The thump again

[inovermyhead]

So having just had the propshaft ( driveshaft ) out and rebalanced plus the diff end coupling replaced, plus both drive shafts ( not sure in US speak but from the diff to the hubs ) completely rebuilt with new cans, cages and ball bearings the car no longer has a steady vibration, hurrah.
But on spirited acceleration in first gear at 4000 revs there is a thump thump sound that appears to be coming from the back of the car and it's loud....

The diff mount is new
The subframe that the diff mounts to has been reinforced with 3 mm steel plate
The engine mounts are new (BMW or Oem) athough on hard braking you can hear the fan hit the radiator, ting ting ting
The centre mount for the prop was preloaded and is the right way round, the doughnut is all good and time was taken to ensure the prop is lined up as perfectly as possible

I was hopeful that this latest work would resolve this thumping issue, but it appears not.
Am I missing something or has anyone else got any ideas.


Cheers. John

 

[Blinkling]

Oh, the DONUT. I recently had to replace mine (flex disk / giubo / guibo /doughnut) unexpectedly. It only lived a short time and as it failed it thumped under acceleration but acted normally above 40 miles per hour and when slowing down. That's what had me confused. And it was supposedly the good, reinforced kind. It was positively oriented the correct way when I put it in. Mine ended up looking terrible by the time I took it out of service but I believe the thumping began before any visual evidence appeared. Do you have a second one you could try?

 

[inovermyhead]

I only get the thump in first gear, I can rev it out in 2nd no problem and while the prop was out we were able to thoroughly inspect the doughnut, nothing to report or raise any issues

John

 

[lloyd]

So having just had the propshaft ( driveshaft ) out and rebalanced plus the diff end coupling replaced, plus both drive shafts ( not sure in US speak but from the diff to the hubs ) completely rebuilt with new cans, cages and ball bearings the car no longer has a steady vibration, hurrah.
But on spirited acceleration in first gear at 4000 revs there is a thump thump sound that appears to be coming from the back of the car and it's loud....

The diff mount is new
The subframe that the diff mounts to has been reinforced with 3 mm steel plate
The engine mounts are new (BMW or Oem) athough on hard braking you can hear the fan hit the radiator, ting ting ting
The centre mount for the prop was preloaded and is the right way round, the doughnut is all good and time was taken to ensure the prop is lined up as perfectly as possible

I was hopeful that this latest work would resolve this thumping issue, but it appears not.
Am I missing something or has anyone else got any ideas.


Cheers. John

The first thing that attracts my attention is the radiator hitting the fan. This may indicate that your transmission mount is kaput, allowing the engine to cant forward. Looks are deceptive. Just because the mount looks intact, doesn't mean it has not sheared. In other words, it may still act as a support, but not prevent some vertical movement such as you have described. It is also possible the mount is old and has softened, so that while it cushions, it permits too much flexing along a vertical axis.

Since you mentioned new motor mounts, are they properly installed? Last time I checked, they are indexed and although it is hard to install incorrectly . . . if one tries hard enough, it can happen. I mention this since I once encountered urethane motor mounts that were installed so that they did not seem completely flush with the corresponding anchor points. I also discovered one of them had sheared as described above.

You might consider lifting the car, and observing the drive shaft operation while being driven by the engine. If something is loose and/or unbalanced, you generally can see it as well as feel it. A new center support bearing and flex coupling generally should be trouble free. However, even new parts can be defective or become defective quickly. If you are using the older style, fat, sold rubber flex coupling, it should resist even severe abuse, but dumping the clutch or missing a shift can quickly compromise a poorly manufactured part. I have heard reports that some off brands are short lived, but I have not personally confirmed this.

You might also consider contacting the party responsible for drive shaft work. There are many things that should not be overlooked, but often are. Are the U-joints phased properly?

What about your differential? If it is an open (non limited slip) style, you might have some significant spider gear wear. Ordinarily, a "clunk" "clunk" sound can be heard on deceleration. Boom is the sound of gear teeth braking.

 

[inovermyhead]

Hi Lloyd,
The transmission support was removed, inspected and replaced as all was good about three weeks ago so I am fairly confident that the problem is not there.
The shaft was balanced this week by a shop that specialises in this work, I saw it run up on the balancing machine after the tail joint had been replaced and it was absolutely spot on. The centre bearing and front rubber disc were closely inspected for wear/ damage and all looked good they were fitted before balancing.

I am leaning towards the engine mounts now as this is really the only thing I think that may be responsible for some movement, as I said when braking hard the motor moves forward enough that the fan contacts the radiator, I'm looking at possibly replacing these with some M3 e30 mounts maybe by Vibra tech .

Other than that I'm stumped

Cheers. John

 

[Henrik]

Hi John!
Since the noise appear to come from the rear of the car I would recommend you to check if the exhaust system is in the correct position.

 

[sfdon]

How about a pic of your motor mounts?
Clear shot of mount showing the pin.
Stock mounts are fine.
I'll be in Brisbane in 4 weeks. Need parts?

 

[inovermyhead]

Henrik, thanks for that that's one thing I haven't checked
Don, I will take some photos tomorrow, be advised though in probably a wasted effort to stiffen the mounts they have been ' reinforced ' with large hose clamps.....
Thanks also for the parts offer, I may pm you

 

[lloyd]

Hi Lloyd,
The transmission support was removed, inspected and replaced as all was good about three weeks ago so I am fairly confident that the problem is not there.
The shaft was balanced this week by a shop that specialises in this work, I saw it run up on the balancing machine after the tail joint had been replaced and it was absolutely spot on. The centre bearing and front rubber disc were closely inspected for wear/ damage and all looked good they were fitted before balancing.

I am leaning towards the engine mounts now as this is really the only thing I think that may be responsible for some movement, as I said when braking hard the motor moves forward enough that the fan contacts the radiator, I'm looking at possibly replacing these with some M3 e30 mounts maybe by Vibra tech .

Other than that I'm stumped

Cheers. John

If virtually everything is new and you are still hearing a thumping noise, then it has to be the installation or, one of those parts. (Or have you forgotten about a marsupial left in the boot.)

Your focus on motor mounts reminded me of the urethane mounts mentioned in prior post. If I recall correctly, someone had installed a makeshift engine restraint with one or two turnbuckles. One can only guess that the engine movement was excessive. Whether the engine movement compromised stock mounts as well as urethane mounst, is unknown. Similarly, it is unknown what might have permitted the excessive engine travel, be it fatigued metal, a previous accident or driving too close to the sun. If you do have excessive movement, that could translate to moving ancillary parts like the exhaust system. Check exhaust pipe placement and hangars for excessive movement and potential contact with other parts, including the car body. The rear silencer and attached tail pipe can easily contact that spare tire well if not securely tethered.

As I said before, I think it behooves you to lift the car and run the drive train in place to go "thunk hunting". Safety is, of course, paramount. Even if you are adept at detecting aberrant sounds, doing so from inside the cabin is not always the same as getting up close. I alluded to the possibility of a problem with the differential. If starved for lubrication, it may "clunk" for help. Unfortunately, once it starts making noise, it can be too late to fully rectify the problem.

The fact that a drive shaft is test spun on a lathe is a good thing and probably eliminates the drive shaft as a likely contributor to the problem. As a weak analogy, tires can be balanced off the car and balanced while mounted on the car. When mounted, there may be a slight difference in balance due to additional attached parts, e.g., rotors, bearings etc. In theory, there should be no difference, reality occasionally proves to be different. When under load, your drive shaft assembly may react differently then when spun unloaded, so observing it on the car might not necessarily be a bad idea. That also applies to the center support bearing. I understand the recommended 2mm in preload, but there is plenty of fore-aft-side-to-side wiggle room to mount or mis-mount that part to the chassis. Maybe it needs a little tweaking to avoid thunking? Although it should not be necessary, I have occasionally seen the center mounts "shimmed" with wide washers. One must assume this was done purposefully, either to align things or maybe it is just a repository for extra washers?

Bet it is something simple.

Best of luck.

 

[Markos]

How about a pic of your motor mounts?
Clear shot of mount showing the pin.
Stock mounts are fine.
I'll be in Brisbane in 4 weeks. Need parts?

That was my first guess. Find a clear parking lot and dump the clutch a bit with the hood open. Watch how much the motor moves. Nothing crazy, use discretion.

Also, you may see similar behavior in reverse...

 

[gazzol]

I had a nasty thumping under real hard acceleration a few years back, it would only do it in 1st and 2nd gear. It turned out that the gearbox mount was incorrect causing the Guibo to "wind up" and it was catching on one of the unused mountings on the back of the gearbox! You had to see it to believe how far it moved, it was incredible but look for marks on the guibo and the unused mounting assuming its there mine was 5 speed . Having said that when I had my problem it felt like it was in the centre of the car.

 

[Bert Poliakoff]

In the E24 world, a cause of thumping upon acceleration had lead to the differential mount.

 

[Mal CSL 3.0]

Hi John,

My 2 cents worth.

I think the fact that you say your fan hits the radiator under braking sounds ominous. This would be a movement of over 5cm+ of your engine (and drive train etc - estimated based on distance between my fan and radiator). Subsequently in first gear there are a lot of G forces as you accelerate so your engine most likely also flexes backwards a few too many cm. This would mean everything moves backwards a bit/warps - trans, driveshaft etc and could cause alignment issues resulting in knocks, thumps (and potential weakening of mounting brackets if you leave it too long)

As you say you have new engine mounts, but are you sure they are correct / tight enough? Are there any other engine/trans mounting plates missed somewhere?

I would start be getting the engine secured in the engine bay very firmly and check all other mounting points along the entire trans and drivetrain so it doesn't move around.

(With my car I can throw it around and accelerate hard in first, brake hard and I get no movement of my engine at all)

Cheers