Occasional clunk on hard braking/acceleration (2012 Mazda5)

[IamFODI]

Recently got our 2012 Mz5 back from having the clutch, RMS, and all 3 powertrain mounts replaced. A few days later, I had to brake pretty hard over a rather bumpy section of road and got a mighty clunk from the front left. Car seemed fine afterward, but then twice more on the drive – once while braking over bumps, and another time when hitting a bump while turning – I got a smaller but pretty sharp clunk from the same corner.

Since then, I occasionally get the same clunk, most of the time from the front DS but occasionally from the front PS. Only happens on sharp acceleration or braking, especially on uneven surfaces. Sounds like a rock hitting the underside. Once or twice, I had it clunk once under braking and then clunk again while setting off under mild acceleration, almost like something had clunked out of place under braking and then clunked back into place when the suspension loaded in the other direction.

Front control arm, strut, strut mount, and all bushings and ball joints on that corner have ~8k miles or less on them.

Thoughts on what to check? The mechanic and I are stumped!

 

[Silentnoise713]

"clunk" sounds different to everyone.

-What was the most recent service/change before the clunking started?
-Does it clunk randomly or only during a certain event, such as while braking or braking while turning/etc.?
-Any lose plastic panels under the car? Hard abs plastic knockings on metal parts/chassis sound just like a clunk

Random suggestions:
Check are front end links and bushings. You can test both by simply disconnecting both front end links from the strut and zip tie them tot chassis. Go for a drive to see if it still happens. Yes, the car is perfectly drivable and it will not lose control and wander into a ditch. That said, you are doing this at your own peril.

Check the brake pads are not moving. Use a flat head to see if you can pry/move it around.

 

[negusm]

Might be a motor mount gone bad too.

 

[IamFODI]

-What was the most recent service/change before the clunking started?
-Does it clunk randomly or only during a certain event, such as while braking or braking while turning/etc.?

Might be a motor mount gone bad too.

Addressed most of this in the OP.

All three powertrain mounts are new OE. Clunk started after a service that included those and the clutch kit.

Happens only sometimes under strong acceleration or braking, e.g. emergency stop, hard braking over bumps, sudden heavy throttle uphill, etc.

Sounds like a rock hitting the underside. Seems to be coming primarily from front left.

 

[IamFODI]

Haven't solved this problem yet but have gathered a bit more info.

Let's say I'm accelerating briskly and hit a bump. At that moment, something will clunk. After that, the next time I brake relatively hard, I'll get another clunk whether I hit a bump or not.

It happens in reverse, too. If I hit a bump while braking hard, it'll clunk; then, next time I accelerate briskly, it'll clunk again even on a smooth road.

Not saying this is what's actually happening, but it feels like like momentary wheelspin plus suspension motion causes something to pop out of place, and applying a lot of force in the opposite direction causes the thing to pop back into place.

It seems to be happening more with winter tires than with my all-seasons, which I take as further evidence that wheelspin is a factor. Wheelspin alone won't do it, though. It has to be wheelspin plus a bump.

Since my previous post, I found that the mounting bolts at the rear ends of the front LCAs had not been tightened to spec, so I tightened them. Didn't seem to make a difference. Subframe mounting bolts were tight. Hoping to check more next time I'm under the vehicle.

If anyone has any further thoughts on what to check, I'm all ears!

 

[Silentnoise713]

Sound travels esp when the car is moving so this is hard to figure out. You'll need to put the car up on all 4 jacks and start knocking things with a rubber mallet.

My "been there, done that" experience on this car...
-Loose ABS plastic panel knocking on any part of the metal chassis will sound like a hard knock. Zip tie works.
-Loose aluminum heat shield (runs along the whole exhaust tunnel, for me it was just fore of the gas tank) also makes a hard knocking sound. Crimp the loose section to a stable section.
-Most common is worn RSB and FSB bushings but I think you replaced those.
-Hawk rear pad 'ears' was juuust loose enough to move and creak a knocking sound under impact.
EDIT: (add 2 more points clunking with this car)
-Loose nut that secure the end link to stabilizer bar. The through bolt is every so slightly smaller than the hole so if not tight, it will clunk.
-Worn end link ball joints. You will not be able to see or shake/bang to find this unless it is really worn. Simply disconnect one end of the link, zip tie out of the way, and go for a drive.