Unusual noise/vibration sound in '85 with the 307 drivetrain ???

[noreastcarnut]

There is an unusual noise/vibration sound in the drivetrain of my '85 with the 307 engine !!  The mileage is only 42,000 original. It comes on when I apply the gas and torque to the engine. It is not stready. It comes and goes.  If I drive lightly touching the gas pedal, it is quiet. The sound comes from the front of the car, maybe in the right front. It is loud as it can be heard echoing off buildings. All was checked over for many hours by my "ace" mechanics, but they can't find out the source. There is nothing loose or rubbing. They even removed the starter to check the flywheel, but found nothing. Sometimes when stopped, standing on the brakes and revving the engine, the noise can be heard. It is an erratic sound.  What can it be ???

[Aaron65]

Have you pulled the drive belts to see if the noise goes away? If the noise can be reproduced with the car stopped, you've eliminated axles/bearings. It could be something like a power steering pump or one of the other ancillary devices on the front of the engine. 

[noreastcarnut]

Thanks for that, I'm sure my "ace" mechanics tried your suggestion.

[60FlatTop]

After doing the things an ace would do, like prying up on the mounting locations, checking for movement in the harmonic balancer, making sure the fan shroud was secure, the routine stuff. I would jack the car up on the floor, not the lift, and support the front end with jack stands under the lower A-arms. That is close to normal support yet takes the weight off the drive system. Being really careful one ace can stand or squat on either side at the front while a third operates the gas pedal, shifter, and all the other controls. Even a third peering into the open engine bay. That would be a peer group of three.

 

I have used that method with only one other person and found it quickly reveals those untenable noises. I remember one of those noises that echoed along guard rails. No doubt about that bearing once the weigh was off.

 

Just screen the ace's names and exclude any named Ferris.

[noreastcarnut]

Thanks, all for your suggestions.  I am working on them.  

[Seafoam65]

If it does it with the drive belts removed it is most likely  to be the harmonic balancer

having torn rubber between the inner and outer discs. I've seen this about twenty times over the years and it is very difficult to diagnose if the outer disc has yet to get off center.

[EmTee]

8 hours ago, Seafoam65 said:

If it does it with the drive belts removed it is most likely  to be the harmonic balancer

having torn rubber between the inner and outer discs.

Set #1 cylinder at TDC by watching the valves and/or monitoring piston travel via the spark plug hole.  Then compare with the timing mark on the balancer.  If there's a discrepancy, then you have likely found the culprit.  It's possible that the rubber hasn't completely failed yet, so even if the mark looks OK the damper could still be bad.  It seems to me that improper torsional movement of the outer ring should be visible using a timing light with the engine held at constant RPM.  "Fluttering" of the timing mark would indicate the outer ring is rotating too far relative to the crankshaft; likely due to disintegration of the elastomeric (rubber) ring.

[NTX5467]

From what I have seen, at least on small block Chevy motors, when the outer ring of the balancer "moves", it also shifts rearward. exposing the front of the rubber in the process.  That movement can be easier to detect, to get the orientation that "something is wrong".  Perhaps other brands' balancers do similar?

[61polara]

I would check the front wheel bearings.

 

[EmTee]

On 6/19/2024 at 8:48 PM, noreastcarnut said:

Sometimes when stopped, standing on the brakes and revving the engine, the noise can be heard.

That seems to eliminate wheel bearings or other driveline components that don't rotate with the crankshaft.

[pfloro]

On 6/19/2024 at 5:48 PM, noreastcarnut said:

On 6/19/2024 at 5:48 PM, noreastcarnut said:

Sometimes when stopped, standing on the brakes and revving the engine, the noise can be heard. It is an erratic sound. 

As I mentioned earlier, this sounds like a flex plate or torque converter issue. 

 

When the THM 325-4L is in gear and the car is not moving, the impeller fins (welded to the torque converter shell) are moving fluid through the stator (not turning), through the turbine (also not turning), and back to the impeller. 

I think that broken impeller fin welds could cause noise & vibration under load.

 

Paul

Edited June 29 by pfloro (see edit history)

[EmTee]

It might be worth dropping the transmission pan to look for any shiny metallic flakes or anything else unusual...

[NTX5467]

Have we determined if it is more of a metal flexing against metal sound, a whirring sound, or something else?

 

Flex plates usually don't just break because they can.  On my '05 Impala 4T65E, it was because the pressure solenoids were aging and caused the line pressure to max-out in normal driving.  Eventually, it stretched the converter to flex plate bolts enough that looseness happened, which caused the orig flex plate to crack.  A "different" noise that came and went.

 

First thing replaced with the new flex plate as a new GM Reman converter.  Same bolts, which appeared to torque down as needed, but were bottoming out.  New flex plate broke 3 months later.  Then careful inspection revealed a shiny area on the bolts' shank.  New flex plate and new bolts fixed it.

 

Default mode . . . until something breaks, drive the car.

 

Just some experiences,

NTX5467

[noreastcarnut]

Thanks again, great suggestions.  I will see a new "ace" mechanic this week.  

 

J.

[noreastcarnut]

Yes, I saw another "ace" mechanic today.   After checking the weird noise, he thinks it may be the flex plate.  I now must find a local mechanical shop to dig into the transmission/ engine area at a scary price, I expect. I'll keep you all informed.

 

Thanks,

 

J.

[pfloro]

The transmission will have to be removed to inspect or replace the flexplate. The engine stays in.  During the 28 years I owned my '84 Toronado (200,000 miles) the transmission was rebuilt once @ 77,000 miles.  It's not a monstrous job to remove & replace it.

 

If you are planning on keeping this car and putting some miles on it, I would strongly recommend having a TransGo "Shift Kit" installed.  These downsized transmissions (THM 325 (3 speed) & THM 325-4L (4 speed)) had soft shifts "engineered in" by GM.  When it "applies", the intermediate band (creates 2nd gear along with other pieces of hardware) takes a beating and burns up over time.  The "shift improver kit" creates "sharper" shifts but they do not become harsh.  The shifts occur at the same time as before, but the "transition" from one gear to another happens more quickly. This reduces heat and wear of the friction materials (the band & the clutch disks).  The kit can be installed without removing or rebuilding the transmission.  However, it is easier with the unit "on the bench".   A reliable transmission shop familiar with these older THMs should install the kit.

 

Due to a burned up intermediate band, my 325-4L was rebuilt @ 77,000 miles. I had the TransGo shift kit installed at that time.  The improvement in shift quality (both upshift & downshifts through all gears) was amazing.  At 200,000 miles, there had been no change in the shift quality.

 

Give it some thought...

 

Paul

Edited July 9 by pfloro (see edit history)

[NTX5467]

Every time you feel a soft, strung-out shift, there's friction material wearing away during that time.  Quicker and firmer shifts "get on with it" and generate less heat and wear.  Such shifts can also make the car feel "more youthful", too.

 

NTX5467

[EmTee]

Before going whole-hog on flexplate replacement, I would mark the converter position relative to the flexplate, remove the mounting bolts and push the converter into the transmission as far as it will go.  Then use a screwdriver or small pry bar to test the plate for lateral movement by prying against the side of the ring gear,  Do this a section at a time and rotate the ring gear through 360°.  Pay attention to the amount of force needed to deflect the plate and note any 'soft' spots, or faint clicks, as metal edges catching and rubbing as might be happening at a crack near the crankshaft attachment.

 

Edited July 10 by EmTee (see edit history)