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Engine Noise Diagnosis
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Diagnosing Engine
Noises can be the most difficult thing a mechanic can do.
Misdiagnosis is the norm rather than the exception.
I almost laugh when people open up and say it's a "rod
knock" for every noise from fuel pump rattle to rocker arm
tapping.
My personal favorite was a Chevette customer of mine who
insisted he had a rod knock when in fact a bulge in one of the
tires was hitting his shock absorber.
You might not have enough money to send your kid to college
after you spend a ton of money attempting to fix an audio
illusion. On the other hand you may spend hours and hundreds of
dollars replacing parts in an engine that is truly shot.
First thing you need to do is spend 20 bucks for a cheap
stethoscope at the auto parts store or if you are going to do
this a lot get the electronic ones from Steelman for about $160.
But, possessing human nature, you will convince yourself that a
hose stuck in your uneducated ear will do just as well. No sense
in arguing with you that the whole idea is to be able to tell
infinitesimal changes in direction and intensity that require
the use of two somewhat experienced ears AND the right tools.
So stick your dumb ol' hose in your dumb ol' ear and we'll start
with some clues.
Remember that diagnosis of noises is nothing more than splitting
a bunch of possibilities down to only one
First off, eliminate all of the accessories like the alternator,
power steering pump, A. C. compressor and vacuum pump by
removing the belts one at a time. If the noise is gone, of
course the problem is a belt driven accessory. If the naughty
noise is still there you should be able to hear it more clearly
by not having the accessories whirring away.
If the engine has a carburetor instead of fuel injection it
probably also has a mechanical fuel pump mounted to the engine.
Before the engine gets too hot, put your hand on it. If it is
making a noise you should be able to feel it.
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Try to track the noise
down with the stethoscope tip or the end of the hose suckered onto the
engine surface, sealing the end. Spend a full ten minutes putting the
hose all over the engine, not just where it is loudest. Try to envision
the parts moving inside the engine. You are training your ear, not just
listening, so don't get in a big rush except to be sure that the engine
doesn't overheat. A trained ear can tell you which piston is slapping or
which rocker arm is clacking from outside the engine so if you come out
from under the car proudly saying, "it's the bottom end" get
your dumb-ass back under there until you can tell me it's coming from
the oil pump or the 3rd piston back on the driver's side or the flywheel
or the camshaft.
Rod knocks are loudest at higher speeds (over 2500 RPM) Feathering the
gas pedal may result in a distinctive back rattle between 2500 and 3500
RPMs.
Bad rod knocks may double knock if enough rod bearing material has been
worn away for the piston to whack the cylinder head in addition to
having inside of the big end of the connecting rod banging on the
crankshaft rod journal. It will sound like a hard metallic knock with an
alternating and somewhat muffled aluminum piston klock sound.
Wrist pin knock on domestic vehicles is very rare today but is a
favorite for the misdiagnosticians.
Determining which cylinder contains the noisy parts may be aided by
shorting out the plug wires one by one with a common low voltage test
light. Now you won't get the bulb to light up but it is a convenient way
to short the cylinders without getting zapped or damaging the ignition
coil.
Attach the alligator clip to a convenient ground, away from fuel system
components, and pierce the wire boots at the coil or distributor end of
the wire.
If the noise is changed when the plug wire is shorted to ground, you can
figure that the problem is in the reciprocating bottom end parts.
(piston, wrist pin, connecting rod or connecting rod bearing)
The reason the sound changes is that when you short the cylinder plug
wire you are stopping the combustion chamber explosions that are
slamming the piston downward making the inside of the big end of the
connecting rod bang against it's connecting rod journal. Or in the case
of piston slap it no explosion changes how the piston is shoved hard
sideways when the explosion
If you get a change in the sound when you short a cylinder out it may
become moot as to what the problem is because the oil pan and cylinder
head must be removed to correct the problem. [Generally speaking, an
engine with damage to reciprocating parts (pistons, rings, connecting
rods, wrist pins or rod bearings) and more than 70 thousand miles is not
cost effective or risk free enough to attempt to repair. Replacing a
crankshaft, for example while the rest of the engine has 70k perfectly
maintained miles on it is risky enough but whatever killed the crank has
scored the rings and packed the lifters with debris and smoked the
piston pin bosses etc.]
If the sound doesn't change, look at parts other than the reciprocating
ones. In many cases of rod-knock or piston slap, more than one is
banging so even if you eliminate the noise from one rod the other one
will still be a-banging away with a different, more singular tone.
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Diagnose
Noises
with a timing light?
Valve train noises occur at half of crankshaft
speed so even if your ear can't tell whether the noise
is happening at 700 rpm (raps per minute) or only 350
rpm, your eyes can. Hook the timing light to any one
cylinder and watch the flash illuminate the timing mark.
Stare at it for a while and see if the flash jives with
the knock. If it does, then it is more likely to be
rocker arms, pushrods, lifters, camshaft, cam bearings,
timing chain and gears. If the noise seems twice as fast
it is probably in the crank, mains, rods, rod bearings,
wristpins and pistons.
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Engines
problems, engine diagnosis, engine repair, engine repairs, rod knocks,
lifter ticks, valve train noises, piston slap, engine smoking, oil
consumption, engine noises, sound diagnosis
There is a real nice yet little
known test for piston slap I'll pass along. Some test results can be
mixed or ambiguous but this one is 100% and I've never seen it wrong
after using it for the last 10 years.
When the engine is cold, the aluminum piston is small in comparison to
it's iron cylinder. Therefore the rather hollow slapping noise will be
loudest first thing in the morning. After the engine warms up, the
aluminum piston heats up faster than it's iron cylinder, cutting down on
the excessive clearance between the piston and cylinder wall.
So, the test is this.
First thing in the morning, start the engine up and run it for 15
seconds while you listen carefully and memorize the sound and it's
intensity. Shut it down quickly, pull the spark plugs and put two
squirts of motor oil into each cylinder. Reinstall the plugs, fire the
engine up again and listen.
If you have piston slap the noise will have been greatly reduced or even
eliminated…..for 15 or 20 seconds that is, and then your nightmare
noise will come back to you like a Marine Corps marching band coming
toward you in the parade.
Valve train noises generally are loudest up to 1500 rpms. Lifters are
also misdiagnosed commonly as the source of many noises when in reality
they are quite trouble free, sorta. Dirt contamination on a sludged
engine is the number one cause of true lifter noises, low oil pressure
is number two, . Whatever you do, don't put engine flush in a sludged
engine! We call it "Instant rod knock" because of the way it
overloads the oil filter to the point of opening the filter bypass valve
flooding and destroying the engine bearings with mud. The only safe way
to clean a sludged engine is to accelerate the oil changes and let the
detergent in the oil do the cleaning at a controlled rate. Like every
500 miles
By the way, if you have low oil pressure, don't bother putzing around
with the valve train because the damage you find will be the result of
low oil pressure and will return after you spend a bunch of money on
valve train parts.
For what it's worth, 19 engines out of twenty that we tear down with low
oil pressure do NOT have bad oil pumps but have worn out bearings and
journals so quit with the wishful thinking about just putting a pump in
it. Think about it, generally speaking an oil pump is two dumb ol' iron
gears spinning around immersed completely in oil. EVERYTHING in the
engine has a tougher time of it than the oil pump.
The engine starts to use oil so a valve job is performed to correct the
problem.
Immediately afterward the engine starts knocking or ticking.
The misdiagnosis is that the valve job gave the engine too much power
making the weak bottom end fail.
What really happens is that mechanics all around the world burp the
heads and timing covers off without draining the coolant from the engine
first. The coolant spills into the bottom end of the engine. Even one
oil change may not get all of the spilled glycol antifreeze out.
When you mix the smallest amount of antifreeze and water with motor oil
you end up with a low grade acid that attacks the bearings. When you
autopsy the engine you will find that the bearings are darkened and with
time the bearing surface becomes rough.
That's why it developed a bearing knock immediately after the heads were
done.
Carelessness or ignorance.
The right way is to open the drains or knock a freeze plug out.
Vee engines have two separate water jackets on each side so don't for
get that you have two drains, one each for the right and left.
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