Where To Begain Your Diagnosis If You Have A Engine That Cranks,But Will Not Start
65The Obvious Problem
If you have a vehicle that cranks over good,has fuel,but will not start here are some simple steps to take to pinpoint the culprit.The first thing to do is check for spark.No fancy tools needed for this test.You can insert a screwdriver into the end of the spark plug wire,ground the screwdriver against the valve cover or exhaust manifold.Crank the engine over like you are going to start it,check for blue looking spark or slight cracking sound.Next step is to check the compression.The tools you will need for this or a compression testor,ratchet and spark plug socket.Remove spark plugs,insert compression test line into spark plug hole and crank like you are trying to start for 3 to 5 seconds.If compression is good,the obvious problem is fuel delivery.The most likely causes are a dead fuel pump,low fuel pressure,plugged fuel filter are no signal to injectors.On most vehicles the fuel pump is turned on by the PCM though a fuel pump relay.The fuel pump circuit may be wired though the oil pressure switch or a interia switch that turns the fuel pump off in case of a accident.
Two tests for diagnosing a faulty fuel pump or checking the dead head pressure and fuel volume test.The dead head test checks the maximum pressure of the fuel pump with the return fuel line blocked.The fuel pump should produce higher than normal pressure with this test.If fuel pressure does not go up the pump will not be able to keep up with engine load demands.The fuel volume test is the best way to evaluate the condition of the pump.This test requires a fuel pump pressure/flow testor.If flow testor is not available you can disconnect the fuel return line from the regulator.Connect a hose from the regulator to a large container .With the engine off energize the fuel pump.As a general rule in @ 30 seconds a good fuel pump will deliver one quart.Make sure there no open flames or sparks.
Misdiagnosis is the leading cause for fuel pump replacement.When the vehicle will not start the fuel pump gets the blame most of the time.If you follow the above tests it will pinpoint the problem.
