We got a Rumble goin down under
11-17-2015, 04:24 AM
#1
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We got a Rumble goin down under
Hi Gents , Im an old school car restorer and finally recieved my beautiful 1984 928S auto / Gold metallic and lots a miles
I absolutely love this Ride with a passion ... I am 65 years young and have restored and owned a ton of older muscle and a grip of 914,s with a few 911,s over the last 40 plus years .. in any case ... here is where I so need your help
and with a lil bit of encouragemnt ... I will perform the miricle fix .. hah ha
My problem is a vibration / a low rumble in my drive line ... driving at normal acceleration a low frequency rumble occurs ... and when I pick my foot off the accelater things smooth right out .. glides smooth as ice
press the accelerator and the low frequency vibration will return
My first thought is a CV imbalance .. these boots are completely torn apart and it appears that they have been like this for a long time ... I purchased all new boots and will repack the CV,s on the rear axles ... no problem .. I will also inspect all the ***** and races inside the joints ...
being very experienced with repacking CV,s since I own a small VW repair shop of 35 years
I need to know if Im looking at the wrong gremlins ( the CV,s and axles )
Could the real sourceof that light vibration be in the drive shaft area which I have no experience whatever
does the main drive shaft have CV,s inside that tube
please help me if you can
repacking the CV,s on the outside rear axles are a peace of cake ... but that torque tube deal is stressing me out .. hah ha eek
cheers
Lawrence from Sunny So Cal / maybe a lil burny to
I absolutely love this Ride with a passion ... I am 65 years young and have restored and owned a ton of older muscle and a grip of 914,s with a few 911,s over the last 40 plus years .. in any case ... here is where I so need your help
and with a lil bit of encouragemnt ... I will perform the miricle fix .. hah ha
My problem is a vibration / a low rumble in my drive line ... driving at normal acceleration a low frequency rumble occurs ... and when I pick my foot off the accelater things smooth right out .. glides smooth as ice
press the accelerator and the low frequency vibration will return
My first thought is a CV imbalance .. these boots are completely torn apart and it appears that they have been like this for a long time ... I purchased all new boots and will repack the CV,s on the rear axles ... no problem .. I will also inspect all the ***** and races inside the joints ...
being very experienced with repacking CV,s since I own a small VW repair shop of 35 years
I need to know if Im looking at the wrong gremlins ( the CV,s and axles )
Could the real sourceof that light vibration be in the drive shaft area which I have no experience whatever
does the main drive shaft have CV,s inside that tube
please help me if you can
repacking the CV,s on the outside rear axles are a peace of cake ... but that torque tube deal is stressing me out .. hah ha eek
cheers
Lawrence from Sunny So Cal / maybe a lil burny to
11-17-2015, 05:37 AM
#2
Rennlist Member
If the boots are torn then CV's are a good start, and need to be done anyway.
It could also be the torque tube bearings, which would mean you would need to remove the torque tube, press out the solid drive shaft and replace the bearings.
Another distinct and likely possibility is a 'rumbling' rear wheel bearing, which will be a relatively easy WYAIT when you do the CV's.
With it safely up on a lift, you could spin up the driveline, and use a mechanics stethoscope or long screwdriver to your ear to isolate the noise.
There are lots of threads on all three procedures ... check Dwaynes garage, which is noted in the FAQ sticky thread, and search will also yield lots more.
... and .. Welcome! ... you've come to the right place.
It could also be the torque tube bearings, which would mean you would need to remove the torque tube, press out the solid drive shaft and replace the bearings.
Another distinct and likely possibility is a 'rumbling' rear wheel bearing, which will be a relatively easy WYAIT when you do the CV's.
With it safely up on a lift, you could spin up the driveline, and use a mechanics stethoscope or long screwdriver to your ear to isolate the noise.
There are lots of threads on all three procedures ... check Dwaynes garage, which is noted in the FAQ sticky thread, and search will also yield lots more.
... and .. Welcome! ... you've come to the right place.
11-17-2015, 01:41 PM
#3
Rennlist Member
Good way to check for TT issue is when sitting, on an auto the TT/drive shaft is directly connected to the Flywheel, so as you rev the engine in Park or Neutral the sound would rise and fall w/ the spinning drive shaft.
Its spinning when in gear too but harder to rev up with wheels on the ground
Its spinning when in gear too but harder to rev up with wheels on the ground
11-17-2015, 02:14 PM
#4
Chronic Tool Dropper
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Your car looks great! Start a picture thread please, and share some history on the car and your recovery efforts.
Noises and vibrations:
The two axle shafts are identical side to side. Wear typically occurs faster on the drive faces than the trailing faces of each CV joint, contributing to vibration on a pulling throttle when pressure is put on the worn surfaces. A good first step would be to completely clean and repack the joints, assemble them with new boots, and swap the axles side to side so you are pushing on un-worn faces in the joint. Follow normal inspection procedures for the ***** and races of course, and replace any joints that show wear or pitting on the ***** or the faces.
The 'torque tube' down the center of the car includes a driveshaft that rides in ball bearings inside the tube. The bearings don't last forever, unfortunately. As the bearings get tired with age, the old grease cakes inside and allows the bearing to rub and make a rumble sound. In your automatic car, the driveshaft is connected to the crankshaft at the front with a flex plate, so it always spins at crankshaft speed. With the car safely on stands, you can start the engine and listen/feel the torque tube for noises and vibration. If you have a mechanics stethoscope, you can listen for the bearing noises along the length of the outside of the tube. Your description of a noise that goes away on trailing throttle would steer me away from the torque tube bearings, but they are easy to listen to and eliminate from your troubleshooting list.
Motor mounts on the car are originally hydraulically-dampened rubber units. Over time and pressure, the fuid eventually leaks out, and the reduced support and dampening allows the rubber to collapse. The mounts have safety hooks that interlock around the mount, intended to keep the engine in place in an accident. The workshop manual guidance suggestss inspection of the mounts by sticking a finger or two between the hooks. The original fitment has almost no gap there. By the time you can get a finger in between the hooks, it's time to think about replacement. If one finger is a loose fit, or you can wedge two fingers in there, your mounts have collapsed completely. With the engine sitting hard on the supports, vibrations are transmitted directly to the chassis/tub, and manifest themselves as noise in the cabin.
Transmission mounts support the gearbox in the rear, transferring the weight of the gearbox to the cast aluminum crossmember. A quick visual check of the height of the gearbox from that crossmember will clue you to the condition of the mounts. There is originally space to place fingers between the bottom of gearbox and the crossmemmber, originally somewhere around 15-20mm. If you can't get fingers comfortably in that space, the transmission mounts should be replaced. Collapsed transmission mounts allow normal driveline vibrations to be transmitted to the chassis/tub, and may also help CV and half-shaft noises find their way into the cabin.
How well does the engine run? Do normal diagnostices suggest everything is OK? A weak or missing cylinder will offer noises and vibration that disappear when load is removed. A good compression test is a good periodic test, and smog test results will tell you a lot about other health issues in the engine itself.
HTH!
Noises and vibrations:
The two axle shafts are identical side to side. Wear typically occurs faster on the drive faces than the trailing faces of each CV joint, contributing to vibration on a pulling throttle when pressure is put on the worn surfaces. A good first step would be to completely clean and repack the joints, assemble them with new boots, and swap the axles side to side so you are pushing on un-worn faces in the joint. Follow normal inspection procedures for the ***** and races of course, and replace any joints that show wear or pitting on the ***** or the faces.
The 'torque tube' down the center of the car includes a driveshaft that rides in ball bearings inside the tube. The bearings don't last forever, unfortunately. As the bearings get tired with age, the old grease cakes inside and allows the bearing to rub and make a rumble sound. In your automatic car, the driveshaft is connected to the crankshaft at the front with a flex plate, so it always spins at crankshaft speed. With the car safely on stands, you can start the engine and listen/feel the torque tube for noises and vibration. If you have a mechanics stethoscope, you can listen for the bearing noises along the length of the outside of the tube. Your description of a noise that goes away on trailing throttle would steer me away from the torque tube bearings, but they are easy to listen to and eliminate from your troubleshooting list.
Motor mounts on the car are originally hydraulically-dampened rubber units. Over time and pressure, the fuid eventually leaks out, and the reduced support and dampening allows the rubber to collapse. The mounts have safety hooks that interlock around the mount, intended to keep the engine in place in an accident. The workshop manual guidance suggestss inspection of the mounts by sticking a finger or two between the hooks. The original fitment has almost no gap there. By the time you can get a finger in between the hooks, it's time to think about replacement. If one finger is a loose fit, or you can wedge two fingers in there, your mounts have collapsed completely. With the engine sitting hard on the supports, vibrations are transmitted directly to the chassis/tub, and manifest themselves as noise in the cabin.
Transmission mounts support the gearbox in the rear, transferring the weight of the gearbox to the cast aluminum crossmember. A quick visual check of the height of the gearbox from that crossmember will clue you to the condition of the mounts. There is originally space to place fingers between the bottom of gearbox and the crossmemmber, originally somewhere around 15-20mm. If you can't get fingers comfortably in that space, the transmission mounts should be replaced. Collapsed transmission mounts allow normal driveline vibrations to be transmitted to the chassis/tub, and may also help CV and half-shaft noises find their way into the cabin.
How well does the engine run? Do normal diagnostices suggest everything is OK? A weak or missing cylinder will offer noises and vibration that disappear when load is removed. A good compression test is a good periodic test, and smog test results will tell you a lot about other health issues in the engine itself.
HTH!
11-17-2015, 06:30 PM
#6
Chronic Tool Dropper
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No clutch pedal on the OP's automatic car.
