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misterp

Diagnosing Engine Sounds.

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A fellow site member called me this morning, and we found ourselves trying to explain motor noises and sounds over the phone, and it dawned on me a write-up on this subject would be helpful. One way that you know a great machinist from a jerk mechanic is by how they use their ears - the sharpest machinist I ever met would expertly listen to any engine, even taking a metal rod or long screwdriver and using it as a stethescope to probe and instantly pinpoint *any* issue underhood; it really was awesome to watch. To me, a watching a person thrashing underhood without using a well-trained ear is like watching a 'mechanic' who only owns 4 sockets - they're missing an essential automotive diagnostic tool.

 

The newest generation of auto enthusiasts has benefitted enormously from new-millenium powertrains which are better conceived, better engineered, better made, and so far advanced in reliability that going a quarter-million miles isn't unheard of; but the downside of all this advancement is that many haven't been introduced to or been trained to know the difference between a rod knock, lifter tick, piston slap, spring noise, pulley squeal, loud timing chain, etc. Back in the days of 'older iron' ALL motors made these sounds, many right off the assembly line :laugh: so pretty much everyone in the autoshop got wide exposure to the art of diagnosing all sorts of mechanical funkiness.

 

Now that these LS-based vehicles are "maturing" and starting to display these issues it's a good time to learn authoritatively how to tell one engine noise from another. THAT WAY the next time you call me or Dan or anyone else, or post on the forum you can say something more intelligible than '..hell I dunno, I just started it and it's making this hellacious clacking racket and 'pfffffttttt' sound!" :jester:

 

==========================

 

Automotive noise is categorized into one of these types:

* Engine Accessories (any belt-driven device)

* Upper-Engine (aka valvetrain)

* Lower-Engine (aka short-block or reciprocating)

* Transmission & Powertrain & Axles

* Suspension & Body (squeaks, chatter, banging, wind rush, rattling)

 

This thread will focus on ENGINE emitted noise, identifying common critical issues, and differentiating between upper and lower engine symptoms.

 

IS IT REALLY ENGINE NOISE?

 

Because cars are mostly metal they transmit (and re-transmit) noises everywhere; many times it is difficult to tell if any specific noise is coming from inside the car, or outside the car; or from the engine, the exhaust, the tires, the transmission, etc. Step #1 is to see if the suspect noise happens when the vehicle is parked; or, if the vehicle must be moving to experience the noise, see if shifting into neutral and coasting the vehicle will help you determine if the sound is coming from the engine or not. If you can still hear the suspect noise while crusing at speed in neutral with the engine off, then the noise is NOT coming from the engine! On the other hand, if you can cause the noise to happen *regardless* of car speed, or ideally when coasting at road speed if you can make the noise speed-up or slow down by reving the engine, you can be sure the noise is being generated from the engine accessories, engine itself, torque converter (or clutch), or (auto transmission) the transmission front pump.

 

IS IT ACCESSORY NOISE?

This is VERY easy to answer and your first diagnostic test - temporarily remove the belts from the front of the engine, and run the engine to see if the noise goes away. There is very little risk running the motor with the belt off as long as you do not operate the engine more than 60-seconds - that is long enough to verify the strange noise you hear isn't a bad pulley, bad water pump, failed power steering pulley, or worn-out A/C compressor; if the mystery noise goes away, you now know the culprit is somewhere in the belt-driven accessories.

 

I would NOT skip this step - a case I personally saw in my demolition derby car days, we had a 1969 Chrysler New Yorker with a 383, after getting battery and gas to it and starting the engine we were convinced from the sound it made that it had a failed rod bearing; a snap decision was made to run straight 50W oil in the motor and enter it in the derby anyways (!) and the next day after removing all the belt-driven accessories the knock went away! The actual culprit was a failed bearing in the power steering pump, it made a knocking sound VERY similar to rod knock, and it fooled ALL of us and we'd been around cars all our adult lives... So just because you hear a noise which you are "certain" is internal to the engine, don't jump to conclusions without first verifying the front acccessories!

 

Also, front accessories are VERY easily diagnosed with a "mechanic's stethescope", you leave the belt in place and probe each component listening for bearing defects. A mechanic's stethescope is available at any auto parts store for $10-15.

 

THE DIFFERENCE BETWEEN VALVETRAIN AND BOTTOM-END NOISE:

OK if you are sure the sound is actually internal to the engine, how do you tell if what you are hearing is valvetrain noise, which will bum your weekend but can be fixed, or if your motor is about to star on Faces Of Death? You have to make a judgement based on tempo (speed) of the knock, as well as timbre (pitch) and intensity (tapping or pounding).

 

Speed of knocking noise - the cam timing gear is TWICE the size of the crankshaft gear, because of this all valvetrain noise will repeat TWICE as often as bottom-end noise. So rule here is, if you hear a slow, rhythmic 'crisp' pounding then it's being generated from the bottom-end of the motor; OTOH if the sound is repeating quickly, it's most likely upper-engine valvetrain related (lifters or rocker arms). Mathematically, if the motor is idling at 650-rpm (stock) then a bottom-end bearing knock will repeat about every 1/10th of a second; valvetrain noise will 'clack' twice as fast, or about every 1/20th of a second.

 

BRING ON THE FACES OF DEATH!

I surfed through Youtube and found some very good examples of carnage.

 

Exhibit #1 - Textbook Rod Knock, this vid is required viewing: http://www.youtube.com/watch?v=8817P2ZJxXo The reason that I chose this link even though it's not an LS engine is because you can *clearly* hear the slow, rhythmic hammering of a failed connecting rod bearing. Also, when you crawl under the vehicle you can CLEARLY hear the hammering ringing the engine block & oil pan like a huge bell. Also the sound is very 'crisp' and it really does sound like a person inside with a 10lb sledge; when you are standing next to a motor with a rod knock you can feel the concussion of each blow the crank has to suffer. Here's annother textbook case -

; and another hurt import motor -
The lesson here is to concentrate on the slow rhythm of this engine failure (compared to the engine RPM).

 

Exhibit #2 - The aftermath of rod knock, this motor is a f'd unit but the owner has posted a great vid that demonstrates exactly WHAT is generating the actual 'bang' or 'knock' sound -

In addition to the gap between the connecting rod and crankshaft, the loud noise is also caused by the top (crown) of the piston physically smacking the bottom of the engine head (in more severe cases).

 

Rod knock in LS motors is much harder to 'hear' for a few reasons, first because the motor has a lot of aluminum parts, in the car applications the entire block is also aluminum so you will not hear the characteristic 'ringing' that iron blocks have. Also the oil pan is very thick aluminum, and this mutes a lot of lower engine noise. And the connecting rod clearances on the LS motors are far more precise than past engines and very tight, so engines just starting to exhibit a rod knock will still run and drive with a faint rod knock. Here is a great example of this, in this vid you cannot even hear the rod knock inside the car but I am sure that you would feel the knock in the floorboard and know something was very wrong - http://www.youtube.com/watch?v=gRSrUQudaP0. Also, notice that you hear the rod knock when the motor RPM is changing, and almost disappears when RPM is steady. BY THE WAY, when you rev a motor with a knocking or tapping sound, SLOWLY and GENTLY increase engine RPM during your diagnosis - popping the throttle as demonstrated in this vid is not necessary or called for and just makes you look like a dumbass about to kick the rod out the side of the block.

 

Here's a more advanced case -

On this car the rod knock sounds more like a 'clack' than a knock, but the slow rythm is there and if you listen very carefully not only do you hear each 'clack' but it masks a second sound that is definitely a KNOCK - like a background harmonic. Plus no valvetrain noise is capable of being that 'intense' you can really hear the concussion of the noise as the owner gets out of the car.

 

OK next rod knock example -

This is rod knock for sure, but what is important in this vid is that you don't really hear it when the motor revs UP, but as the motor RPM is returning to idle the knock is more pronounced - that is another rod knock trait, that it is always most pronounced as the RPM is falling/changing.

 

And another one - even Miss Cleo can predict a motor pull and teardown in this car's future (rod knock) - http://www.youtube.com/watch?v=plxNKkw7_Aw

 

WHAT DOES VALVETRAIN (LIFTER) TICK SOUND LIKE?

THIS is "lifter tick" -

The rythm of valvetrain noise is TWICE as fast as RPM (hence twice as fast as rod knock) and the intensity is far less, you can describe it as a 'tick' or 'clack' instead of a knock or hammering sound. Some old-timers will call it "the Singer Sewing Machine sound", if you've heard a 1940's Singer you know what they mean.

 

Lifter tick is a VERY common issue on LS-motors, many people are driving with it right now. Here's an example of a sick factory lifter - http://www.youtube.com/watch?v=TJYX2OfTRQY

 

Another great vid of lifter tick -

- in this case the lifter tick is not as lound or pronounced as the first two examples, and some guys I know would probably think nothing of the noise and drive the motor as-is. The dead giveaway is not only the speed of the ticking (twice as fast as engine RPM) but ALSO that when the RPM is revved UP you can still hear the ticking, in fact it gets lounder/pronounced as RPM changes UPWARD because this is when load is most increased on a lifter, and is exactly the opposite behavior of a rod knock.

 

Here is a case of many lifters clattering at the same time -

If you close your eyes and concentrate you can hear the rhythm of 3 (or more?) lifters.

 

This is a perfect case of FACTORY LIFTER TICK, chances are a lot of guys are driving an engine that sounds just like this -

The problem is more prominent in LS2 motors, Dan's TBSS *brand new* sounded just like this GTO, and on an educated hunch Dan changed the oil filter to a Mobile-1 unit made the noise GO AWAY. :thumbs:

 

ROCKER ARM NOISE EXAMPLE -

Here's an example of a single valvetrain issue making itself aware because you can hear a gap between the rocker arm tip and the top of the valvestem - the rocker arm is 'slapping' down onto the valve before opening it:

The problem with this is that there will be serious metal wear of the tip of the rocker arm, and a loss of performance because the valve is being opened late (loss of duration) as well as not getting all the lift out of your cam profile. The cause of this excessive play is usually a frozen/inoperative hydraulic lifter plunger. I call this a valvetrain 'clacking' noise, and you can definitely pinpoint it to the affected valvecover; in fact, with a mechanic's stethescope if you put the probe on each intake port runner and exhaust port (header) runner you WILL find the exact valve with the issue, the clacking is very easy to trace to its source.

 

AND NOW A WORD ABOUT 'PISTON SLAP'

According to GM, "piston slap" is normal in LS motors, and it aggrivates the hell out of some people. Piston slap is the noise made by cold pistons before they have become hot and expanded to their 'running' size; because pistons are made of aluminum they must have a very loose fit in the cylinders, during the first 60-seconds of running the pistons expand in the motor until they have achieved their final 'warm' running size - it is this plus a couple other reasons why you should not drive a vehicle until it has warmed-up for at least 40-60 seconds, or else the piston rings are not very well supported in the bores and you also get excessive blowby and cylinder wall & piston skirt scuffing until the pistons have expanded to size; this also happens with forged aluminum pistons, in fact with forged pistons the problem is worse because they have 3-4 times more cold clearance than factory cast pistons. Like the noisy LS lifter tick issue, a lot of LS-powered vehicle owners are stuck living with this 'non-issue', here's an excellent xample -

Notice that you can hear the motor getting quieter as the pistons expand to size; to really hear the difference, move the video slider to compare 0:30-35 seconds to 1:40+ and hear the difference - the faint hollow 'hammering' you hear at 0:30 and 0:45 is piston slap.

 

Mr. P. :)

Edited by Mr. P.

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hey the member that called was me :D scared the hell out of me that morning and wait a minute, it DID turn out to be a problem we need to fix. lol

 

great write up steve.

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One way that you know a great machinist from a jerk mechanic is by how they use their ears - the sharpest machinist I ever met would expertly listen to any engine, even taking a metal rod or long screwdriver and using it as a stethescope to probe and instantly pinpoint *any* issue underhood; it really was awesome to watch. To me, a watching a person thrashing underhood without using a well-trained ear is like watching a 'mechanic' who only owns 4 sockets - they're missing an essential automotive diagnostic tool.

 

example:

 

CRAFTSMAN 1/2 x 24-INCH SCREWDRIVER:

A very large pry bar that inexplicably has an

accurately machined

screwdriver tip on the end opposite the handle.

 

one of these great techs Mr. P is talking about not only has a stethoscope...but taught me that the tool stated above could be used as one in certain circumstances...not just a pry bar, punch, chisel or heaven forbid, a screwdriver.

Edited by WODY™

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Did you list the common gm engine rattle that you get when it shifts gears with very little engine load, or holds the gear with no engine load? I heard my friends 5.3L make the same noise. Seems to only go away once the engine is HOT. Even if the temp just got to normal op range itll still do it for a while.

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Did you list the common gm engine rattle that you get when it shifts gears with very little engine load, or holds the gear with no engine load? I heard my friends 5.3L make the same noise. Seems to only go away once the engine is HOT. Even if the temp just got to normal op range itll still do it for a while.

Can you share a link to a vid of the problem?

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first time I have seen this thread..wow where have i been.......Good write up Steve..Later I will sit down and watch clips

  • Upvote 1

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Can you share a link to a vid of the problem?

 

Ill see what I can manage to pull off. Being that I have to be moving it may be difficult. But leave it with me.

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You got a couple dead video links there btw,

 

First and Fifth video under "faces of death"

 

Second video under "WHAT DOES VALVETRAIN (LIFTER) TICK SOUND LIKE?"

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So after testing today, A video of this sound isnt gonna be too easy to make happen. I bet its quite common. Surprised no one else has mentioned the noise before.

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Speed of knocking noise - the cam timing gear is TWICE the size of the crankshaft gear, because of this all valvetrain noise will repeat TWICE as often as bottom-end noise. So rule here is, if you hear a slow, rhythmic 'crisp' pounding then it's being generated from the bottom-end of the motor; OTOH if the sound is repeating quickly, it's most likely upper-engine valvetrain related (lifters or rocker arms). Mathematically, if the motor is idling at 650-rpm (stock) then a bottom-end bearing knock will repeat about every 1/10th of a second; valvetrain noise will 'clack' twice as fast, or about every 1/20th of a second.

 

 

You have some very useful information here, and I agree with what you're trying to do. I disagree with some of your basis, however.

 

You are correct that the cam gear is TWICE the size of the crank gear, but that also means the camshaft revolves Half the speed of the crank.

Mathematically, this is a 4-stroke engine which means the intake valve for a given cylinder only opens every other crank revolution, but you don't have to take MY word for it.

http://en.wikipedia.org/wiki/Four_stroke_engine<br class="Apple-interchange-newline">

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