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Engine diagnostics

The internal combustion engine type of which most people have experience is the gasoline or Otto cycle engine. Unfortunately these are substantially harder to understand than the other primary variant of vehicle engine (diesel engines).

In terms of diagnosing problems diesels are simpler because fewer things need to happen correctly for them to work. A diesel uses a compression ratio of 15:1 or higher. Compressing air this much raises its temperature to something in the neighborhood of 2000°F, which is substantially higher then the flash point of diesel fuel. Thus as long as the engine is not so worn that it cannot fully compress the air, and fuel is actually delivered to the combustion chamber diesels pretty much run.

The necessary conditions for gasoline engines to operate are more complex. First, the air:fuel mixture ratio has to be just about 14:1, too high or low and at best the engine will run poorly, if at all. Second, an electrical spark must be delivered to ignite the mixture. This spark must occur at the correct time, which is different for different engine RPM values.


The carburettor or fuel injection system may deliver a mixture which is either too lean (too much air} or rich (too much fuel). Running an engine lean causes it to run hotter than desirable. This can sometimes be recognized by popping sounds from the exhaust when decelerating at high RPM. Running and engine rich causes incomplete combustion and the excess fuel to form carbon deposits (which may be observed in the tailpipe. These carbon deposits can coat the spark plug insulator, which as carbon is conductive can create a short circuit.

Lean running is aften caused by conditions such as extra air entering the intake via an open vaccum hose or loose hardware or the carburettor or fuel injection not being properly jetted or adjusted. This condition can result in premature wear of the exhaust valves and valve seats, damage to the pistons and especially in air cooled engines, reduced lubrication due to overheated oil.

The idle speed of a gasoline engine is controlled by the closed throttle valve which creates a vacuum which the engine must work against to keep running. This means that lean running due to air leakage will be accompanied by an increased idle RPM or rough idling behavior.

Rich running can also be caused by poor adjustment of the fuel system or by a restricted air filter - these need to be changed per the maker's stated intervals, or more often in dusty enviornments. Leakage of oil into the cylinders, can cause symptoms which look like rich running. This can be leakage past worn piston rings or valve guides, or a damaged head or head gasket.


If spark is delivered to the combustion chamber too early (advanced) it will result in knocking - a loud noise, potentially damaging to the engine If the spark arrives too late (retarded) it will result in the engine running with reduced power.

If the coil output voltage is low (due either to failure of the ignition system or low battery voltage) the plugs may appear to generate a spark when tested outside the engine. However the compressed air:fuel mixture (charge) has considerably higher resistance to electricity, and so no spark may form. When the spark is marginal the engine may run but audibly start missing when the throttle is opened (creating higher pressure in the cylinders).

The most difficult element of diagnosing an engine which is not working (or working well) is that the symptoms of one system can look like the symptoms of another. Unfortunately there is a strong tendency on the part of most mechanics and operators to develop an idea of what's wrong prior to having adequate diagnostic data. Of course this is more problematic when the vehicle is actually needed for some purpose, where panic leads to rushing to a diagnosis.

One of the best things the operator of a vehicle can do is pay attention to the sounds the engine and other machinery generate. Unfortunately, when things break it can be a gradual process, and a change in behavior which develops over a period of weeks or more can be hard to catch. Also, the imagination of a worried operator can easily invent bad sounds. Still, paying attention to the sound of something running right usually pays off in an early warning of a developing problem.

Loud noises should always be investigated. Noise is often created by things either hitting each other or rubbing upon each other with less lubrication than they need. Sounds which get progressively louder may be far less costly to fix when noted early.

Just a few diagnostic details are included above. They are limited to the engine, of course other parts of a vehicle must be working correctly for it to serve as working transportation (powertrain, brakes, steering, and generation of electricity to name a few).

The rest of the vehicle:

The other systems of an auto are (in terms of acute problem diagnosis) less essential to engine operation, yet they are still necessary for the vehicle to be of use.


Correct lubrication is essential to both the engine and the various gearboxes (transmission) and differential(s). In a vehicle with automatic transmission the transmission fluid level is checked with the engine running and transmission in neutral or park position, similarly if there is a power steering pump, its oil fill needs to be checked with the engine running.

Loss of lubrication in an engine running under load can cause expensive failures in a minute or less. If the oil pressure ever drops below its normal level this is cause for concern. If the oil warning lamp signals or the pressure gauge reads less than 1/2 of what is normal the engine should be shut off immediately. Engine oil level is checked with the engine off.


Because all vehicles require electricity to run, they use alternators to power the electrical components. The battery can typically store enough energy to keep a vehicle running for a couple of hours (far less if the headlights are on) or to power the starter motor for a total of perhaps 5 minutes of "cranking" (avoid continuously operating the starter for more than 15 seconds, it is not designed for continuous operation).

Insufficient electrical power can stem from either failure to generate enough power for operation and battery charging or from the battery not taking a charge. Sometimes batteries will fail to charge simply becuase the battery connectors have become corroded and no longer conduct electricity. Most batteries fail in 2-6 years of operation, less if they are discharged often.

The alternator and voltage regulator both have to be operating correctly to maintain charge in the battery. Also the belt which drives the alternator needs to be intact and sufficiently tight. Squealing sounds when reving the engine (most often with the headlamps on or immediately after starting) is often an indication of a loose belt.


Gasoline (and diesel) engines create large quantities of heat (55-75% of the fuel energy is lost as heat). This heat must be dissipated into the air, and unfortunately air is both a poor conductor of heat and has low heat capacity. This is why most engines use water (and ethylene glycol with anti-corrosive additives) as an intermediate coolant.

The coolant is circulated through the engine by a vane pump, usually driven by a belt. The most common causes of overheating are a low supply of coolant, failed or slipping belts and, a stuck thermostat valve. (Note: overtightening the drive belt can often cause an early failure of the water pump.)

The thermostat valve is usually located near the front of the engine. When closed it causes a nearly complete restriction of coolant flow. This allows the engine to warm to its optimal operating temperature quickly. The valve typically opens at about 140°F, allowing coolant to circulate through the radiator and cool the engine. When it fails it usually sticks closed resulting in an overheated engine. If no spare is immediately available it can usually be safely removed.

At some higher temperature, a second thermostat will usually engage a cooling fan motor, which draws additional air through the radiator. In the event of overheating it is usually possible to partially cool an engine by running the passenger compartment heater set to maximum heat and blowers also set to maximum, preferably with windows open to help the waste heat leave the vehicle.

Never pour cold water directly into an overheated engine with low coolant. This can cause the block to crack. Also allow an overheated engine to cool before opening the fill cap, steam can easily cause 2nd degree burns.

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