The engine in your car is designed to run smoothly with maximum power output
and produce as minimal emissions as possible. When your car's engine is not performing
properly it can cause, low gas mileage, low power output, increased emissions and
possible internal engine damage if left untreated. This troubleshooting guide is
designed to isolate the malfunctioning cylinder and troubleshoot to repair the problem.
Before we start we need to know one of two things, is the engine running poorly
at idle only and seems to be ok under power? Or does the engine running fine and
it's just the engine idle condition that is the problem. If your engine is
idling rough please visit,
why does my engine run rough
at idle? If your engine cranks over but won't start
visit
why won't my engine start? If your engine won't crank over visit
why won't my engine crank over? If your engine is running rough all of
the time or intermitted you are in the right place. Below we have created a guide to aid
diagnoses and repair procedure for most popular rough engine running problems.
Car Repair Guide - READ COMPLETELY BEFORE BEGINNING
Step 2 - To check for problems with electronically controlled
components such as an
engine,
transmission,
ABS
brake, or
SRS (supplemental
restraint system, Air Bag) and the fuses test ok a
trouble code scan is needed to identify any system trouble. Use a simple
scanner 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
a 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 can not detect the failure because the problem is not sensor
related, so the engine doesn't run smooth and the computer thinks everything
is ok with no codes. If the trouble code retrieved relates to a cylinder misfire
like a injector driver or ignition coil failure first these problems and re-test
system. If no trouble codes are present proceed to the next step.
How do I scan my service engine soon light?
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 hose
are typically connected to the engine intake manifold and will supply engine
vacuum to various accessories like power brakes. Some cars are designed with
a larger vacuum transfer hose like Ford that connects the intake manifold to the
IAC (idle air control) motor. If a broken or dilapidated vacuum line
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 "ok" proceed to the next step.
Step 4 - A
pocket IR thermometerlaser
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 isolate the
following tests to that cylinder, if haven't found the misfiring cylinder continue
with the following steps in this guide: (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 shoot 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 way different
then the remaining spark plug the cylinder the different plug came out of 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 the ignition spark will travel to
the engine block (ground) instead of the spark plug gap (between the electrodes)
How do I change my spark plugs?
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 exits 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 inspection
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 "ok" 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.
Typical Ignition System in Operation
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 ECM (engine
control module) controls the engine ignition timing by advancing and retarding
the primary trigger signal. In older cars points and condenser with a vacuum
advance unit performed this job. We have listed the most popular causes for
an ignition system to fail or misfire visit -
How do I test the engine ignition system?
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 an
online 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 the relay 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 an
online auto repair 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 the
test 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 use an
online auto repair manual to trace the power source to the relay.
How do I check my fuel pressure?
Fuel Injection Pressure and Pressure Regulator Test
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
or
multimeter (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 ECM 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 popular reasons
that can cause this condition include a shorted fuel injector, injector wiring
damage or shorted ECM/PCM. While the injector trigger wire is of 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 a 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 present 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 it's original position (closed) after the force is removed. If a valve spring
has broken or a cam lobe worn down it will cause the engine to misfire and run
rough because it causes the
engine looses 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
effect 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 help 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 to aid
in the inspection. If a broken valve spring is discovered it will need to be
replaced to correct the problem.
Typical Camshaft Configuration
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 camber 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 exactly the same in both cylinders.
Step 12 - Engine mounts are used to minimize
engine vibration by isolating the
engine from the car frame. These mounts are constructed by using rubber
vulcanized between to metal plates with threaded studs that bolt to the
engine and the frame of the car. When motor mounts become worn they allow
the engine the contact the frame causing a noticeable vibration. Inspect the engine
mounts for integrity, look for any oil, cracks or separation
of the mount. Remove the failed engine mount and replace if needed. For
exact instructions how to replace the engine mounts in your vehicle consult an
auto repair
manual.
New Engine Mount (appearance will vary)
Step 13 - A balance shaft belt is used inside some designs of engines to counter
act vibration created by the natural engine operation. Many balance shafts
are operated by a timing belt that can break or become out of correlation
with the engine either by a worn timing belt or misalignment when replaced. To
inspect for this problem, remove the timing belt cover and inspect timing
belt marks for proper alignment. All engine are different, consult a car
repair manual for exact belt alignment specifications.
For exact specific repair procedures for any of the steps above visit our
online repair
manual page.
WARNING! Always have the vehicle under inspection on level
ground, in park with the emergency brake on. Always wear protective eyewear, gloves
and necessary clothing before inspection or work begins. Never crank an engine over
when anyone is near the battery or engine. Always have a operational fire extinguisher
close by, obey all first aid instructions in the event of an injury. Never stand
in front or behind a vehicle when starting or running. When engine is cranked over
keep hands and clothing away from rotating components. Anyone with a heart pacemaker
should not perform these test.
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