any available test equipment to further isolate the problem.
Again if you didn't experience the problem personally, operate the equipment yourself or attempt to start the engine. Even if the engine won't start, you can learn a lot about its condition by attempting to start it. Check for unusual sounds, smoke and gauge readings, among other considerations.
* List possible causes of the problem. You usually can locate a general area of difficulty by looking at several causes that result in similar problems. Always check the easiest items first. * Decide what the problem's most likely cause is from the list of possibilities. Make further tests of specific items to reduce the number of possible causes, if you've not yet pinpointed the problem. *Repair the items, then check to make sure your conclusion is correct. Double check your list of problems. Keep in mind that some engine troubles result from a primary and one or more secondary causes of a cumulative nature. Therefore, correcting the primary cause can return the engine to service without correcting the underlying cause. An early, second failure then can result. Also consider that the following list contains only the most common malfunctions and causes. You cannot limit your troubleshooting procedures to only those described.
Many repair shops have found the following procedures to be good methods for quickly determining the cause of engine trouble. As suggested previously, don't always adhere strictly to the following troubleshooting procedures. In many instances, the operator's comments on when he or she encountered trouble will indicate its cause. Also, you'll eventually develop a diagnostic technique that can only come with experience.
If the engine won't start-- or is hard to start A variety of causes can keep an engine from starting. To determine the actual cause, you need to look at several aspects of the engine system. Consider each of the following when you first encounter starting problems: * Fuel. All engines need fuel to start and run. Check that the fuel tank is filled with the proper type of fuel. Water or other fluid in the tank can prevent the engine from running and may cause extensive damage. Water doesn't mix with gasoline or diesel fuel and appears as different-colored areas in the fuel at the bottom of the tank. Clean water is clear (lighter in color), but rust can color water and make it darker. In addition, a large amount of water in the fuel tank can cover the bottom of the tank uniformly and may be difficult to identify. * Controls. Check that controls are positioned properly. Often manufacturers provide equipment with interlocks to prevent starting unless the safety switches are set correctly. Likewise, it is important to properly set the choke or other starting-enrichment system before attempting to start an engine. * Spark. To operate, spark-ignition engines need a correctly timed spark and a proper mix of fuel and air in the combustion system. Other problems can prevent the engine from starting or make it difficult to start. However, most probable causes are a lack of fuel or a lack of spark. Remove the spark plug and check its condition after attempting to start. The spark plug should smell of gasoline and may appear damp. It should not be fouled or obviously damaged.
You can purchase a test plug from an ignition-service tool supplier. It is probably the most valuable tool available for a quick diagnosis of engine problems. Simply detach the wire from the spark plug and attach the test plug. Then attach the test plug to a suitable engine ground. Attempt to start the engine while observing the test plug. You should see a bright electrical spark jump across the gap and cause a snapping sound. A spark indicates the ignition system is operating. Thus the problem is likely improper fuel/air. If the ignition will not cause a spark regularly at the test plug, check the ignition system further to locate trouble. Especially check the condition of the ignition stop switch and associated wiring on magneto-equipped models. Other problems could cause the failure, but remember to check the simplest items first.
* Compression. On diesel (compression-ignition) engines, the engine must compress the air sufficiently. It then sprays fuel into the compressed air. Failure or difficulty in starting is related to the compression or injection. Check the exhaust while attempting to start. Light-colored smoke indicates some injection is occurring. Therefore, a failure to properly compress the fuel is the probable cause. Low cranking speed also can make starting difficult. * Injection. On diesel engines (and a few spark-ignition engines), if compression appears normal, loosen a high-pressure fuel line at one of the injection nozzles. Then attempt to start the engine. If fuel regularly spurts from the loosened connection while attempting to start, then the engine is attempting to inject the fuel. If fuel does not flow from the loosened connection, begin at the fuel tank and follow the fuel's path through the filters and transfer pump to the injection pump. Keep looking until you find the cause. Some causes of the fuel not flowing are: * Air in the lines * Plugged fuel filters * Incorrectly adjusted fuel shut-off controls. Be sure that you've loosened the correct line for this check. If fuel flows from the loosened injection line, remove the injection nozzle from the cylinder head. Attach the nozzle to a tester. If you don't have a nozzle tester, you can reconnect the nozzle to the engine's injection line and direct the nozzle spray outside the cylinder. Again attempt to start the engine or operate the tester and observe the nozzle spray pattern. The nozzle should regularly emit a finely atomized spray. The shape of the spray cone will depend on the nozzle's design. (Caution: The nozzle releases fuel with sufficient force to penetrate the skin. Do not risk injury by permitting the high-pressure spray to contact any part of the body.)
If fuel spurts from the loosened high-pressure fuel-line connection--but it does not spray from the exposed nozzle--service the fuel-injection nozzle. A ragged spray that does not properly atomize can cause difficulty in starting. If fuel sprays from the nozzle, check compression pressure using a high-pressure gauge. Also inspect for mechanical damage such as a burned valve or a leaking head gasket. If compression is satisfactory, check the injection timing.
* Starter. On all models, the starter must turn the engine fast enough to start. A damaged starter, or any other problem that slows down the starting speed, can prevent the engine from starting. On engines with a manual starter, check for wire, rope, string or debris wrapped around the engine's crankshaft. A bent crankshaft also can slow starting speed. Do not attempt to start direct-drive lawn mowers in tall grass. Tall grass impedes the blade so much that you can't crank the engine fast enough to start it--or it causes you to break the rope.
On engines with an electric starter, be sure that safety interlocks operate properly and will permit starting. The battery also must be in good condition before you attempt to start the engine.
If the engine starts, then stops This complaint usually is due to fuel starvation. However, a faulty ignition system also can cause it. To determine which is the problem, follow these troubleshooting procedures: * Check the spark plug. Use a test plug, as mentioned. If you get a good spark, then your problem is probably the fuel:air ratio. If not, the problem is probably in the ignition system. * Remove and inspect the fuel-tank cap. On all except a few early engines, a vent--or breather--in the cap allows air to enter the tank as the engine uses fuel. If the engine stops after running several minutes, a clogged breather may be the culprit. To discover if this is the cause, remove the fuel cap and start the engine again. If the engine then runs well, you simply need to clean the cap or purchase a new one. Caution: Be sure to observe safety precautions before running an engine without the fuel-tank cap in place. If any danger exists that fuel could spill on the engine or a spark could enter the open tank, do not attempt to run the engine without the fuel-tank cap in place. If in doubt, try a new cap. * Check the fuel filter or fuel line. Once you've eliminated a clogged breather as a potential cause of trouble, suspect a partially clogged fuel filter or fuel line. Remove and clean the fuel tank and line. If your engine is equipped with a shut-off valve or a fuel-tank filter, clean those as well. On some engines, a screen or felt-type fuel filter also is located in the carburetor fuel inlet. (Refer to the engine-repair section in your manual to see if this is the cause on your engine's make and model.) Clean this too. * Consider the carburetor. If you still encounter trouble, a sticking or faulty carburetor-inlet needle valve, float or diaphragm may be the problem. Remove, disassemble and clean the carburetor. (For more information, see "Carburetor cures are fundamental," July 1996). * Check the ignition. Once you eliminate the fuel system as a cause of trouble, test the magneto or battery-ignition coil. If you don't have equipment to check this, look for an ignition spark at the test plug immediately after the engine stops running. If you don't see a spark, replace the coil, condenser and breaker points. * Check for compression. On 4-cycle engines, check for engine compression immediately after the engine stops. A sticking intake, exhaust valve or cam follower (tappets) can cause problems. If you find that the engine exhibits little or no compression immediately after it stops running, refer to your engine-repair manual.
When an air-cooled engine overheats When air-cooled engines overheat, check for: * Debris. Check that grass, leaves, dirt or other debris is not plugging the air-inlet screen on the blower housing. Also remove the blower housing and shields and check for dirt or debris accumulated on or between the cooling fins on the cylinder. * Missing or bent shields or missing blower housing. Besides causing an engine to overheat, the lack of these components is dangerous. Never attempt to operate an air-cooled engine without all shields and the blower housing in place. * A too-lean main fuel-air adjustment of the carburetor. * Improper ignition-spark timing. Check the breaker-point gap. On engines with unit-type magnetos, check the magneto-to-engine timing. On battery-ignition units with a timer or distributor, check that breaker points open at the proper time. * Engines under excess load. When you operate an engine in excess of its rated engine horsepower, it's going to overheat. This is true, also, when operating equipment in extremely high ambient air temperatures. * Lack of lubrication. Two-cycle engines operated with an improper fuel-lubricating oil mix may overheat due to a lack of lubrication. Refer to your service manual for the recommended fuel-lubricating oil mix for your engine.
When the engine surges when running Improper adjustment to the carburetor or governor typically is the trouble with engine surging. Refer to your engine's manual for adjustment, inspection and repair procedures. A worn governor linkage also can cause surging.
On spark-ignition engines, check for a vacuum leak between the carburetor and the engine. If you can't locate the cause, overhaul the carburetor.
The governor for some diesel engines is designed so that it is integral to the fuel-injection pump. Therefore, service to the governor will require an overhaul of the injection pump.
When the engine misses If a 2-cycle engine misses only at no-load, high-idle speed, check first that the engine is equipped with an ignition cut-out governor. If it is, the engine will miss at high speed due to cut-out action. If the engine is not so equipped, refer to the appropriate engine-repair section in your service manual and adjust the carburetor as outlined. Some 2-cycle engines will miss when not under load, even though you've properly adjusted the carburetor. If a 2-cycle engine fires evenly under normal load, it usually is okay.
You usually can isolate a continuous miss on one cylinder of a multi-cylinder engine by observing the items listed under the previous subheading, "If the engine won't start...," page 64. On many 2-cylinder engines, the ignition system for each cylinder is independent. Therefore, one cylinder can run perfectly while the other does not fire. On some 2-cylinder engines, however, a short at one set of points will cause the engine not to run. On other models, the engine will not fire on either cylinder if one set of points is stuck in the open position.
You typically can trace complaints of too little power or speed to improper tuning. Make sure the air filter is clean and in good condition. Ensure that the exhaust pipe and muffler are open and not clogged or restricted. You also must correctly adjust the ignition timing and the carburetors. Plus, you must synchronize both the carburetors and ignition systems on 2-cylinder models. Make the proper altitude adjustments for mountain running if the altitude nears or exceeds 5,000 feet.
When the engine uses too much oil Excessive oil consumption easily can lead to early engine destruction. The loss of only a small amount of oil increases the operating temperature of both the oil and the engine. The localized high temperatures--especially around bushings and bearings--may be enough to weld pieces of metal together. In addition, rapid wear is a sure result if the engine operates without enough oil.
Engine oil may leak out of the engine or enter an area of the engine where it doesn't belong. Refer to the figure on page 64, which lists causes of oil loss.
Once you've recognized any engine problem, don't jump too soon to a conclusion. Remember the other two steps you need to take--locating the source of the problem and correcting it—before you forge ahead with the repair. Taking the procedure on a step-by-step basis will be more efficient in the long run.
* Rotary-mower engines. On most rotary-mower engines, the flywheel is light-weight, and the mower blade provides the additional inertia necessary to turn the engine through the compression stroke. Therefore, these engines will not start without the blade attached. When checking rotary-mower engines for starting complaints, first make sure the blade is mounted and solidly attached. To do so, disconnect and ground the spark-plug wire so the engine will not accidentally start. Then hold the engine crankshaft from turning and see if you can turn the blade on the shaft. If you can, you need to tighten the slip clutch or replace the shear key.
If a spark test does not indicate a spark (see subheading, "If the engine won't start...," page 64), remove the flywheel and check the condition of the flywheel key. Some rotary-mower engines are equipped with a very soft flywheel key to prevent damage or injury if the blade stops suddenly from hitting a solid object.
Loose mounting bolts or other loose bolts can cause knocking or rattling noises. Sharp knocking noises that appear to come from within the engine usually indicate serious trouble. Stop the engine and carefully examine the unit to determine the cause.
A bent engine crankshaft or a bent blade can cause extremely loud noises. A bent blade or crankshaft is caused when the mower strikes a solid or nearly solid object. Severe vibration usually accompanies the noise. The noise may be a rapid knocking or a scraping sound.
Diagnose the trouble by removing the spark plug, grounding the spark-plug wire and turning the blade slowly by hand.
Caution: You must disconnect the spark plug to prevent the engine from accidentally starting. Make sure you securely attach the detached spark-plug wire to an engine ground because if it remains loose the spark could jump back to the spark plug and allow the engine to start again. You also could remove the entire spark plug, rather than detaching and grounding the wire. Doing this permits the crankshaft to turn more easily without the interference of the compression cycle. Grounding the spark-plug wire also eliminates the possibility of coil damage from an incomplete secondary circuit.
* Two-cycle engines with reed valve. On 2-cycle engines, the engine crankcase must compress the incoming fuel-air mix for it to properly reach the engine cylinder. On engines using a reed-type valve between the carburetors and the crankcase, a bent or broken reed will not allow compression to build in the crankcase. Thus, if such an engine seems otherwise okay, remove and inspect the reed-valve unit. Refer to the engine-repair section in your owner's manual for information on individual 2-cycle engine models.
* Two-cycle-engine exhaust ports. Two-cycle engines--especially those operating on an overly rich fuel-air mix or with too much lubricating oil mixed with the fuel--tend to build up carbon in the cylinder-exhaust ports. You should periodically remove the muffler and scrape the carbon from the exhaust ports. Recommended procedure varies somewhat with different makes of engines; therefore refer to your engine-service manual for specific instructions.
Similarly, on 2-cycle engines that are hard to start or experience a loss of power, remove the muffler and inspect the exhaust ports for carbon buildup.
* Four-cycle engines with compression release. Many makes of 4-cycle engines have a compression release that reduces compression pressure at cranking speeds. Be sure to use this feature to make engine-cranking easier. Most models with this feature develop full compression when turned backward.
Robert K. Mills is senior technical writer for Intertec Publishing Corp., the parent company of Grounds Maintenance. He is the author of Engine Fundamentals of Operation and Service, an Intertec/K-III publication on which this article is based.
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