We are a team of ASE certified mechanics that have created this guide to help
you save money while doing the job yourself, or at least see what you are paying
for when having the job done at a shop. An engine is designed to operate smoothly,
as with any engine a slight vibration is normal, but a notable vibration is telling
you something is wrong, which can be a minor issue like a vacuum leak or a more
serious problem such as low compression or balance shaft issues.
Isolating the Vibration
Vibrations can be caused by internal engine factors or external problems, first
remove the serpentine belt and then start the engine to see if the vibration goes
away, if so, you know the problem is external such as a serpentine belt tensioner.
If the problem is still there you know the problem is internal such as a balance
shaft issue. Also, the vibration could be more noticeable when the engine is under
Vibration (Serpentine Belt Installed)
A serpentine belt tensioner is designed to keep the belt tight during the
engine operation and to take up the slack as the belt naturally wears. This
spring loaded pivot based part can wear allowing the belt to "bounce" as it
loses and gains traction among the drive and accessory pulleys which can be
seen by observing the tensioner while the engine is running, it should be relatively
smooth. If you notice a large vibration the
serpentine belt tensioner
needs to be replaced, the serpentine
belt should be replaced as well.
An engine cooling fan can be driven by the engine or powered electrically,
both configurations can cause a vibration. Inspect the fan's integrity to ensure
the blades are intact and there are no missing parts, this will cause the fan
to rotate out of round which produces a vibration. Also, when a
fan clutch or
fan motor wears it can fail
to hold the fan blade true, with the engine not running (key off) grasp the
fan to check for play.
When an air conditioner is overcharged it can cause an engine vibration
when the HVAC system is on due to higher than normal pressures, this causes
the compressor to overwork itself creating a vibration. This can also be caused
by too much compressor oil in the system.
Check the internal pressures
of the air conditioner system and adjust and lower the high side pressure.
With the belt removed, grasp the remainder of the accessories driven by
the belt and spin by hand to check their rotating ability, this can detect a
bad bearing which can cause a vibration and warrants replacement.
Vibration (Serpentine Belt Removed)
Engine mounts are a vulcanized rubber mount that the engine and transmission
rests upon to shield the car frame and body from the natural operational vibrations.
These mounts are two pieces of preformed metal held together by a piece of vulcanized
rubber and are subject to wear and stress as the engine torques when accelerating
allowing the engine and transmission to rest against the frame of the vehicle
causing a noticeable vibration. The best way to inspect an engine mount is from
under the vehicle using a flashlight to check the integrity of the mount, the
failure will be obvious as the rubber part will be blown out, torn or missing
which warrants an engine mount replacement
to fix the problem. Some mounts are oil filled which will be leaking, another
indication of failure.
A balance shaft mechanism consists of two counter weighted shafts driven
by the crankshaft, a balance shaft belt or chain is used to rotate the shafts
to counteract vibrations created by the natural characteristics of the engine's
operation. The belt or chain can break or become out of synchronization with
the crankshaft either by worn timing components, or a possible misalignment
when assembled. To inspect for this problem, remove the timing belt cover and
inspect timing belt or chain marks for proper alignment. These shafts can also
be driven from an independent balance shaft belt or chain which can be separate
from the camshaft timing belt
or chain. If the balance shafts are non-operational due to mechanical failure
or wear the engine will vibrate, here is an image of the engine balance shafts
which can cause a rattling noise when bad as well.
Engines are designed to be internally or externally balanced, this is achieved
by counter balances in the harmonic balancer or the flywheel-flex plate. If
one of them has a problem or the incorrect parts have been installed the engine
will have a noticeable vibration. Below is an image that shows a harmonic balancer
that has come apart which can produce an abnormal noise.
Vibration Under Acceleration
When an engine misfire
occurs it will cause a vibration which will begin happening when the engine
under load or acceleration, all engines are symmetrical and when part of the
cycle is missing the engine becomes off balanced. This can trigger a check engine
light which can help detect the particular cylinder having the issue.
Front wheel drive vehicles are designed with an additional engine mount
called a torque mount, this mount helps control side movement of the engine
while under acceleration. When a torque or support mount fails it will start
allowing the engine to move excessively causing damage to the
CV axles while producing a vibration.
To check for this problem lift the hood and observe the engine, then have a
helper start the engine and while applying the brakes shift the transmission
from reverse to drive, the engine should move slightly. If excessive movement
is noticed the torque mount is bad
and needs to be replaced.
Vibration at Idle Only
Vacuum leaks are a major source of engine idle vibrations, this is because
when an engine is at idle it depends on the intake system to hold said vacuum
more than any other time of the engine's operation, (when an engine is under
load, the vacuum is reduced). Most of the time a broken vacuum line can be easily
observed. To test for
a vacuum leak start with the engine cool and allow to idle, use carburetor
cleaner and lightly spray around the vacuum hoses (avoid exhaust manifolds)
if the idle speed increases you have located the source of the leak.
Low compression in one cylinder can cause an engine vibration at idle, this
is because an engine depends on an equal amount of compression in each cylinder
to run smoothly. If a tune up
has been done recently and you think this can be the problem a
compression test needs
to be performed.
The intake valve must seal completely for the engine to run smoothly at
idle, if an intake valve leaks it will allow exhaust gases to enter the intake
manifold and into the intake charge of the additional cylinders causing a vibration
at idle. To test this condition a leak down test must be performed, if positive
results are produced for leakage, a valve job is required.