The Doctor and Dabney Track Down Another Noise
Dabney drove his 1948 Pontiac station wagon over to the Doctor's house one morning. He was in the neighborhood, after all, and figured that the Doctor might be able to cast an opinion on the noise Dabney was hearing in the engine. Dabney knew the Doctor could always be counted on for an opinion...
The old Pontiac engine had a noise. It was a dull, heavy metallic knock that increased in frequency as the speed of the engine increased, and it got more noticeable when the engine was under load. It wasn't making any noise when the Doctor came out of the house.
"What's up?" asked the Doctor.
"The engine is making a noise that I can't pin down, Doc," said Dabney. "I think something's beginning to fail inside — maybe a lifter — but it only makes noise in certain circumstances. Let's take a drive if you have a few minutes."
"It seems the Pontiac's engine makes noise at idle, under load at mid ranges and particularly when the clutch is let out. I'm about ready to open up the top of the engine and check the valve clearances," said Dabney
"There's no reason to do that now," said the Doctor. "First, let's take a ride and pin down the exact circumstances of the noise, then do some checks with the stethoscope. Then we'll decide whether to open the engine or not. No sense in doing the work if we don't have to..."
They drove off, chatting about the weather and the old Pontiac's history. They noted all
the circumstances surrounding the noise and verified that it only took place as described.
"Well," said the Doctor. "That tears it. Let's give a listen with the stethoscope."
Dabney pulled the Pontiac into the garage and they started listening to the engine noise through the mechanic's stethoscope, moving the tip from point to point over the engine's surface. The noises were definitely coming from the bottom of the block.
"Well, Doc, it appears the sound isn't coming from the top of the engine. Is there any way the valves could still be the problem?" asked Dabney
"There's always a slight chance of harmonics being generated somewhere in an engine and the sounds coming from somewhere else," said the Doctor, "but in this case I think we've got a bottom-end problem. You see, the key issue here is that the noise gets worse when the clutch is let out."
"Why is that so important," asked Dabney.
"Because, my boy, only one thing can cause the noise to get worse when the clutch is released," replied the Doctor. "Let me explain a little about engine noises."
With that, Dabney poured himself a cup of coffee, knowing there would be a lecture.
"Engine noises generally come from just a few sources," said the Doctor. "There's oil failure, where bearings lose their protective film of lubrication. In that case there will initially be loud banging as the rods hit the crank journals on the power stroke, followed by screeching sounds as the bearing material heats up and grinds off, and eventually followed by silence as the engine freezes up and is destroyed," grinned the Doctor.
"Then you have higher-pitched noises that come from too great a mechanical clearance, such as the valves, lifters, pushrods and rocker arms. Piston ring failure and piston slapping are also noises that have particular frequencies that are easily detectable by the stethoscope," he continued.
"Well, Doc," said Dabney, "the sounds we're hearing are coming from the crankshaft - according to your description. But how come the bearings aren't being chewed up if oil failure is the cause? We've driven the car over 50 miles now."
"Because, dear Boy," said the Doctor. "The oil pressure hasn't failed. Remember, I told you that the key here is the release of the clutch. You see, the clanking noise that comes from oil failure can also be the noise that comes from too great a thrust clearance of the crankshaft. As the crankshaft moves from front to rear (or reverse) it clanks against the engine block. Releasing the clutch makes the crankshaft move, of course, and that's when we're getting the worst noise."
"That's a bit of great diagnosis, Doc," said Dabney. "It fits the circumstances perfectly and it also means all we have to do is take off the oil pan and insert the correct thickness thrust bearing on the crankshaft."
"That's almost correct, Dabney," said the Doctor.
"Why is it ALMOST," asked Dabney. "Isn't that all we have to do?"
"It's almost correct because of the word "we," replied the Doctor, as he walked off toward home...