Troubleshooting 120Dear Ole Mechanic;

When I see something broken, I usually have no trouble fixing it.  What I have trouble with is figuring out what is broken.  How do you do troubleshooting--say on an engine that won’t start?  If I could learn to troubleshoot a problem, I think I could become a good mechanic.

Please help me learn troubleshooting.  Thanks.

Fix It
 

Dear Fix It;

Troubleshooting is not hard, but it does require some basic knowledge.  I’ll use your example of a gas engine that won’t start.  The basic knowledge you need is: fuel, compression and spark.  If you are missing any one of these three, then I can guarantee that the engine won't start.  So, where should you start?  With the easiest thing to check.  That can vary but on most engines with a carburetor the air cleaner is in the way, so I take it off.  Just under it is the carburetor, so while looking at it, it is a good time to work the throttle linkage to see if the accelerator pump squirts gas into the throat of the carburetor.  That is the simplest check for fuel.  If there is fuel, there then there is no need to check the fuel pump or the fuel filter--at least not for a no start problem.  The other quick thing to check for at this time is the smell of the gas.  Today’s gas can go bad in a very short time.  The smell of good versus bad gas is one thing that you will need to learn for yourself.  If the accelerator pump is not sending fuel to the carburetor, then you may have found the problem that is causing the "no start" problem.  Now other checks will need to be made to the fuel system to determine why gas is not getting to the engine.  Some of those checks would be looking for a plugged up fuel filter, a bad fuel pump or even for gas in the gas tank.  I know that sounds too simple, but I have fixed several no-start problems with a little gas.  Remember, gas is one of the three must have things to get an engine to run.

Troubleshooting 2What if the engine is equipped with an electronic fuel-injection system?  That means that checking for gas is going to be more difficult, so I usually go to something that is easier to check for--such as spark.  All that is needed is to pull a spark plug out, reattach the plug wire, ground the outside of the plug and look for spark while cranking the engine.  If there is no spark, then the engine will not run!  Again, there will need to be additional checks to see why there is no spark.  On an electronic fuel-injection engine, additional checks will require that an electronic scan-tool be connected to the engine's electronic controls. The scan-tool can show trouble codes that may indicate why there is no spark or it may take a direct observation of the RPM sensor signals.  All you would be trying to do is to figure out what is keeping the spark from happening.  By the way, there is usually an access port in the fuel injector fuel line to determine if gas is getting to the injectors at the proper pressure.  Also, if the spark plug that was pulled out is wet with gas when it was pulled out, that is a good indication that gas is getting to the engine.  A wet spark plug is enough to get a smell test for bad gas.  If there is no spark on a carbureted engine, then the needed checks will be for a bad coil, bad wires, a bad rotor and--if equipped--bad points.  Of course, the timing of the spark must be at least close enough to get the engine to run.  A simple timing-light check can verify the correct timing.

If there is good fuel and spark, then it is time to check the compression.  One spark plug should already be out from the spark test.  Pull the rest of the plugs and proceed with a compression-gauge test of all cylinders.  If there is not at least 50 PSI of pressure in all cylinders, then it is time to see why there is low compression.  The head gasket may be bad, piston rings may be worn, or the camshaft may have jumped its timing--thereby causing valves to open at the wrong time.  I have even seen bent valves that wouldn’t seat because the camshaft jumped its timing and valves got bent when they hit the pistons.

The key is to start with the basics and to proceed systematically until you find the problem.  This has been a very brief look at troubleshooting, but it should keep you from tearing the carburetor apart when the ignition coil is bad.

Herr Professor Nuzanbolts