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  • Car Engine Runs Rough or Misfires

Car Engine Runs Rough or Misfires

Car Engine Runs Rough or Misfires

Car engine running  poorly at idle only and under power?

Or does the engine run fine  and it's just the engine idle condition that is the problem. If your engine is vibrating  at idling please visit, engine vibrates  at idle. If your engine cranks over but wont start visitengine wont start. If your engine  won't crank over visit engine  wont crank over. If your engine is running rough all of the time or intermittently  you are in the right place. Below we have created a guide to aid diagnoses and repair  procedure for most common rough engine running problems.

Car Repair Guide

Step 1 - Anytime you have a problem with electronically controlled  components such as an engine, transmission, ABS brake, or SRS (supplemental restraint  system, air bag) inspect all fuses  in the under hood power distribution center and under dash fuse panels using a test light. If all  fuses test okay continue to the next step.

Step 2 - To check for problems with electronically controlled  components such as an engine, transmission, ABS brake, or SRS (supplemental restraint  system) and the fuses test ok a trouble code scan is needed to identify any system  trouble. Use a simples canner tool to retrieve trouble codes and  check if they relate to the specific problem, like an ignition coil failure code.  If a trouble code is present but does not pertain to the immediate problem like  an EVAP code ignore it until a later time, after the engine is running properly.

The reason we repair non-related codes after the engine is running properly is because  sometime false codes can be triggered by a rough running engine. Once the engine  is running properly the code present might cycle and turn itself off. You might say "if the engine isn't running right shouldn't it have a check engine  light and a trouble code?"

Sometimes conditions occur that will not be detected  by the computer, example: if the intake or exhaust valve operation fails the computer  cannot detect the failure because the problem is not sensor related, so the engine  doesn't run smooth and the computer thinks everything is okay with no codes. If the  trouble code retrieved relates to a cylinder misfire like an injector driver or  ignition coil failure first repair these problems then re-test system. If no trouble  codes are present proceed to the next step.

Step 3 - Check for broken or dilapidated vacuum hoses on and  around the engine, your car's engine is designed to run on a system that can hold  vacuum. Vacuum hoses are typically connected to the engine intake manifold and will  supply engine vacuum to various accessories like the brake system. Some cars are  designed with a larger vacuum transfer hose like Ford that connects the intake manifold  to the IAC (idle air control) motor . A broken or dilapidated vacuum line or air  intake boot can cause the engine to lose vacuum which will allow the engine to run  rough and die. Inspect all engine and accessory vacuum lines to look for missing,  torn or dilapidated lines and replace as needed.

Also have a helper rest their foot on the gas pedal just enough to keep the engine  running. Check the engine when it is running to listen for any whistling noise coming  from the engine that is not usually present. Follow the noise and inspect vacuum  lines in that area. Also, when the engine is running it will pull inward a broken  or weak piece of the hose to create a larger vacuum leak. Check the integrity of  all vacuum hoses at each end of the hose. Typically this is where a vacuum hose  fails. If all vacuum hoses check "okay" proceed to the next step.

Step 4 - A pocket IR thermometer  laser temperature reader is handy for detecting a cylinder misfire. If you have  visual access to all cylinder exhaust  manifold ports you can use a pocket IR thermometer temperature detector to detect  a cylinder misfire. Start with the engine cold, then start the engine, take temperature  reading on all cylinders, if one cylinder is considerably lower temperature then  the other cylinders you have found the cylinder that is malfunctioning. Example:  on a four cylinder three of the exhaust ports test at 190 degrees and one is at  81 degrees. The cylinder at 81 degrees is misfiring. Once you have found the cylinder  that is misfiring use the following tests to find out why it is misfiring, if haven't  found the misfiring cylinder continue: (the next step can be combined with step  6 and 7)

Step 5 - The spark  plugs in your engine can give you valuable information about how the engine  is running. The chemical reaction inside the combustion camber will leave deposits  on the spark plug that can tell you the condition inside the engine's combustion  chambers. For example: if the engine is running rich all of the spark plugs will  have a black or grey soot covering the spark plug electrode. If the engine is running  lean all of the spark plugs will be clean and white (check your spark plug condition).  If one of the spark plugs looks considerably different from the remaining spark  plugs that cylinder might be having a problem.

Before you remove the spark plugs, mark the plug wires to identify their positions  in the firing order, this will help to make sure they do not get mixed up. Next,  use a spark plug socket to remove the spark plugs, we use a spark plug socket because  the inside of the spark plug socket is lined with rubber to help cushion the spark  plug insulator, which is made of porcelain and is easily cracked or broken. If a  spark plug insulator is cracked or broken the engine will misfire because theignition spark will Travel  to the engine block (ground) instead of the spark plug gap (between the electrodes)

Step 6 - Engine compression is very important for a smooth running engine. An engine  depends on an equal compression reading in each cylinder to run smoothly. If poor  compression exists in one or more cylinders it can cause a rough idle condition  and low engine power output. A compression test of all cylinders is needed to check  for engine wear and internal damage. Remove ignition coil connector or ignition  system or fuel pump fuse to disable  power to the ignition or fuel system to avoid fire.

Compare cylinders compression  reading, typical compression readings are between 125 p.s.i. and 160 p.s.i. all  cylinder readings should be within about 5% of each other. If low compression exists  a cylinder malfunction exits and further troubleshooting is required. Possible causes  for a low compression condition are: burned intake or exhaust valve, broken piston  or piston ring, broken valve spring or a blown head gasket. If engine compression  tests "okay" proceed to the next step.

Step 7 - The ignition system in your car  ignites the fuel inside the engine's combustion chamber at the optimal time in the  piston stroke to produce the most power while emitting the least amount of emissions  as possible. There are many configurations of ignition systems but all operate on  the same principle, create a low energy field and collapse it onto a high energy  coil and that transfers the electrical energy into the secondary ignition system,  i.e. coil wire, distributor cap and rotor (if equipped)plug wires and finally the spark  plug.

This system is triggered by the primary ignition system, this system varies depending  on manufacturer but all operate on the same principle, use some kind of low voltage  trigger system i.e. crankshaft  position sensor (CKP), camshaft position sensor (CAS). This low voltage system  (1.5 to 3.0 volts) is amplified to 12 volts by using an ignition module (amplifier)  and then transferred to the primary side of the ignition coil. The PCM (Powertrain  Control Module) controls the engine ignition timing by advancing and retarding  the primary trigger signal. In older car points and condenser with a vacuum advance  unit performed this job. We have listed the most common causes for an  ignition system to fail or misfire.

Step 8 - Test for proper  fuel pressure with a test gauge on the fuel rail or in line somewhere in the  system, most throttle body injection  cars (TBI) are between 13 psi and 17 psi. and most (DPI) direct port inject systems  are between 40 psi and 55 psi. If good fuel pressure is present continue to next  step. If no or little fuel pressure is present check the fuel pump fuse and fuel  pump control relay located in the fuse panel, you can find this fuse and relay by  checking your owners manual, back of the fuse panel cover diagram, or anonline auto repair manual,  if the fuse or relay has failed replace it a new unit and re-test. Have a helper  crank over the engine while you place your fingers over the relay, does the relay  click under your fingers? If so the relay could be working, there is a chance therelay has burned contacts inside causing the problem but we will get back to  that.

Next, access the fuel pump power  feed wire, there are a few ways to do this, first you need a wiring schematic to  find the color wire needed for testing, the best way to do this is with aservice manual.  Once you have found the color wire it should be located in the wiring harness near  the fuel tank were the pump is located. Ground thetest light and probe  (pierce the wire's outer coating with the test light point) the wire, have a helper  crank the engine over. If the test light illuminates and you have no fuel pressure  the fuel pump had failed and needs to be replaced.  If the test light doesn't  illuminate the fuel pump control relay has probably failed, replace it with a new  unit and re-test, in most cases this relay is under thirty dollars. There is an  outside chance the power feed to the relay has failed but it doesn't happen very  often. If this is the case trace the power source to the relay.

Step 9 - While an engine is running you should be able  to hear the injectors clicking as the electronic  valve opens and closes. To aid in this inspection use a stethoscope and touch it  against each injector. If no audible sound is heard test fuel injector pulse and  supply voltage output (this test is used for most cars).  This test will tell  you if the computer system has operating voltage and injector trigger signal. Remove  an electrical connector from a fuel injector (all injectors need to be tested) probe  both sides of the connector with a grounded test light ormultimeter (voltmeter) (black lead)  switched to DC voltage (there are only two terminals).

Have a helper turn the key to the "on" position without cranking the engine  and observe the test light. The test light should illuminate one side of the connector  only and the multimeter should be at about 12 volts. Next, switch the test light  or multimeter (voltmeter) lead (black) or to the positive side of the battery to  test the system ground injector trigger, probe the side of the connector that did  not light up, have a helper crank the engine over and observe the test light, it  should blink on and off or the multimeter (voltmeter) should bounce from 0 to 12   volts. (Note: if no injector pulse is present try disconnecting the remainder of  injectors and re-test, if a fuel injector is shorted it can shut down the injector driver causing no injector pulse.

If injector pulse returns plug the injectors electrical connectors in one at  a time until the pulse fails and then replace that injector). If this test revealed  that there was no pulse but system has power, the PCM is not generating a fuel injector  trigger. If there is no trigger to the fuel injector it will not allow fuel to enter  into the engine. Some of the most common reasons that can cause this condition include  a shorted fuel injector,  injector wiring damage or shorted PCM. While the injector trigger wire is off, test  the fuel injector windings by setting the multimeter (voltmeter) to ohms and attach  the leads to either side of the injector.

Your reading should be between 11 and 18 ohms. Test all of your injectors; if  one injector reading is considerably different from the others replace that injector.  Check the fuel pressure regulator, remove the vacuum line feeding the fuel pressure  regulator, check for the presence of fuel, if fuel is present the diaphragm inside  the regulator has ruptured and is feeding raw fuel into the engine causing it to  run rough.

Step 10 - A camshaft is commonly  used to operate poppet valves in a piston engine. A cylindrical rod is situated  in the cylinder block or cylinder head which has oblong lobes or cams which push  the intake and exhaust valves. This force is applied on the valve directly or through  an intermediate mechanism such as a rocker arm, lifter (cam follower) and push rods  are used to press against the valve for movement. Each valve utilizes a spring which  will return the valve to its original position (closed) after the force is removed.  If a valve spring has broken or a cam lobe has worn down it will cause the engine  to misfire and run rough because it causes the engine to lose compression. You might  say" wouldn't either of these conditions show up in a compression test?" and the  answer is not always. The problem with a compression test is that it is performed  at engine cranking speed. If you have a worn camshaft it will show up only when  the engine is running.

In some conditions a broken valve spring can effect different aspects for example:  If just one coil of spring breaks off, the spring can still close the valve but  only at low RPM's, causing a high RPM engine misfire if the spring breaks somewhere  in the middle it will affect both idle and power conditions. To test for this condition,  remove ignition coil connector or ignition system or fuel pump fuse to disable power  to the ignition or fuel system. Remove the valve covers to gain access to visually  inspect the valve train.

Have a helper crank the engine over while you watch the rocker arms or cam lobes,  make sure they are all going up and down the same amount, if one or more lobes are traveling less than the others you have a flattened cam lob and the camshaft needs  to be replaced. Inspect the condition of the valve springs, use a flashlight and  small mirror too aid in the inspection. If a broken valve spring is discovered it  will need to be replaced to correct the problem.

Step 11 - The head gasket in your engine is used to seal the  cylinder head to the engine block. The cylinder head is fastened to the engine block  using head bolts that are tightened to a specific torque. When a head gasket wears  it can allow coolant or exhaust to enter the combustion chamber causing a cylinder  misfire. To test for a blown head gasket, have the engine cold, then remove about  a half a gallon of coolant from the radiator. Next attach a chemical tester to the  top of the radiator were the radiator cap attaches and fill the tester with testing  solution.

Start the engine and watch the chemical tester it will start to bubble as the  engine warms up, if the chemical turns blue to yellow the engine has a blown head  gasket. If the head gasket is blown between two cylinders it will allow exhaust  to enter the cylinder that is not in the power stroke causing a cylinder misfire.  To check for this condition perform a compression test of all cylinders. If the  head gasket is blown between to cylinders the compression reading will be a little  low in two cylinders that are next to each other and compression reading will be  close to the same in both cylinders.

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