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Engine Noise Diagnosis 101
Diagnose Noises with a timing light?
Old Wives Tale number 53.
|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. The right way is to open the drains or knock the back freeze plugs out. Vee engines have two separate water jackets on each side so don't forget that you have two drains, one each for the right and left.
SO how come millions of mechanics will lean over a fender and in 5 seconds, confidently spew, "yup, it's a rod knock", like they were looking at a big wart in a good light.
I'm sure that if a buffalo ate a burrito, Magnifico Mechanico would brand the resounding report a rod knock!!!
When your engine is cold, the aluminum piston is small in comparison to it's surrounding iron cylinder. Why? Because with heat, aluminum expands roughly 5 times as much as iron. Therefore the rather hollow piston slapping noise will be loudest first thing in the morning. After the engine warms up, the aluminum piston expands more than it's iron cylinder, reducing 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.
"It's a Doozie"
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, ahem, 20 years.
1929 Duesenberg Model J
|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.
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.
Some guys will use straight pins stuck in the ankle of the wire boots in the distributor. You know. The guys with tattoos and key rings stuck in their eyebrows. Then they touch a grounded jumper wire to each one. 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, 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.