Hard Problem - Manual Transmission Noise
03-23-2017, 09:07 PM
#1
Hard Problem - Manual Transmission Noise
I'll apologize up front for the long post... I'm just trying to get everything I know about this problem up front. I'm getting desperate and I'm hoping someone has seen this before...
I recently bought this car, a 94 corvette, my first. Got an incredible deal on it (78k, looks great inside and out, runs great, black on black, manual, $2200) but at this point the bottom is being sucked out of my wallet. I'm at about $7000 now.
Here's the problem... I bought the car knowing it needed a clutch. I had a new clutch, pressure plate and release bearing installed. Test drove it and discovered that 1st and 2nd gear synchronizers were weak and the transmission had a bad noise coming from it. Found a used transmission with only 42k on it. I paid $1900 for it. Had it installed and other than the synchronizers, it has the EXACT same issue. Getting desperate now. Got a refund for the transmission and then found a BRAND NEW transmission (for $2750) that had been sitting on a guys shop floor for 5 years. It looked new and I could tell from the zero wear on the input shaft that it had never been installed.
Just got it back from the shop and it still has the same EXACT same noises coming from it. I'm guessing now that the problem is not in the transmission.
The transmission shifts and pulls perfect. All three of them did. Car runs great!
When under load, it makes a noise that sounds like buzzing or scrapping, like a bearing running dry.
You can hear it in all gears but it's worst in 5th and 6th gear. 4th sounds the best.
When accelerating in the lower gears, the noise will disappear between 2 and 3 thousand RPMs.
At idle, with the clutch out, there is a rattling, like "rocks in a can".
Press in the clutch, and the rattle stops.
Both noises are bad. The buzzing sound makes it hard to talk in the car while in 5th or 6th gear.
Doing research, I found that this car has a dual mass fly wheel. These fly wheels have a spring that absorbs the irregular rotation of the engine. When the springs gets weak it can cause the rattling sound. I haven't seen anything that indicates this would cause the buzzing sound though.
One more thing that may indicate something... I bought the car from a retired guy who only put 250 miles on it because of all the trouble he had with it. I have the receipts that show he had a new clutch and water pump installed in the car. The clutch was bad when I bought it. He was pretty frustrated with it. When we took the clutch out of the car, it was shredded... I mean nothing but scraps and pieces left , not worn out, just tore up. There was a lot of hard residue on the flywheel. It looked like it had cooked and melted to the flywheel. We used a wire brush on a drill to clean it up and it looked ok. This was the remains of a NEW clutch so it's a real mystery what happened to it.
One more little thing... After we installed the new clutch, when pressing the clutch pedal all the way to the floor,with the car running in neutral, we would hear a scraping sound. Kind'a like a bolt head was scrubbing something as the clutch turned. It only happened when the clutch was pressed to the floor and we sure didn't want pull the transmission again, so I opted to drive it. Strangely, the noise disappeared after driving it few days and it hasn't come back.
So, I've visited zfDoc and read about one problem that sounds kind'a like the buzzing/scraping noise I'm hearing. He called it "Gear Buzz", but I just cannot believe that I have acquired three transmissions, one of them NEW and all three could have this exact same problem.
Is there any chance a bad flywheel could be causing both these noises? Could it have been damaged or installed improperly and destroyed the first new clutch?
Again, sorry for the long post... Any insight or help would be greatly appreciated!
I recently bought this car, a 94 corvette, my first. Got an incredible deal on it (78k, looks great inside and out, runs great, black on black, manual, $2200) but at this point the bottom is being sucked out of my wallet. I'm at about $7000 now.
Here's the problem... I bought the car knowing it needed a clutch. I had a new clutch, pressure plate and release bearing installed. Test drove it and discovered that 1st and 2nd gear synchronizers were weak and the transmission had a bad noise coming from it. Found a used transmission with only 42k on it. I paid $1900 for it. Had it installed and other than the synchronizers, it has the EXACT same issue. Getting desperate now. Got a refund for the transmission and then found a BRAND NEW transmission (for $2750) that had been sitting on a guys shop floor for 5 years. It looked new and I could tell from the zero wear on the input shaft that it had never been installed.
Just got it back from the shop and it still has the same EXACT same noises coming from it. I'm guessing now that the problem is not in the transmission.
The transmission shifts and pulls perfect. All three of them did. Car runs great!
When under load, it makes a noise that sounds like buzzing or scrapping, like a bearing running dry.
You can hear it in all gears but it's worst in 5th and 6th gear. 4th sounds the best.
When accelerating in the lower gears, the noise will disappear between 2 and 3 thousand RPMs.
At idle, with the clutch out, there is a rattling, like "rocks in a can".
Press in the clutch, and the rattle stops.
Both noises are bad. The buzzing sound makes it hard to talk in the car while in 5th or 6th gear.
Doing research, I found that this car has a dual mass fly wheel. These fly wheels have a spring that absorbs the irregular rotation of the engine. When the springs gets weak it can cause the rattling sound. I haven't seen anything that indicates this would cause the buzzing sound though.
One more thing that may indicate something... I bought the car from a retired guy who only put 250 miles on it because of all the trouble he had with it. I have the receipts that show he had a new clutch and water pump installed in the car. The clutch was bad when I bought it. He was pretty frustrated with it. When we took the clutch out of the car, it was shredded... I mean nothing but scraps and pieces left , not worn out, just tore up. There was a lot of hard residue on the flywheel. It looked like it had cooked and melted to the flywheel. We used a wire brush on a drill to clean it up and it looked ok. This was the remains of a NEW clutch so it's a real mystery what happened to it.
One more little thing... After we installed the new clutch, when pressing the clutch pedal all the way to the floor,with the car running in neutral, we would hear a scraping sound. Kind'a like a bolt head was scrubbing something as the clutch turned. It only happened when the clutch was pressed to the floor and we sure didn't want pull the transmission again, so I opted to drive it. Strangely, the noise disappeared after driving it few days and it hasn't come back.
So, I've visited zfDoc and read about one problem that sounds kind'a like the buzzing/scraping noise I'm hearing. He called it "Gear Buzz", but I just cannot believe that I have acquired three transmissions, one of them NEW and all three could have this exact same problem.
Is there any chance a bad flywheel could be causing both these noises? Could it have been damaged or installed improperly and destroyed the first new clutch?
Again, sorry for the long post... Any insight or help would be greatly appreciated!
Last edited by mwester; 03-25-2017 at 09:04 PM.
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Nilak (01-22-2022)
03-26-2017, 07:46 PM
#2
Instructor
Its always possible that the bellhousing is not true and the tranny is not sitting square with respect to the engine. I am going to assune any clutch job included a pilot bearing.
If it doesnt have the original dual mass flywheel maybe you could have a clearence issue.
I would also double chech every part number including the clutch fork and stud. It is always possible it was never correct from the factory. Check the back of the block for machining issues as well.
Maybe it was in an accident and something isnt right? Grasping at straws but just trying to help
If it doesnt have the original dual mass flywheel maybe you could have a clearence issue.
I would also double chech every part number including the clutch fork and stud. It is always possible it was never correct from the factory. Check the back of the block for machining issues as well.
Maybe it was in an accident and something isnt right? Grasping at straws but just trying to help
03-26-2017, 08:58 PM
#3
Thanks for the ideas Bwbob. Some of what you've noted may well be what's going on. We did replace the pilot bearing.
I was doing some investigating today and discovered that when the clutch was replaced before I bought the car, the fly wheel was replaced as well.
Doing some more research I found that a dual mass flywheel should have some play in it before the springs begin dampening. I put my car on jack stands and took off the access plate, then tried rocking the wheels in gear and found that my flywheel has no play at all.
This made me take a second look at the invoice where the guy before me had the clutch and flywheel replaced. There was a part number on the invoice for the flywheel. After a bit of searching I found that the replacement flywheel was a single mass.
This probably explains the "rocks in a can" sound I'm hearing at idle.
Does anyone know if the increased rattle produced by using a single mass flywheel could translate into the buzzing/scraping sound that I'm hearing up to 3000 rpms? Maybe there is some kind of alignment issue that's causing the single mass flywheel to make more noise than usual.
I was doing some investigating today and discovered that when the clutch was replaced before I bought the car, the fly wheel was replaced as well.
Doing some more research I found that a dual mass flywheel should have some play in it before the springs begin dampening. I put my car on jack stands and took off the access plate, then tried rocking the wheels in gear and found that my flywheel has no play at all.
This made me take a second look at the invoice where the guy before me had the clutch and flywheel replaced. There was a part number on the invoice for the flywheel. After a bit of searching I found that the replacement flywheel was a single mass.
This probably explains the "rocks in a can" sound I'm hearing at idle.
Does anyone know if the increased rattle produced by using a single mass flywheel could translate into the buzzing/scraping sound that I'm hearing up to 3000 rpms? Maybe there is some kind of alignment issue that's causing the single mass flywheel to make more noise than usual.
03-27-2017, 12:41 PM
#4
Instructor
With the car on the stands, any strange noise as you spin the wheels with the tranny in neutral? Also check it with someone pushing the clutch in to check for binding.
There was a guy with a dual mass flywheel in the for sale section. Not cheap but it may be the way to go for you if that noise is just too much.
In neutral you should be able spin the wheels easy and some of the guts of the tranny will spin. In different gears with the clutch all the way to the floor you should be able to make all the parts in the tranny spin including the clutch disk. You might hear a very light scrape sound but it shouldnt really drag. Doublr check wheel bearings, brakes, and u jounts. Driveline noises can travel. Ive seen muffler systems flex under load and touch parts, made me chase motor mounts etc.
There was a guy with a dual mass flywheel in the for sale section. Not cheap but it may be the way to go for you if that noise is just too much.
In neutral you should be able spin the wheels easy and some of the guts of the tranny will spin. In different gears with the clutch all the way to the floor you should be able to make all the parts in the tranny spin including the clutch disk. You might hear a very light scrape sound but it shouldnt really drag. Doublr check wheel bearings, brakes, and u jounts. Driveline noises can travel. Ive seen muffler systems flex under load and touch parts, made me chase motor mounts etc.
03-28-2017, 08:49 PM
#5
I have an update on my transmission problem after taking it to a transmission shop for a diagnostic.
There are two noises causing concern... one is the rattling sound at idle. The transmission guy says this is almost certainly caused by the single mass flywheel that has been installed and is not really a problem, just annoying.
The other noise is like a buzzing or dry bearing sound that is heard under acceleration. The tranny guys tells me he thinks this noise is normal but that I shouldn't be hearing it. He thinks that I am missing a heat shield that is supposed to be covering the whole transmission. He noted that there is a LOT of heat and noise coming up through the console. I asked my mechanic about it and said it had no heat shield.
I went to a local dealer and they were able to confirm that there should be a heat shield that covers the entire transmission. (It's no longer available for purchase though.)
Can anyone with a C4 Manual trans confirm ever seeing this? Is there someone selling these after market?
There are two noises causing concern... one is the rattling sound at idle. The transmission guy says this is almost certainly caused by the single mass flywheel that has been installed and is not really a problem, just annoying.
The other noise is like a buzzing or dry bearing sound that is heard under acceleration. The tranny guys tells me he thinks this noise is normal but that I shouldn't be hearing it. He thinks that I am missing a heat shield that is supposed to be covering the whole transmission. He noted that there is a LOT of heat and noise coming up through the console. I asked my mechanic about it and said it had no heat shield.
I went to a local dealer and they were able to confirm that there should be a heat shield that covers the entire transmission. (It's no longer available for purchase though.)
Can anyone with a C4 Manual trans confirm ever seeing this? Is there someone selling these after market?
03-29-2017, 02:08 PM
#7
I have an update on my transmission problem after taking it to a transmission shop for a diagnostic.
There are two noises causing concern... one is the rattling sound at idle. The transmission guy says this is almost certainly caused by the single mass flywheel that has been installed and is not really a problem, just annoying.
The other noise is like a buzzing or dry bearing sound that is heard under acceleration. The tranny guys tells me he thinks this noise is normal but that I shouldn't be hearing it. He thinks that I am missing a heat shield that is supposed to be covering the whole transmission. He noted that there is a LOT of heat and noise coming up through the console. I asked my mechanic about it and said it had no heat shield.
I went to a local dealer and they were able to confirm that there should be a heat shield that covers the entire transmission. (It's no longer available for purchase though.)
Can anyone with a C4 Manual trans confirm ever seeing this? Is there someone selling these after market?
There are two noises causing concern... one is the rattling sound at idle. The transmission guy says this is almost certainly caused by the single mass flywheel that has been installed and is not really a problem, just annoying.
The other noise is like a buzzing or dry bearing sound that is heard under acceleration. The tranny guys tells me he thinks this noise is normal but that I shouldn't be hearing it. He thinks that I am missing a heat shield that is supposed to be covering the whole transmission. He noted that there is a LOT of heat and noise coming up through the console. I asked my mechanic about it and said it had no heat shield.
I went to a local dealer and they were able to confirm that there should be a heat shield that covers the entire transmission. (It's no longer available for purchase though.)
Can anyone with a C4 Manual trans confirm ever seeing this? Is there someone selling these after market?
There is no heat shield above the transmission. This is not a C5 or newer with the tunnel plate. There is a shifter boot that if missing or torn can allow heat and noise into the passenger cabin.
As far as the rocks in a can sound, it is normal for a single mass, and the noise should go away when pressing in the clutch. The other Boise you are hearing sounds like the throw out bearing may be excessively worn or not the proper size. Or the inspection cover or some other piece is loose.
What is your location? As somebody might be nearby to help with a look over.
03-29-2017, 02:15 PM
#8
Drifting
The ZF trans in my '90 made noisy gear sounds from the day I bought it new. Having driven 4 speeds for many years it was disconcerting to the degree I took it back to the dealer several times. They assured me it was normal for this unit & accept it for what it was. Today it is just a normal performance sound and I'm happy with it. Perhaps you could find an original ZF C4 and see how it sounds.
Regarding the wrapping I don't recall the ZF covered with anything.
Regarding the wrapping I don't recall the ZF covered with anything.
03-29-2017, 11:07 PM
#9
Thanks for all the replies!
Just to answer some of the comments... the clutch, pressure plate, pilot bearing, release bearing and flywheel have all been replaced. The transmission is new with less than 100 miles on it.
I'm feeling a bit better about it now because the consensus seems to be that the noise it's making is normal for the single mass flywheel and this transmission. I was afraid something was wrong and I was going to destroy the transmission.
It's still something I'm going to have to improve though... it so noisy in 6th gear that it's hard to talk comfortably inside the car. Even with the windows down I can still hear it. There is a LOT of heat under the cup holder too, not so much coming from the shifter.
I going to find some heat shield\noise dampening material and get it installed in the tunnel. My mechanic says he can do it without removing the transmission again, so maybe I won't be totally broke when this is done.
I'll post the results here when I get the insulation in.
Just to answer some of the comments... the clutch, pressure plate, pilot bearing, release bearing and flywheel have all been replaced. The transmission is new with less than 100 miles on it.
I'm feeling a bit better about it now because the consensus seems to be that the noise it's making is normal for the single mass flywheel and this transmission. I was afraid something was wrong and I was going to destroy the transmission.
It's still something I'm going to have to improve though... it so noisy in 6th gear that it's hard to talk comfortably inside the car. Even with the windows down I can still hear it. There is a LOT of heat under the cup holder too, not so much coming from the shifter.
I going to find some heat shield\noise dampening material and get it installed in the tunnel. My mechanic says he can do it without removing the transmission again, so maybe I won't be totally broke when this is done.
I'll post the results here when I get the insulation in.