bright_medal

Hello RL,

I wanted to see if you could help me diagnose a possible problem.

The Car:
I recently purchased a 2012.5 CPO C2S. It's a 7spd MT, with not too many fancy options (which was the way I liked it) and about 24K miles. It has the standard PASM suspension, on the standard 20" wheels and factory PZero tires.

The (possible) problem:
I was taking the car for a hard drive through the Angeles Crest canyons this weekend. The car was performing like a champ, and I was really enjoying the drive. On the return drive (which is largely a downhill ride), I experienced car bumping around far more than usual. My first though was that perhaps I had blown a tire, to the point where I eventually pulled over the check. The tires were fine, so I jumped back in and kept going. Still, the car felt strange.

Since then, I've been driving the car, and it largely feels normal, but it does feel like the bumps are noisier than usual. Hard to describe here, but bumps feel like they're making a "ka-thunk" sound vaguely towards the rear of the car. On a smooth road, the car is as quiet and smooth as can be.

I tried to go back to the canyons on Sunday late-afternoon, but ran into so much traffic that I wasn't able to get enough speed to see if it's hard driving that causes the bump and "ka-thunk" noises or not. Please note, that I can't seem to tell any difference if the "Sport" button is turned on or not. But, it was on during my hard canyon drive.

The context:
I've owned several "performance" cars including an RX7, an MR2, an E36 M3, and an E46 M3, several of which were significantly lowered so I'm very familiar with bumpy rides. In general, I'm amazed at how comfortable the ride is on the 991 considering how capable the car is. Also, I wouldn't by any means consider myself an automotive hypochondriac, but I'm questioning myself this time.

The question:
What do you all think? Does it sound like I'm imagining things or did something relatively subtle break? Thanks for reading this super long thread and thanks in advance for your advice

96redLT4

Maybe high tire pressures or P Zeros that need to be swapped out?
Jim

bright_medal

Thanks for the advice, I don't think it's the PZeros. The front are brand new, and the rears are in good condition.

What tire pressures does everyone run on the front and rears?

dawgsontop

So if the feeling is consistently rhythmic and speed related that would lead me to believe that it's rotating mass related and it sounds awfully similar to what happened to me on my 335i. There ended up being 4 large bubbles on the inner sidewall of the back right tire and it was causing the car to vibrate at speed. Might not be a terrible idea to have the alignment/tire pressure/tires checked.

Al.Fresco

The comfort pressure recommendations are....
31 Front
34 Rear

2001f4s

If you need new tires, switch to Michelin PSS2's. Much quieter tires.

LexVan

I'd start here. How old are the rears? Rarely are the rears ever newer than the fronts.

As for the tires pressures, you may have worked them, and heated them up on the way up the canyon, then on the way down, they might be a bit (1-2 PSI) over inflated from optimum. Don't let em get much over 40-42 hot. 40 is pretty ideal (hot).

Jnosol

I run 31 front and 39 rear cold. Consider MPSS as your next set of tires, quieter and lasts longer.

bright_medal

Thanks everyone for your advice. I went ahead and lowered my tire pressure to comfort settings and was pretty amazed by the difference.

However, I don't think that tire pressure was quite the "problem". The outside temp on Saturday during my canyon run was probably around 100 degrees, so I do wonder if I may have overheated my tires.

Does anyone think it might be suspension related? The harsh hits seem to be more harsh than ever and make a "thunk" like sound.

Thanks all.

Jnosol

It might be good to have your dealer to take the look. It's not out of the ordinary for struts to give out pre-maturely or you have a rock stuck in the springs/suspension parts.

Larry Cable

check your rear droplinks are properly torqued, also check that you are not missing any wheel/ tire balance weights... check that your rear shocks are not leaking

Also remember if your tires were significantly over pressure this would impact high velocity compression cycles of your shocks, since the effective spring rate of the tires (over inflated) would be higher...

If the problem is still manifest, its a h/w problem, if its gone, then it was s/w (tire pressure etcs)

jimbo1111

Check out the shocks. Maybe the gas seals are gone.

Jsonorous

I recently had a "thunk" coming from my rear suspension. Brought it in for service, turns out it was the sway bar clunking away.

Larry Cable

OP might also try driving with PASM in "normal" and "sport" to see if the frequency of "thunks" changes with this setting ...

Joec500

2nd on the the torquing sway bar connections. I had a rattle thump after I lowered my car. Turns out my drop links were a little loose.