Rod Knock Diagnosis and FAQ

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Mar 30, 2005
General Rod Knock Info:

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 tap.

Edit: Here's an example from right here on Supramania. Guesses as to the source of the sound included rod knock, valve lash, something caught in the fan amongst others:


My personal favorite was a customer of mine who insisted he had a rod knock when in fact a bulge in one of his 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 $120.

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'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 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 burning of fuel in the combustion chamber that is 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 combustion 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, you must rebuild. 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. You need to find out why things went bad to start with. Engines don't just fail. There are reasons for the failures.]

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.

Commentary on rod knock after a Head Gasket swap

I've seen dozens, no probably HUNDREDS of cars develop rod knock after a head gasket repair.

The common (incorrect) assumption is that the new head gasket provided higher compression and the additional force on the worn out old bearings cause a failure. In a word, bullshit.

What the real cause is, 99.9999% of the time is coolant in the oil. Now before you go and discount this, understand a few things:

Many tests have shown that as little as 0.04% (400 PPM) water in lubricating oil can cut the fatigue life of bearings by as much as 48%.

Moisture is generally referred to as a chemical contaminant when suspended in lubricating oils. Its destructive effects in bearing applications can reach or exceed that of particle contamination, depending on various conditions.

Water may cling to metal surfaces or even form a thin film around solid contaminants such as silica particles. But by far the most damage is done when 'etching' occurs.

Water etching is a common type of corrosion occurring on bearing surfaces and their raceways. This corrosion is caused primarily by the generation of hydrogen sulfide and sulfuric acid from water-induced lubricant degradation. Yes, you read that right water (and anti-freeze) when mixed with oil and heated inside an engine combine to form acid. This eats away the soft surfaces of your bearings in no time.

(Ever notice that a Supra with a BHG for any length of time will have rust in the coolant? - Guess what - the coolant has become acidic by mixing with the exhaust gasses that are getting into the water jacket!)

Remember your bearings are never supposed to "touch" anything but oil. The bearings ride on a very thin layer of oil. Once there is some etching, the oil pressure drops (since the etching has provided the oil with a place to go other than where it is supposed to be) and eventually the bearing touches the rotating surface. After that it's all over.

Did you do your head gasket replacement with the motor out of the car? Did you drain 100% of the coolant from the motor before you removed the head? Did you pull the oil pan while changing the gasket? If your answer to any or all of these is no, I'll bet coolant contamination in the oil was the culprit.
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