Understanding how a car works and identifying what it's telling you brings peace and confidence in driving old cars. As much as I love using all of the latest tools and diagnostic equipment, there's something very satisfying about listening to your engine with a stethoscope and nailing the problem. The purpose of this article is to set up an orderly approach, from the simplest to the worst-case-scenario, to solving many engine problems by process of elimination. Most of our old cars are non-computer, and this is the way the old-time mechanics did it, using their five senses: Sight, Smell, Hearing, Taste and Touch, as well as a sixth--Common Sense! It is important to focus on the main problem, but to be aware of everything around it at the same time.
I remember one time we were working on our '46 Packard 356 Straight 8. The car had been in storage for several years, and everything needed servicing. One man was under the hood listening to the squeal of the water pump. I was behind the wheel, and had to re-start the engine several times when finally the starter hung up. The Bendix didn't kick out, and before I could get out of the car, the starter had caught fire! The guy under the hood was still listening to the water pump squeal, oblivious to the fire. Using the battery quick disconnect, I was able to arrest the problem, only losing a $600 starter and not the entire engine, car and garage! You have to use all of your senses and be aware of your entire work area.
When we are scoping out a noise, we consider the whole engine, but divide it into zones: Zone 1-HEAD; Zone 2-BLOCK; Zone 3-CRANK CASE; Zone 4-FRONT; and Zone 5-REAR. Then, using a stethoscope, listen to each zone to help narrow down the problem. Once, while working on our ' 49 Buick 320 Sedanet, we heard a light rapping coming from the head area. We had just done a valve job, and knew everything was right, but what was that noise? We moved the stethoscope around, stopping on a loose bolt that held the spark plug cover plate on. Once it was tightened down, the noise was gone. It had sounded like a rod bearing or loose valve. The noise even increased when the engine was revved, but it was just a loose bolt! Sometimes you get a break!
After setting up our zones, we make a list of what the possibilities could be. For example, General Problems--Carbon, Detonation, Spark Knock, etc.
HEAD: Bad valve guides, sticking valves, out-of-adjustment valves, broken or weak valve springs, ridge worn on valve seat or work push rods, excessive camshaft end play or worn bearings. With the modern oils, and the elimination of ZDDP, camshaft lobes are beginning to show premature failure, especially in the high-revving engines.
BLOCK: Worn bores, loose pistons or rings, rings stuck, a ridge in the cylinder, piston pin loose (this one can be hard to identify--we had one that sounded like a valve out of adjustment).
CRANK CASE: Crankshaft main bearings loose, excessive end play, shaft out of alignment, bad connecting rods or bearings, bent rod.
FRONT of the engine: There could be worn gears or chain, end play, broken bolt in the harmonic balancer (we had this on our ' 54 Cadillac 331 and it could be heard through the radiator, sounding like a bad rod bearing), cracked harmonic balancer, fan belt or fan, also check generator and water pump.
REAR of engine: Crankshaft end play, flywheel, or starter. To begin testing, if it' s at all possible, have it timed and firing on all cylinders, and warmed up to operating temperature. It is also helpful to try different RPM' s to see if the noise changes.
LET'S START TESTING! Detonation which is commonly known as fuel spark or compression knock, is really incomplete combustion. The sound is a sharp knock with a metallic ping, heard especially when the engine is under a heavy pull, or accelerating in high gear. The sound of the knock will change when carbon builds up, when mixture is too rich, when the timing is off, or if the fuel is too low in octane for the engine's high compression. It can also result from a lean carburetor adjustment.
Carbon makes an erratic knock that shows up on a hot engine, and is called pre-ignition. The sound becomes more pronounced as the spark is advanced. It also changes with carburetor adjustment. It is similar in sound to Detonation and is loudest under heavy engine pull or acceleration. The difference comes when retarding the spark changes the tone and may make it go away altogether. The engine will always lose power.
Spark Knock also produces a pinging sound. It can be caused by a faulty vacuum advance, an overheated engine, faulty ignition wiring short-circuiting, or too hot of spark plugs.
Cylinder wall problems can make a ' hissing' sound in the crank case on the compression stroke. This also can be evident by a loss of power, and crank case breather smoke. It almost always indicates scored cylinder walls when new plugs are put in an engine where the cylinders have not been reconditioned and have ridges, and new rings. Besides making the hissing noise, which results from loss of compression, the new rings might break on the ridges during ring travel.
Cylinder Head: Water in the oil pan can be the result of a cracked of loose cylinder head. It can also be caused by a bad head gasket. As the compression is lost, it produces a hissing sound, along with loss of power. Many times the coolant can be seen seeping out around the head gasket.
Main Bearing Knock : You can hear and feel this one. It's a dull, heavy knock that can be felt with your foot on the accelerator, always louder under a load. When the bearings are about to go, it can be felt and heard at an idle, but generally is most dominant at 25-30 mph. The knock is usually periodic, because of unequal wear on the main bearing. A single knock can indicate a single bad bearing, and might show up at a certain speed, go away, then show up again at a higher speed, producing a deeper tone. Know how many main bearings your engine has. Loose main bearing knocks, especially the center one, can be diagnosed by the way it sounds and the tone it produces. When all mains are loose, the tone of the knock will remain the same, but the timing of the knock may vary, depending on the number of main bearings, as the number of knocks per revolution depends on this number. To isolate the bad bearings, shorting out the cylinders adjacent to the bad bearing will quiet the noise.
Crankshaft End Play: Excessive end play allows the crankshaft to float endwise, creating an irregular knock. The sound is similar to a loose main bearing. This does vary with different engines, and you can not short this noise out, but you can make it disappear by forcing the crankshaft endwise. Also, the intensity of the knock is only slightly affected by load. When building an engine, always check your manual to correctly set end play. An old mechanic friend of mine told me that many Buick Dynaflow transmissions' front pumps were pounded out due to excessive end play on the 322' s. It' s crucial!
Flywheel: This produces a knock with a slight metallic ring. It is loudest when the engine is revved, and seems to move through the engine, and can easily be confused with a bad rod bearing. It can vary in sound from a thump with vibration, to a continuous rattle.
Rod Bearing: A burnt bearing can be heard at all speeds, but is most prevalent at around 25 mph. Sounds range from a light pound to a series of short, sharp pops. It is not always heard at an idle, but is heard best with no load on the engine at part throttle. On this, you can short out the cylinder, and it can be heard on deceleration after revving the engine. It is lighter in tone in comparison with a main bearing knock.
Connecting Rod: Noises usually result from being bent, twisted, or not correctly lined up with the crankshaft and cylinder bore. It is usually heard at idling, or around 10-15 mph, and sounds similar to piston slap, with a slightly sharper tone. The tone can be somewhat diminished by shorting out, but the sound will not be eliminated. If left alone, it will eventually cause the rod bearing to wear, producing a rod bearing knock.
Piston: A worn or loose piston knock will usually be heard at an idle, on acceleration, and a higher speeds. This is called piston slap,and can be heard best using a stethoscope at idle. Of course, the degree of looseness affects the sound it produces. Generally, the knock is a hollow, bell-like sound, sometimes reaching a severe clanking sound when accelerating. At low engine speeds, it can click like a valve tap. With the engine warm, it can be checked by shorting out the cylinder. If the knock is from piston slap, shorting out will usually cause the knock to entirely disappear. Another test is to put a heavy oil in the cylinder, filling the clearance space. This usually will stop the slap for a few minutes. In a straight eight engine, the noise is loudest when the engine is cold, and lessens as the engine warms, although it can still be heard. If the piston is broken or cracked, the knock is present at all speeds and is loudest at idle and low speeds. At high speeds, it produces a heavy pounding.
Piston Pin: This sound is not limited to, but is best heard at idling. By retarding the spark, you can make it less audible. This is a dull, metallic sound, although high pitched, and not very loud. The intensity is less under load, and under quick acceleration. Shorting the cylinder will confine this knock to the upper part of the cylinder. If compression is relieved, the knock will usually go away. If the piston pin is at fault, the knock will always synchronize with the power impulse stroke of the piston.
Piston Ring: The cause of ring noise may vary from being too loose in the grooves, to broken rings, to hitting a ridge in the cylinder. A tone at regular intervals with the cylinder shorted, is usually a light, rapping sound or a clicking, or a sharp rattle. The noise created by a broken ring depends on the location of the break, and the amount of looseness in the groove. A loose fit with regular piston movement would produce a distinct rattling sound. A broken ring permits excessive blow-by and this can be checked by placing a rubber hose at the end of the breather and listening at the end of the hose.
Front End Noise: On chain-driven cars, there can be several factors causing the noise; 1. The chain might be improperly adjusted for tension; 2. The chain might be worn; 3. Sprocket teeth might be worn; 4. Sprockets might be loose or out of alignment. The chain must also be getting correct lubrication. Front end noise is usually a rumbling sound, due to too much slack in the chain. It is mostly heard at idling, or under acceleration. When the chain is too tight, you will hear a humming sound. To correctly adjust the chain tension, tighten it until you hear the humming sound, then back it off until it disappears. As the chain wears on engines with limited clearance, it may strike the housing, making a clanking sound at regular intervals.
Camshaft: Camshaft noise may be caused by excessive bearing clearance or end play. A loose shaft will produce a dull thump, sharper than a main bearing knock. A loose front bearing might produce a noise in the front end drive on straight eight engines. A loose cam bearing can be located by listening to determine the approximate location and applying downward pressure on the push rods riding the cams on each side of the suspect bearing. Usually when one bearing is loose, the push rods riding the cam on either side of the bearing will also be noisy. Depending on the type of engine you have, the tone of the noise may vary, but generally it is a sharp rap that usually occurs one or more times during a revolution of the camshaft.
Push Rods: Push Rod noise is usually accompanied by loss of engine power, and is caused by too much clearance between the valve stem and the push rod. With the roller type, it may also be caused by a worn roller or pin. It is a taping or sharp metallic knock, becoming a rap when the parts are worn. The noise is heard at regular intervals at camshaft speed. Excessive clearance can be temporarily taken up using a feeler gauge held between the valve stem and push rod. If clearance is decreased by a noisy valve, it will result in a popping in the carburetor. A valve with too much clearance will quiet upon taking up the excessive clearance.
Valves:Valve noise may come from several factors. Bent valve stems can produce a drum-like knock, sometimes accompanied by a pop, causing the valve to stick in its guide. This hollow, drum-like knock with popping in the carburetor is most noticeable under acceleration, but can also be heard at an idle. This can also be the result of a late-closing valve. Worn guides usually cause clicks and rattles, plus carburetor popping. Shorting out will not eliminate this noise, but will diminish it. Valve noise is generally heard at all speeds, especially under load when the engine is accelerating. A weak or broken valve spring can also produce a light knocking sound at idle and up to around 40 mph. Engine performance will be irregular, and the engine will usually backfire, especially at higher speeds. A broken valve spring can easily be seen upon inspection. To check for spring fatigue, insert a screw driver between the spring coils to increase tension and see if the sound changes. A jingling noise is associated with a broken inner spring.
I hope this helps in a quick check of internal engine noises. Watch for further articles on external noises (generator, fuel pump, etc.). During tough times, it is more important than ever to try and keep it simple! Enjoy your cars and support our vendors. Keep ' em driving!