An Engine Noise Mystery Quiz
August 12th, 2012, 07:50 PM
#1
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An Engine Noise Mystery Quiz
Okay, here it is, a new game!
It goes like this: I describe a strange problem as best I can, and you try to come up with the cause. In this case, I already know the cause, but I'm interested in seeing how hard it is to guess what it is.
If this proves popular, then I'm sure others can add interesting problems they've had to the challenge.
So, nearly two years ago, for reasons known only to the Heavens, I bought a 1973 Delta 88 convertible with a 1978 260 motor and a TH400 transmission from one of the local flea market characters (our local flea market is a community event, and those of us who buy, and occasionally sell, there know each other fairly well). The car was running, registered, and on the road as a daily driver, and the price was right.
As soon as I got it home, I noticed this noise. It was a sort of staccato, irregular, crunching, dull, slightly metallic sound which occurred as the RPMs built when I floored it (not immediately upon pressing the pedal), and which I could sometimes also produce by revving it at idle.
I listened to it carefully, and could not for the life of me figure out what it was.
When driving, it sounded like it was coming from inside the middle of the engine, but also like it was coming from the transmission or under the car.
With the hood up, it never sounded like it was coming from the side you were on, and always like it was coming from the other side, or from the middle, and maybe toward the back.
The character of the sound was a little bit like a rod knock, but not quite as dull, and not as regular.
It did not appear to affect the engine's function at all - power was smooth (but not much - it's a 260), idle was smooth, everything else sounded good.
I checked a number of things, but could find nothing wrong, so, since the engine seemed to be running well, and since it was a 260, which I planned to replace anyway, I figured that the worst case scenario was a rod knock, and that if the rod went, that would mean it was time for a new motor.
It didn't sound like a rod knock, though.
Fast forward nearly two years.
I took the car on a 350 mile trip in 100° weather, had the two-core radiator first prove inadequate, and then spring a leak, necessitating a drive for a couple of miles badly overheated before I could install my spare radiator (yes, I was a Boy Scout
).
When I arrived at my destination, the knock seemed louder than before.
It also now occurred at low RPMs, especially when the idle was rough because I had let the choke off too soon. It seemed to have a random character that matched the roughness of the idle.
Performance appeared unaffected.
A minor diagnostic test was performed which I felt reduced the possibility of a rod knock to a fairly low level.
At this point, several hundred miles from home in an old car making a bad noise, I called an old buddy who has owned over a hundred cars, each one more of a rat than the last, and asked his opinion.
He nailed the problem immediately (said he'd had two cars with the same thing), and further examination without need for disassembly proved him right.
I have not included every step of troubleshooting I performed, so as not to lead anyone to the answer, but I will answer any questions posed. The information I have already provided was enough for my buddy to figure it out.
When it seems as though discussion of this quiz question has been completed, I will provide the answer in the form of a photograph.
Let the games begin!
- Eric
It goes like this: I describe a strange problem as best I can, and you try to come up with the cause. In this case, I already know the cause, but I'm interested in seeing how hard it is to guess what it is.
If this proves popular, then I'm sure others can add interesting problems they've had to the challenge.
So, nearly two years ago, for reasons known only to the Heavens, I bought a 1973 Delta 88 convertible with a 1978 260 motor and a TH400 transmission from one of the local flea market characters (our local flea market is a community event, and those of us who buy, and occasionally sell, there know each other fairly well). The car was running, registered, and on the road as a daily driver, and the price was right.
As soon as I got it home, I noticed this noise. It was a sort of staccato, irregular, crunching, dull, slightly metallic sound which occurred as the RPMs built when I floored it (not immediately upon pressing the pedal), and which I could sometimes also produce by revving it at idle.
I listened to it carefully, and could not for the life of me figure out what it was.
When driving, it sounded like it was coming from inside the middle of the engine, but also like it was coming from the transmission or under the car.
With the hood up, it never sounded like it was coming from the side you were on, and always like it was coming from the other side, or from the middle, and maybe toward the back.
The character of the sound was a little bit like a rod knock, but not quite as dull, and not as regular.
It did not appear to affect the engine's function at all - power was smooth (but not much - it's a 260), idle was smooth, everything else sounded good.
I checked a number of things, but could find nothing wrong, so, since the engine seemed to be running well, and since it was a 260, which I planned to replace anyway, I figured that the worst case scenario was a rod knock, and that if the rod went, that would mean it was time for a new motor.
It didn't sound like a rod knock, though.
Fast forward nearly two years.
I took the car on a 350 mile trip in 100° weather, had the two-core radiator first prove inadequate, and then spring a leak, necessitating a drive for a couple of miles badly overheated before I could install my spare radiator (yes, I was a Boy Scout
). When I arrived at my destination, the knock seemed louder than before.
It also now occurred at low RPMs, especially when the idle was rough because I had let the choke off too soon. It seemed to have a random character that matched the roughness of the idle.
Performance appeared unaffected.
A minor diagnostic test was performed which I felt reduced the possibility of a rod knock to a fairly low level.
At this point, several hundred miles from home in an old car making a bad noise, I called an old buddy who has owned over a hundred cars, each one more of a rat than the last, and asked his opinion.
He nailed the problem immediately (said he'd had two cars with the same thing), and further examination without need for disassembly proved him right.
I have not included every step of troubleshooting I performed, so as not to lead anyone to the answer, but I will answer any questions posed. The information I have already provided was enough for my buddy to figure it out.
When it seems as though discussion of this quiz question has been completed, I will provide the answer in the form of a photograph.
Let the games begin!
- Eric
August 12th, 2012, 08:06 PM
#3
same but different
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Location: Central Missouri
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Perhaps a tool or device used in hope of a quick road side repair had finally found its way down to the thing that goes round and round. But only after overheating and of course a dose of murphys law. 
Happens all the time.
Well...thats my guess.
Happens all the time. Well...thats my guess.
August 12th, 2012, 09:06 PM
#12
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August 12th, 2012, 09:14 PM
#17
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I wanna say it had a cracked flexplate...but then why would overheating make that worse? IDK...still thinking about it.
I'm not familiar with the 260 so I'm kinda lost here. Could it have been a converter bolt hitting a freeze plug on the back of the engine?
I'm not familiar with the 260 so I'm kinda lost here. Could it have been a converter bolt hitting a freeze plug on the back of the engine?
August 12th, 2012, 10:30 PM
#18
alternator maybe waterpump. My dad's alternator on his 77 poncho started to sound really bad at first i feared it was a rod knock once i pulled the belt off it went away. I know you said center of engine maybe towards back but engine noises could be misleading . I have had cheap waterpumps make some gnarly noises when the bearings failed. That's all i could think of . i just also thought maybe a bad clutch fan. if it got worse with over heating im guessing when it would engage it would not perform properly making noises.
Last edited by coppercutlass; August 12th, 2012 at 10:38 PM.
August 13th, 2012, 04:24 AM
#20
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Nope. Doesn't have one.
Nope. Water pump started squirting 2 days after I got the car, replaced it, checked alternator and P/S bearings, all fine. Actually changed P/S pump later on, too, because of pulley alignment (and re-checked the alternator and water pump bearings then).
The fan clutch was fine and the fan was not banging into the shroud.
Nope. P/S pump fine, core support and shroud on tight.
I had also checked (and re-checked) the exhaust system and the A/T cooling lines for banging / vibration.
Nope. The sound would have been too regular.
I had checked the tightness of all the converter bolts, though, and they were tight.
DING DING DING DING!!
Steve gets it, less than four hours into the contest!
This was easy to diagnose once I got the idea, proving the old adage that if you're not thinking about it, you won't find it.
All I had to do was jack up the car and crawl under with a flashlight.
I firmly grabbed the converter and turned the engine with it from side to side. I could feel about 1/16" of easy play before turning the converter made the engine start turning. Also, when looking up with the light at just the right angle, I could see hairline cracks like these from every hole in the flexplate going in toward the center.
Repair was straightforward:
Raise car, remove driveshaft, move transmission and crossmember back a few inches and support front of trans and back of engine, remove and replace flexplate, reassemble.
No more noise, and a mystery solved!
Thanks for playing!
Any other mysteries for us to solve out there?
- Eric
ps: No. Steve, I have no idea why overheating made it worse.
My suspicion is that it didn't, but that 6 hours at 70-75 MPH did, and the two things just happened at the same time.
And, yes, I had been driving the car 70 miles round trip to work almost every day for 2 weeks before the trip to be sure all the bugs were out of it, and checking the coolant level every day, and it never went down, so the radiator hole was blown out during the trip.
It's good to be prepared, though .
alternator maybe waterpump... I know you said center of engine maybe towards back but engine noises could be misleading. I have had cheap waterpumps make some gnarly noises when the bearings failed... i just also thought maybe a bad clutch fan. if it got worse with over heating im guessing when it would engage it would not perform properly making noises.
The fan clutch was fine and the fan was not banging into the shroud.
I had also checked (and re-checked) the exhaust system and the A/T cooling lines for banging / vibration.
I had checked the tightness of all the converter bolts, though, and they were tight.
Steve gets it, less than four hours into the contest!
This was easy to diagnose once I got the idea, proving the old adage that if you're not thinking about it, you won't find it.
All I had to do was jack up the car and crawl under with a flashlight.
I firmly grabbed the converter and turned the engine with it from side to side. I could feel about 1/16" of easy play before turning the converter made the engine start turning. Also, when looking up with the light at just the right angle, I could see hairline cracks like these from every hole in the flexplate going in toward the center.
Repair was straightforward:
Raise car, remove driveshaft, move transmission and crossmember back a few inches and support front of trans and back of engine, remove and replace flexplate, reassemble.
No more noise, and a mystery solved!
Thanks for playing!
Any other mysteries for us to solve out there?
- Eric
ps: No. Steve, I have no idea why overheating made it worse.
My suspicion is that it didn't, but that 6 hours at 70-75 MPH did, and the two things just happened at the same time.
And, yes, I had been driving the car 70 miles round trip to work almost every day for 2 weeks before the trip to be sure all the bugs were out of it, and checking the coolant level every day, and it never went down, so the radiator hole was blown out during the trip.
It's good to be prepared, though
.
August 13th, 2012, 04:57 AM
#21
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Eric, I'll play
69 442 drop top 400/400 buckets console
Drive 30 miles to work on rainy morning. head lights on.When I arrive at my destination, console shifter won't budge. Will not come out of drive.
Drive 30 miles to work on rainy morning. head lights on.When I arrive at my destination, console shifter won't budge. Will not come out of drive.
August 13th, 2012, 05:45 AM
#23
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- Eric
August 13th, 2012, 08:06 AM
#25
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442, so it had dual exhaust...either the cable melted onto the header or you bent the rod at the end of the cable preventing it from going back into park.
August 13th, 2012, 08:50 AM
#26
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Or the neighbors cat was hiding under the car to stay out of the weather and was jamming the back drive linkage.
August 13th, 2012, 02:28 PM
#28
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Here's one. I'll give you everything up until my aha! moment where I figured it out...
1972 Cutlass Supreme 468 CI BBO (Silver Car) had been running hot, but not overheating for about a month. Replaced thermostat, and refilled with distilled water and Water Wetter. That seemed to help.
Got in the car one Sunday morning and drove it to FIL's house (Father In Law) to pick up my kids that had spent the night. On the way over the car started to miss but nothing drastic...I figured it hadn't had time to warm up completely.
I parked the car in FIL's driveway and shut it off. Nothing out of the ordinary. I went inside for about an hour and then was getting ready to leave. I cranked the car but nothing happened. I heard a single deep clunk but then nothing followed. I tried again, and again I got the same clunk of the starter engaging but not turning.
So I popped the hood, gave everything a once over and didn't notice anything unusual. I put a voltmeter on the battery and it read 12 volts. However, when I put a Schumacher Electric battery charger on it the charger said it was only at 60%. So we let it charge until it read 100%.
Again, I tried starting the car. Again, I got a clunk and nothing else. So fearing that my almost out of warranty Optima battery was taking a dump I took it, and my alternator in for a test at O'Reilly. The alternator showed failed, but I figured it would since it's a one wire and the dork up there at the auto parts store didn't know how to configure the damn machine to read a one wire alternator. Anyway, they sort of tested the battery but ended up hooking me up with a new battery.
I installed the new battery and alternator and had FIL turn the key while I watched for the starter gear to come out and engage the flexplate, which it did. So the battery was new, the starter was working but still the engine wouldn't crank.
It was at this moment I knew that something was wrong...I did one more test to check my theory and then another to confirm my suspicion.
What was wrong?
1972 Cutlass Supreme 468 CI BBO (Silver Car) had been running hot, but not overheating for about a month. Replaced thermostat, and refilled with distilled water and Water Wetter. That seemed to help.
Got in the car one Sunday morning and drove it to FIL's house (Father In Law) to pick up my kids that had spent the night. On the way over the car started to miss but nothing drastic...I figured it hadn't had time to warm up completely.
I parked the car in FIL's driveway and shut it off. Nothing out of the ordinary. I went inside for about an hour and then was getting ready to leave. I cranked the car but nothing happened. I heard a single deep clunk but then nothing followed. I tried again, and again I got the same clunk of the starter engaging but not turning.
So I popped the hood, gave everything a once over and didn't notice anything unusual. I put a voltmeter on the battery and it read 12 volts. However, when I put a Schumacher Electric battery charger on it the charger said it was only at 60%. So we let it charge until it read 100%.
Again, I tried starting the car. Again, I got a clunk and nothing else. So fearing that my almost out of warranty Optima battery was taking a dump I took it, and my alternator in for a test at O'Reilly. The alternator showed failed, but I figured it would since it's a one wire and the dork up there at the auto parts store didn't know how to configure the damn machine to read a one wire alternator. Anyway, they sort of tested the battery but ended up hooking me up with a new battery.
I installed the new battery and alternator and had FIL turn the key while I watched for the starter gear to come out and engage the flexplate, which it did. So the battery was new, the starter was working but still the engine wouldn't crank.
It was at this moment I knew that something was wrong...I did one more test to check my theory and then another to confirm my suspicion.
What was wrong?
August 13th, 2012, 03:14 PM
#29
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Blown head gasket caused rough running, overheating.
Adding straight water and water wetter may have lowered the boiling point, raising cooling system pressure a bit (assuming the pressure cap wasn't releasing, or the pressure had been less than the cap blow off during previous drives).
With increased pressure from higher heat / different coolant, small head gasket leak allowed coolant into cylinder --> cylinder filled with water --> hydro-lock.
Do I win, do I win?!?
- Eric
Adding straight water and water wetter may have lowered the boiling point, raising cooling system pressure a bit (assuming the pressure cap wasn't releasing, or the pressure had been less than the cap blow off during previous drives).
With increased pressure from higher heat / different coolant, small head gasket leak allowed coolant into cylinder --> cylinder filled with water --> hydro-lock.
Do I win, do I win?!?
- Eric
August 13th, 2012, 03:19 PM
#30
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Something tells me you read my post about it lmao! That's almost too perfect an answer Yes, you win lol!
Realized it wasn't the electrical system, it was the engine itself. I put a breaker bar on the crank bolt and couldn't physically turn engine over in direction of running rotation but when I turned it the opposite way it would turn. I started pulling spark plugs and finally got to #8 which was clean. With all the plugs out I tried starter again and it turned, spitting coolant out of the #8 cylinder.
$600 later I now have Cometic gaskets and higher compression...and that sweet sweet smell of C-12 burning in the exhaust
Yes, you win lol!Realized it wasn't the electrical system, it was the engine itself. I put a breaker bar on the crank bolt and couldn't physically turn engine over in direction of running rotation but when I turned it the opposite way it would turn. I started pulling spark plugs and finally got to #8 which was clean. With all the plugs out I tried starter again and it turned, spitting coolant out of the #8 cylinder.
$600 later I now have Cometic gaskets and higher compression...and that sweet sweet smell of C-12 burning in the exhaust
Last edited by ah64pilot; August 13th, 2012 at 03:22 PM.
August 13th, 2012, 03:23 PM
#31
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I may have, but if I did, I don't remember doing so.
I just thought about what would suddenly freeze up a motor without it being blown, and water was the obvious choice. Since you hadn't just driven through a lake (as I did once in a Scirocco, which lived to tell the tale, and did, in fact, float), the water had to come from inside. If it came from inside, and was probably caused by the same thing that caused the rough running, then a head gasket seemed like the number one choice.
- Eric
I just thought about what would suddenly freeze up a motor without it being blown, and water was the obvious choice. Since you hadn't just driven through a lake (as I did once in a Scirocco, which lived to tell the tale, and did, in fact, float), the water had to come from inside. If it came from inside, and was probably caused by the same thing that caused the rough running, then a head gasket seemed like the number one choice.
- Eric
August 14th, 2012, 04:35 AM
#32
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I have 2 that I'll offer and I'll post one at a time. I might be a little slow to respond today given several meetings but I'll try to get back when I can.
This goes back a number of years ago when I was working at a local gas staion while in school. Car was a 71 Nova 350 2 bbl/350 auto. Car was about 9 years old and had average miles. All in all the car was in good condition. Customer brings the car in complaining the car would just shut off while driving and was running fine up to that point. The owner would coast to the side of the road and within a minute or two the car would start back up and would seem fine. The problem was intermittent and it may be several days before it did it again. The car was checked over and everything seemed fine. It was again checked over after it came back the second time and we couldn't duplicate the problem. Everything was checking out fine which was frustrating as the owner was getting concerned about driving the car. The problem was now starting to happen more frequently. After the 3rd visit to the shop, the problem was found. I will say that it was not an issue related to boiling fuel in the fuel lines.
This goes back a number of years ago when I was working at a local gas staion while in school. Car was a 71 Nova 350 2 bbl/350 auto. Car was about 9 years old and had average miles. All in all the car was in good condition. Customer brings the car in complaining the car would just shut off while driving and was running fine up to that point. The owner would coast to the side of the road and within a minute or two the car would start back up and would seem fine. The problem was intermittent and it may be several days before it did it again. The car was checked over and everything seemed fine. It was again checked over after it came back the second time and we couldn't duplicate the problem. Everything was checking out fine which was frustrating as the owner was getting concerned about driving the car. The problem was now starting to happen more frequently. After the 3rd visit to the shop, the problem was found. I will say that it was not an issue related to boiling fuel in the fuel lines.
August 14th, 2012, 05:21 AM
#33
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Let me add this info to weed out the usual issues. This car had recently received a major tune-up by another shop (points, condenser, rotor, cap, wires, plugs and filters) and the fuel pressure from the pump was on spec. The dwell and timing were also set correctly. I replaced all of the rubber fuel lines since they were original and that didn't help. There was a lot of rainy weather during the time this was going on. But the wet weather actually helped with the problem. It was during the nice days that the car would shut off but it ran fine when it rained.
August 14th, 2012, 08:20 AM
#35
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August 14th, 2012, 10:18 AM
#38
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August 14th, 2012, 10:41 AM
#40
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My best guess - Did this car have a bimetalic spring control for the choke? I think choke related anyway. Or the H2O hydraulic screwtinater resivour was low on water. Mind you these require non chlorinated water so it must have distilled water or rain water.
