87 300d: Chasing down annoying front end pop
 

Hi All,

I've been trying to chase this popping sound on the front passenger side of my 87 300d for awhile now. Here's a brief overview:

Only on the front passenger side, never the driver side. The pop happens when going over bumps, hard braking (but won't pop again with hard breaking until after I have come to stop, after I've stopped it'll pop again under hard braking, but not again until I stop), turning the steering wheel while idle, turning the steering wheel while backing up and turning (produces a noticeably louder pop). These noises are not consistent neither, sometimes it's all of the above all the time, sometimes there's no sound, sometimes there's a mix.

I took the car into the mechanic and they could not find the source of the sound, though they did find two other things that needed attention. I had them fix those but then took the car back because the bill was getting too high for that month.

All of the suspension and steering components were replaced 3-4 years ago. I asked the mechanic specifically about the ball joints on the control arms (replaced three years ago with lemfoerders) and he assured me the control arms were fine.

In the report, the mechanic mentioned slight play in the idler arm bushing (replaced two years ago, I've had to replace that three times since 2013 due to using cheap parts, last one was lemfoerder). I tested it, but there doesn't seem to be much play.

If anyone could give me advice about what you think the culprit could possibly be, or what more tests I should run, please let me know. This popping sound is driving me crazy, especially since I have been able to successfully diagnose and solve every other problem on my car, thanks in large part to the fine folks on this board.

Anyhow, I was bored so I took a video of me testing the tie rods, center link, and idler arm. Does the play look normal in these? Could these be responsible? Thanks for all the help!

https://www.youtube.com/watch?v=YC7Ou2Y0OEM

I would look for play in the brake caliper and rotor before anything else. And dumb stuff like a loose wheel lug. I’m guessing that the pads or guide pins are shifted when you first brake, and only return to position when you’re stopped. Tough to give you more than a guess over the web.

Can you hear it when you pump the front end with your hands? Maybe have someone pump it and someone else listen around for the sound.

I’ll check the brake components tomorrow. Philly do you mean pump the front end by pushing down on the hood? If so, I’ve done that and haven’t heard anything at all...

Mine was bad guide rod mounts on the 84SD. Were they replaced with the rest of the front end work? What brand parts were used?

Does the w124 have a guide rod? I have never come across a guide rod when rebuilding my suspension and steering...maybe it goes by another name?

I had a thunk under braking on one of my 123 cars...I never did figure it out and it never caused any problems. Also had one on my Miata that I autocrossed vigorously...never figured it out either and never it caused a problem.

There's no guide rod on a 124

Can you pump some grease into the ball joint boots?

Check the supports for the anti-roll bar. They crack, and then you get the pop that you describe.

Max, that’s the sway bar right? I’m about to head out and check a few things on the car, including the ball joint

yes.

As mentioned double check the sway bar brackets. Lift the front of the car to remove load and check. I initialy checked mine loaded (on ground) and the break was not visible until I lifted the front end.

Here's an update...

I checked everything mentioned above (caliper, brake pads, sway bar brackets) I even checked the steering damper because it was easy and, heck, I need to be thinking outside the box.

I did, or might have, discovered something. With the vehicle up, I turned the wheels so they were pointing to the driver side as far as possible and then I shook the wheels by the 3 and 9 o'clock further towards the driver side to put tension on the idler arm, drag link, and tie rods. I noticed play and heard some pop/clunking. Here's the video, sorry about the sun but I couldn't help it none.

https://www.youtube.com/watch?v=NnhjyMpN2DE

Is the idler arm supposed to move that much?

Could it actually be the idler arm that is the culprit?

Thanks for the help!

Yes.