No Start Diagnosis

This article will help you learn to diagnose a non-starting gasoline engine.   There are many different fuel and ignition systems used for gasoline engines however this article should apply to 99% of the engines that are on the road today.   If you see anything in this article that you do not understand, disagree with, or know is just plain wrong PLEASE contact me.   Phil Krolick (January 2012)

An engine that will turn over or crank, but does not start, has a defect in one of three basic areas: #1 compression #2 ignition #3 air/fuel.   All three can be quickly checked to identify which vehicle system is defective.

#1 No or low compression can result in an engine that cranks over, but not start.   Compressing the air/fuel mixture is required to get it hot enough to reliably burn once the spark plug fires.  A cylinder with low compression will not raise the air/fuel temperature enough to start the combustion process.   Low cranking compression can be caused by slow starter motor speed, improper cylinder sealing, a broken timing chain/belt, or a defect in camshaft timing.   Most of these problems can be detected by listening to how the engine sounds as it is cranking over.   Identifying problems just by listening takes some experience, especially as different engines sound different when they are cranking.

Measuring intake manifold vacuum, while cranking the engine, is a very easy test that can eliminate compression as the cause for a no start.   Gasoline engines that have at least three inches (3") of manifold vacuum when the engine is at normal cranking speed should have enough compression to start.   Slow cranking speeds will cause low compression as the air has more time to leak past the piston ring end gaps.   To maintain normal cranking speed the battery should have at least 10 volts while the starter motor is turning.   The throttle should be closed (foot off the gas pedal) during a cranking vacuum test.   Because of idle air bypass valves some vehicles will have very little cranking vacuum.   Low cranking vacuum does not guarantee that there is a compression problem.   Cranking vacuum is a quick test only and if you have over 3" of vacuum there is enough compression to run the engine.   If you have low cranking vacuum, place your hand over the main air intake tube that runs between the air filter and the throttle body. If you feel good strong suction, continue with the spark and air/fuel testing.

Minor defects in the compression of an engine will allow the engine to start, but cause the engine to run poorly.   Weak compression in one or two of the cylinders may cause the engine misfire and to run rough. An engine with low or weak compression in all the cylinders may run smoothly, but it will have low power.   Learning to diagnose engine compression will improve your skills at figuring out why an engine is not running properly.

In the lab you will have the opportunity to measure cranking R.P.M.   and cranking vacuum to find out what is normal.   Once you are good at doing this you will be able to quickly test for defective cranking vacuum and R.P.M.

#2 A defective ignition system will cause an engine to crank over but not start.   This could be due to no spark getting to the spark plug, a spark that is too weak to sufficiently burn the compressed air/fuel, or a spark that occurs at the wrong time.

An easy way to check for no spark, or weak spark, is with a spark tester that forces the spark to jump a large gap.   This device may look like a spark plug but it is designed to demand a firing voltage of at least 20 KV.   Another style has an adjustable air gap.   Be sure to open this air gap far enough to force the ignition coil to put out over 20 KV.   This is important because a weak ignition coil can easily generate enough voltage to jump a normal spark plug gap outside of the cylinder at atmospheric pressure.   Inside the cylinder, where you can not see, the spark must jump the gap under compression pressure and this requires more voltage.   A spark plug that fires outside of the cylinder may fail to spark when it is installed in the cylinder head.

Install the spark tester on a plug wire that has just been disconnected from one of the spark plugs.   Be sure to clip the other end of the tester to the cylinder head to provide a ground for the spark.  Using a spark tester is simple and easy, but it only tells you if spark is available to one cylinder at a time.   If you have a good spark at your spark test plug, it might be the only cylinder that is getting a good spark.   So when you are using your spark tester, pay attention to all the clues.

If the engine cranks over at normal cranking speed, but engine is not trying to start (no sputtering or popping or backfiring) you can assume that whatever you find at one cylinder will probably be the same for all cylinders.   So, if there is no spark at one cylinder, the ignition system is probably why the engine will not start.   If some cylinders have spark, and others do not, the engine will at least try to run.   A spark tester will only tell you if a spark is found at the cylinder you have hooked the tester to.   If the engine tries to start during cranking, maybe it sputters or pops or backfires, you can bet that at least some of the spark plugs are firing.   If this is the case there are better ways to discover what the problem is.

Improper ignition timing, or incorrect firing order, will cause a no-start even though the spark plugs are sparking.   If the spark plugs or wires have been changed just before the vehicle fails to start you should suspect the firing order may be wrong.   Carefully check that each plug wire goes to the proper spark plug, and is in the correct coil or distributor cap tower.   If the distributor has just been re-installed, or the timing chain or belt was just replaced, and the engine does not start, you can suspect the camshaft or ignition timing is incorrect.   The first time this happens to you I suggest asking an experienced technician to help you out as this can get very confusing.

One other problem is the spark tester may spark perfectly but the spark plug still does not fire.   This is most commonly caused by too much fuel in the cylinders causing the spark plugs to short out or get fouled.   If you remove the spark plug and see that it is wet (or covered in oil), find and fix that problem.   If the engine still does not start after fixing that problem, replace all the spark plugs.   In my experience, fouled spark plugs should be replaced as cleaning spark plugs is unreliable and not cost effective.

In the lab you will have the opportunity to practice using the spark tester on a variety of engines and learn to verify the operation of both the primary and secondary ignition system

#3 A defect in the air/fuel ratio can cause the engine to not start.   The engine can get no fuel, or not enough fuel, or it can get way too much fuel.

A fuel injector is a simple solenoid including a coil of wire designed to build a magnetic field when turned ON. This magnetic field pulls open a spring loaded valve or pintle inside the fuel injector.   When the pintle is open, fuel will spray into the intake manifold.   When the voltage available to the injector coil is removed (turned OFF) the fuel stops flowing.   As voltage available to the coil inside the fuel injectors gets lower, it takes longer for the magnetic field to get strong enough to open the pintle.  For many fuel injection systems, cranking voltage must be at least 10 volts or the electric coil inside the injector will take too long to open the pintle and the injectors will not provide enough fuel.  It is very difficult for an engine to start when the fuel mixture is too lean.   Always make sure the cranking voltage is over 10 volts before blaming the fuel system.

A lean air/fuel ratio is caused by not enough fuel, or too much air.   Not enough fuel may be caused by a vehicle that is out of gas, a fuel pump that does not turn on, low fuel pressure, or a defect in the fuel injection system.   Too much air can be caused by a leak in the air induction or intake system between the airflow sensor and the cylinders.   Any of these problems may cause an air/fuel ratio that is too lean.

A quick test to see if a lean fuel mixture is causing the engine to not start, is to add propane while cranking.   If the engine now tries to start the problem is no fuel, or not enough fuel.   Check to see if the fuel pump is turning on.   The easiest way to do this is to listen at the fuel filler neck when you first crank the engine.   A funnel inserted into the fuel filler neck can make it very easy to hear if the fuel pump is running.   Once you get good with a lab scope it is not hard to use a low amp probe to get a fuel pump pattern.   This will show if the pump is running, and if it is pumping fuel or just air.   If the fuel pump is running then the injectors may not be turning on and off, the fuel pressure could be too low, or the vehicle could be out of gas.   Adding a gallon of gas is pretty easy to do.   Testing the injectors can be done in a number of ways.   One is to listen for clicking at the injectors using a stethoscope.   Using a lab scope can give you much more information on the injectors.   Fuel pressure and volume can also be checked.

A rich air/fuel mixture is caused by too much fuel, or not enough air.  A common reason for not enough air entering the cylinders is a plugged air filter and checking that is easy.   If the air filter looks dirty try starting it with the filter removed.   Quick tests for too much fuel are a little more difficult.   First you should use a spark tester.  If the ignition system is working, remove one or two spark plugs and see if they are covered in gasoline.   High fuel pressure or leaking fuel injectors can cause the spark plugs to become fuel fouled.   This is called flooding and will keep the engine from starting.   You can find both high fuel pressure and leaking injectors by performing a fuel pressure test.

This is a simplified version of how to discover if a no-start is caused by an improper air/fuel ratio.   There are many more tests to diagnose the air/fuel ratio.

Minor defects in the air/fuel ratio will allow the engine to start, but not perform properly.   A defect in the air/fuel ratio can cause drivability problems such as an engine that runs rough, or has low power.   Learning to diagnose the air/fuel ratio will help you to figure out why an engine is not running properly.  

In the lab you will have the opportunity to use propane to test for an excessively lean air/fuel ratio, properly remove and replace spark plugs, and test fuel pressure and volume.

Is there a proper order to perform these tests to diagnose a vehicle that will not start?

#1) Hook a volt meter to the battery.

Vehicles that will crank over but not start will quickly run down the battery.   A weak battery will have low cranking voltage that will result in slow starter motor speeds (compression), a weak spark (ignition), and a lean mixture (air/fuel).   If you ignore the weak, battery you will go in circles trying to diagnose compression, ignition or fuel systems that do not properly work due to the weak battery.   If the voltage at the battery goes under 10 volts while the starter motor is cranking, the battery is getting weak.   Hook up a booster battery to keep the cranking voltage above 10 volts and then charge the battery before returning the vehicle to the customer.

CAUTION: hooking up a booster, or charging the battery can cause the battery to explode.   Always use safety glasses when working and take special care to properly use the booster battery or battery charger.   Never hook or unhook battery charger cables directly to a battery if the charger is plugged in.   When using a battery booster or jumper cables always make the negative booster cable connection last, and unhook it first.   Be sure to connect the negative booster cable to a good ground away from the battery.   The last connection you make when hooking up booster cables will always create a spark.   The first cable you disconnect after boosting a battery will create a spark.   Make sure this spark is away from the battery or it just might cause the battery to explode.   You must demonstrate this procedure to the instructor the first time you try this in the lab to make sure you know the proper battery safety.   If a battery explodes the primary danger is to your eyes.   Be sure to wash your eyes with clean running water for at least 15 minutes if acid gets on your face or in your eyes.   I have witnessed three battery explosions in the past 15 years and it has been about five years since the last one blew up.   Do the math.   Wear the glasses.   Keep the sparks away from the battery.

#2) Do not crank the starter motor for more than 10 seconds.

Starter motors are designed to deliver maximum power for a very short time.   If they are run for longer than 15 seconds without time to cool down, the starter will overheat and eventually self destruct.   Keep your cranking times as short as you can.

#3) Check for compression and spark.

To preserve the battery and starter motor, perform the cranking vacuum and spark test at the same time.   Hook up the vacuum gauge to manifold vacuum and the spark tester before performing either test.   Get both the gauge and the test plug where you can easily see them.   If either fails the test (low vacuum or no spark) diagnose that system.

#4) Add propane.

If the engine has good compression (cranking vacuum) and spark, and does not seem to be over-fueling, add propane to the engine while cranking the motor.   If the engine now starts and tries to run, check for fuel pump operation, injector operation, and test fuel pressure and volume.

#5) Check for fouled spark plugs

If you have compression, and spark, and no change when adding propane you may have a "flooded" engine.   Take out one or two spark plugs to see if they are fuel fouled.   A plugged air filter can cause this and it is usually quick and easy to inspect the air filter.   If the spark plugs are fuel fouled and the air filter is O.K. be sure to check the fuel pressure.

#6) Check for codes, check the Technical Service Bulletins, follow the vehicle specific diagnostic chart for an engine that cranks but will not start, start asking questions.   Actually checking codes is a good FIRST STEP as this may quickly direct you to the problem.