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How to find noises and causes in your Corvette engine

What’s That Noise?

Jim Smart May 25, 2018
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Engine noise makes us more nervous than any other automotive sound because noise can mean expense and inconvenience. And, sometimes engine noise is nothing more than a normal dynamic of the engine’s design. We often make assumptions about engine noise based on what we’re told and who we talk to instead of taking our troubleshooting step-by-step until the source of the noise is discovered and corrected. What’s more, noise doesn’t always need to be corrected.

When you evaluate engine sound, remember there are two basic sources of engine noise because you have two different motion events going on at the same time: camshaft speed and crankshaft speed, which are different. The crankshaft turns at twice the speed of the camshaft. Crankshaft noise is typically a whizzing or rapid clicking sound where the camshaft is more of a tapping or clicking at a slower tempo.

Where engine sound becomes confusing is rod journal noise, which moves at the same speed of the crankshaft yet the sound is in rhythm with the camshaft and valvetrain. Rod bearing noise is in rhythm with the valvetrain because rod journal noise happens on the power stroke only, and only at the cylinder affected. The same can be said for pistons and pins, which will knock or click at the same clip as rod journals.

Timing and valvetrain components are another source of engine noise and often easier to correct than those dreaded sounds from down under. Because Chevrolet has chosen to stay largely away from overhead cam technology in its V-8 engines, you’re less likely to be plagued with valvetrain noise with the exceptions being a collapsed lifter, bent pushrod, busted rocker arm, or fractured valvespring. Valvespring keepers and locks can fail, too, causing their share of headaches and engine failure. Typically, valvetrain failure and engine damage occur at high rpm.

There are dozens of other noise sources, too. Vacuum leaks can cause a whistling or hissing and poor performance. Most of the time you can’t even hear a vacuum leak, yet the engine will run poorly and with a faster-than-normal idle you can’t get under control with the idle speed adjustment screw. Electronically controlled engines will idle at a higher speed with a vacuum leak, which cannot be adjusted out. Idle speed on these computer-controlled engines is adjusted via an idle-air-controlled solenoid, which can stick and keep the engine at a faster idle and make a lot of noise.

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Fuel pumps and eccentrics can make a noise in rhythm with the valvetrain, which can sound like a knock or click. You can correct this problem with a new fuel pump or by addressing camshaft eccentric wear.

Harmonic dampers get noisy when the rubber ring becomes dry-rotted and the outer ring becomes loose on the hub causing a rattle in rhythm with crank rotation. When dry-rot becomes really bad, the outer ring can leave the damper and go right through your hood. The harmonic damper is the crankshaft’s shock absorber designed to damp crank twist as eight pistons hammer away on it with a consistent beat. The harmonic damper minimizes the risk of crank breakage.

Crankshaft counterweights have been known to contact the block, oil pan baffles and oil pump pickup causing an obnoxious knock in rhythm with the crankshaft and connecting rod journals.

Your engine’s front dress can also be a source of noise from the generator or alternator, power steering pump, water pump, air-conditioning compressor or smog pump. Generator/alternator bearings wear out and create a hiss or groan. The same can be said for power steering pumps, which whine when they’re low on fluid (cavitation) or worn out. Power steering pumps tend to whine under a load when you apply steering input. Water pump bearings will hiss or squeal and typically cause leakage from the weep hole because the seal can also fail at the same time.

Air-conditioning compressors get noisy and rattle when the compressor clutch or cam plate inside become excessively worn. The quickest way to find compressor noise is to cycle the switch on and off to see when there’s noise. Then, examine the compressor for evidence of lost refrigeration oil and refrigerant. If it is damp with oil you are losing both.

There’s also flywheel, flexplate, clutch and torque converter noise that happen in rhythm with the crankshaft. This sounds more like a rattle whether it’s the clutch or torque converter. Troubled torque converters will set up a vibration as well as noise.

Establishing and solving engine noise is a matter of finding the source and figuring out how to solve it. The key to solving the problem is not to panic or guess. Know where the noise is coming from and zero in on a solution. Find the fault and then find the fix. Vette

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1. When you have a rod knock, the news is rarely good. To establish rod bearing noise or knock, pull one spark plug lead at a time with the engine at idle and see if the knock goes away. If the knock goes silent, you have piston/rod bearing issues on that particular bore.

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2. Rod bearing wear or improper journal machining can cause rod knock. If you’re befuddled by a knock in a fresh engine, keep in mind that mistakes happen. Never assume because an engine is new there aren’t mistakes. Main bearing journal noise will tend to be a squeal or a hiss, especially if you have a spun bearing that has gone dry.

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3. Piston noise is more common with cold engines with forged pistons. Expect to hear a mild rap or rattle on one or more cylinders until the engine reaches operating temperature. If you hear piston slap after the engine gets hot, it indicates excessive piston-to-cylinder wall clearances or a damaged piston skirt. Again, pull one spark plug wire at a time and see which bore goes quiet.

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4. Piston noise can also come from tight deck height clearances where the piston is a pinch out of the bore and the head gasket thickness isn’t enough. You also need to ascertain valve-to-piston clearances hot. You want at least 0.080-inch (intake valve) hot and 0.100-inch (exhaust valve) hot.

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5. Precision piston-to-cylinder-wall clearances will yield a quieter engine. The cylinders must be match-honed to each piston to get this spot on. Not all engine builders follow this regiment, which can result in piston noise because no two pistons are the same exact size. Forged pistons call for more generous clearances due to a greater expansion rate. You don’t want excessive clearances with cast or hypereutectic pistons, which yield a less aggressive expansion rate.

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6. Crankshaft endplay can cause engine noise centered mostly around the rod journal-to-rod relationship and only were it excessive. Acceptable crank endplay is from 0.002-0.006-inch. You don’t want any more than 0.0010-inch. Crankshaft endplay-related noise would be more common with a manual transmission as the clutch is engaged and released.

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7. Crankshaft thrust bearing thickness affects endplay. If you have excessive crankshaft endplay in excess of 0.010-inch, you could get noise and may have to look to another crank or a machine shop that can weld up your flange and machine it to size. Some oversize thrust bearings are available from Summit Racing Equipment, but very few for undersized cranks.

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8. These excessively worn and scored rod bearings generated quite a rattle in a small-block Chevy, not to mention low oil pressure.

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9. Harmonic dampers will create a rapid-fire rattle when the outer ring becomes disconnected from the hub and rubber ring. This, of course, depends on the design. Examine the rubber’s condition and when in doubt, replace the damper.

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10. Engine noise sometimes isn’t engine noise at all. Because the torque converter rotates with the engine’s crankshaft, any malfunction in the torque converter can be perceived as engine noise. Lie underneath the vehicle with the engine at idle and place your hand on the bellhousing. Listen at the bell with a mechanic’s stethoscope, which you can get from Harbor Freight. Do you hear the noise? And do you feel the noise?

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11. Torque converter noise is challenging to diagnose because it comes from a maze of components inside. Noise typically comes from the sprag or perhaps the clutch (if a locking torque converter). Another source of noise is the automatic transmission’s front pump, which can whine.

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12. Clutches are another source of perceived “engine” noise when it is not the engine at all. A defective pressure plate, clutch disc or release bearing can make a lot of noise. Again, check the bellhousing as a potential source for the noise, especially if clutch pedal movement changes the noise.

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13. This is a hydraulic clutch release slave cylinder and release bearing, which can also be a source of noise in rhythm with the crankshaft. Gently work the clutch pedal and listen for changes in the noise. A defective transmission input shaft bearing will make a lot of noise in rhythm with the crankshaft with the clutch engaged.

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14. Bad water pump bearings typically make a squeal or a hissing sound because the water pump turns at roughly the same speed as the crankshaft, depending upon application. By the time a water pump bearing starts making noise, the seal will also be shot.

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15. Serpentine belt drive accessory noise can be like looking for a needle in a haystack because most of the accessories turn at the same speed as the crank. The alternator is the exception because it sports a smaller pulley and spins much faster than the crank. When you have noise in the front accessory drive, zero in on each accessory with a mechanic’s stethoscope.

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16. When a power steering pump is in trouble, it will whine, especially when you run the steering wheel from lock to lock. Pump whine can be an indication of low fluid (cavitation) or a defective pressure regulator.

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17. Fuel pump noise will always be in rhythm with the camshaft, which works hand-in-hand with the valvetrain. Fuel pump noise will be in unison with the rocker arms.

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18. Camshaft noise of any kind will be at half the speed of the crankshaft; a clicking or tapping instead of a whizzing sound like you’d hear from the crank.

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19. Lifter noise is easy to spot because it happens in rhythm with the camshaft and rocker arms. In fact, a defective hydraulic lifter can collapse causing excessive valve lash, which is easy to spot. When you have established which bank the noise is coming from, remove the valve cover and check valve lash. A loose rocker indicates a collapsed lifter or damaged fulcrum. Another issue can be a failed lifter roller and pin, which can tear up the lifter bore.

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20. Roller rocker arms are well known for noise and that’s the nature of the design. Roller tappets and rocker arms give an engine a mechanical lifter sound, which is normal and nothing to worry about. Extraordinarily loud rocker arms indicate improper valve adjustment.

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21. You’d be surprised how many enthusiasts have had spark plugs pop out of cylinder heads, especially on late-model engines with aluminum heads. When a spark plug pops out of a cylinder head it sounds like a loud knock when you’re behind the wheel.

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22. Starter noise can be unsettling, especially when you’ve released the key and the engine is running. A failed starter drive can hang-up in the flywheel or flexplate and make a disgusting racket. Solenoids can stick in the closed position and keep the starter from operating. This is a sound easily mistaken as “engine” noise.

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23. Exhaust header gasket leakage at the header or manifold gasket will sound like a clicking or popping sound. This sound is often mistaken for a blown head gasket. Look for carbon staining around the header flange at the cylinder head.

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24. Header collector flanges are notorious for leakage due to blown gaskets. Expect to hear a popping noise at the collector. Exhaust leaks beyond the header or exhaust manifold make the engine sound louder inside the cabin yield a buzzy sound at the leak

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25. Vacuum leaks cause a buzzing, whistling or hissing sound combined with poor performance due to a lean condition. Leaking carburetor base gaskets, vacuum hoses and canisters and intake manifold gaskets will buzz and whistle. Look at the damage to this carburetor base gasket. Spray carburetor or brake cleaner where the leak is suspected and watch what the engine does. If it runs rough, you’ve found the noise and the leak.

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26. A popular old misconception has been “your valves are rattling” or “my valves are knocking,” when they rattle under acceleration; it’s actually spark knock, or pre-ignition. Spark knock on start-up or under acceleration is caused by pre-ignition or lean mixture. If you own a late-model Corvette, it isn’t as simple as moving the distributor or swapping jets. You need a professional tune. Ray McClelland of Full Throttle Kustomz custom tunes, improving performance and eliminating spark knock.

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27. Older engines with distributor ignition systems have to be curved the same basic way McClelland does on late-model Corvettes. When there’s a distributor and carburetor, he does it by curving the distributor and re-jetting the carburetor on a dyno. Custom tuning gets rid of spark knock.

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28. Distributors should be curved on a distributor machine to get a baseline, then tuned again in the engine to eliminate spark knock. Curving is performed by adjusting the mechanical advance as well as vacuum advance. The vacuum advance gets us started, handing off to mechanical advance as rpms increase.

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29. Another oft-mistaken noise is the electric fuel, which makes a buzzing or humming sound that can radiate throughout the vehicle. If your pump is loud enough to hear over the engine and stereo, it should be pulled and inspected.

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30. If you don’t have a mechanic’s stethoscope, you can check noises with a long-handle screwdriver or a piece of electrical conduit. Put your ear to the screwdriver or conduit and put the other end to the engine where the noise is. You will be able to hear everything. And remember, each region of the engine has its own unique sound.

Photography by Jim Smart

Sources

Summit Racing
Akron, OH
800-230-3030
http://www.summitracing.com
Harbor Freight Tools
800-423-2567
www.HarborFreight.com
Full Throttle Kustomz
www.ftkustomz.com

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