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Old 12-23-2003, 10:55 PM   #1
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Default Engine Noise Diagnostics

Here'* a little blurb I found online that may help you in determining the source of that annoying noise. It was taken from remanufactured.com:

Diagnosing Engine Noises can be the most difficult thing a mechanic can do. Misdiagnosis is the norm rather than the exception. It'* almost laughable when people invariably open up and say it'* a "rod knock" for every noise from fuel pump rattle to rocker arm tap.
My personal favorite was a Chevette customer of mine who insisted he had a rod knock when in fact a bulge in one of his front tires was hitting a shock absorber.
You might not have enough money to send your kid to college after you spend it fixing an audio illusion. On the other hand you may spend dozens of 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 discern 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 stupid ol' ear and we'll start with some clues.
Remember that diagnosis of engine noises is nothing more than splitting 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 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.

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'* the bottom end" get your dumb-*** back under there until you can tell me it'* coming from the oil pump or the 3rd piston back on the driver'* 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 allowing the piston to whack the cylinder head in addition to the big end of the connecting rod banging on the crankshaft rod journal. It will sound like a hard metallic knock (rod) with an alternating and somewhat muffled aluminum (piston) klock sound.
Wrist pin knock in modern engines 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 coilpack 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'* connecting rod journal. Or in the case of piston slap, no explosion changes how the piston is shoved hard sideways against the cylinder wall.

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.

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'* 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'* 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'* 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 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.
Over nineteen 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, usually, an oil pump is two dumb ol' iron gears spinning around immersed completely in oil. EVERYTHING else in the engine has a tougher time of it than the oil pump.

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.

There ya go, that should help with all those "whats this noise?!" posts.
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