Stutter/ABS engages in corners

Hi everyone, hoping someone can help me out with some direction with a new problem on an 07 Cayman S . I've driven the car about 34k of it's 40k miles since buying it in late 2013. The car is a manual with PASM with 19" rims. Last month I got a 40k mile fluid flush, new front brakes (rotors, pads, sensors), and fresh rear tires (Bridgestone S-04s, replacing the same model. Kept the half worn fronts that have plenty of treads left). I've got most of my maintenance done at a Porshe dealer but this time selected a highly rated local European shop to do this fairly straight forward maintenance. Everything feels pretty good in a straight line or in turns that wouldn't fluster a Corolla. I'm just about to finish the rest tires' 500 mile break in period and they're starting to feel in balance with the worn fronts after feeling just a little unbalanced while the treads were still slick. Here's where the problem starts. I took the car up to my favorite mountain road a few days after the rear tires were installed and the other work was completed. The first corner on this road is a 2nd or 3rd gear right hand sweeper. Since I was still working in the tires I didn't want to push too hard so I braked early and lightly, lifted before turn in, and eased into the corner at maybe 10 mph over the posted limit. Nothing that's ever been a problem before on this road. As I swept the apex the car suddenly felt like it was passing an oncoming semi truck in a gust of wind, the car slowed momentarily and I felt a ratcheting sensation from somewhere, similar to ABS modulation at slower speeds. It occurred twice in that long corner and I reproduced twice more in similar right hand corners later up the hill. During at least one of these shudders i saw the ABS light flash while I felt the ratcheting. After that I slowed and started down the hill under the limit. Since then, I've reproduced this sensation a couple more times in right corners but only the smallest of hitches both times, not the distinctive ratcheting feeling that felt like a huge gust of wind and no ABS warning that I can see. Never left hand corners, only rights, but I just haven't got many quick left turns around me that are fast enough to try reproducing it at all. I took the car back to the service shop but he was unable to reproduce while he test drove it (probably wasn't going quite quick enough in spite of my urging) and found no error codes. Today I felt the stutter on a 65 mph freeway transition ramp while driving exactly that speed. Again, I have NEVER been braking when this occurred, only loading up the front left tire under moderate cornering in fairly long right corners. Could this be PSM kicking in when it feels irregular imbalance because of the front and new rear tires having varying wear? It doesn't feel bad but the car didn't feel as stable as usual after the tires were installed, but again I put a lot of that down to different tread depths coupled with the slippery new tire curing compound. I've yet to reproduce the amount of ratcheting as I felt when the tires had no mileage but I also haven't returned to the road where it first happened either. It's hard to believe the PSM would have interpreted that I had a dynamic problem in any of these incidents. I can't say I've ever actually known if I deployed PSM before but if it did would it feel like ABS was activated by itself? Would it engage on a dry road at only what you'd call "brisk" driving? It never felt overly aggressive or even perceptible to my knowledge if it had ever kicked in before. It's a very disconcerting feeling and is not at all confidence inspiring. The odds and behavior tell me somehow I have a problem in the braking system. However, it occurred to me that (please no, please no, please no) this could be a precursor to an IMS breakage. Luckily I do have an extended warranty, but still, I'd like to keep this engine, thanks. Had anyone experienced this before and have any idea how to address it? Sorry for the long post but wanted to provide as much info as I know about. Thanks! Sent from my Pixel 2 XL using Tapatalk

This is what will happen when the tire diameter size from front vs. rear is out of spec and the traction control (PSM) is being fooled. The rear tires are typically taller than the fronts, and system may think one of the tires is slipping when cornering if not within a specific ratio the because the RPM of the wheel is faster than what the system expects to see. Did you put smaller diameter tires on the rear when you had them changed? This would explain the car thinking the rears are spinning in corners if so, or on the other hand new taller fresh rears with worn fronts may be causing the car to think the fronts are slipping

Certainly sounds like the PSM. But shouldn't that flash a indicator on the dash? And, yes it would feel like auto ABS activation because that is exactly how the car pulls itself back on line. i.e. by tapping the appropriate brake. If it is the PSM then whether it is an out of range difference in front to rear tire diameter or traction difference is what needs to be answered. Check out your manual to see if the PSM flashes when activated. My guess is it should. Also check to see if there is a specification on tire wear difference front to rear. If not the PSM, i.e. there is an indicator and it is not lighting up, then pray. lol Good luck!

Thanks for the feedback! The new rears are the same size as the previous pair, 265/35 r19, but I guess the extra tread depth (and possibly some slip from the curing) is causing PSM kick in to correct some instability that's it's sensing. I think the ABS sensation is startling enough though that it works better to get me to lift than to bring the car in line. Lol I feel just a little offset towards the front too but not enough to make me feel stability management is required. I'll believe what it's trying to tell me though. Amazing how sensitive these cars are. It does seem to be getting better and harder to duplicate as the rears wear in a little more. I'll let you know if anything changes! Sent from my Pixel 2 XL using Tapatalk

This explanation seems perfectly plausible. I wanted to do the same with tires as OP and was told that is a no-no. It is incredible how fast and sensitive the PSM is on these cars. Hopefully the problem will disappear with the rears breaking in.

There is a roughly 1 inch/25.4 mm difference between the height of the back tires versus the front. You can change anything you want theoretically as long as this ratio is still maintained. You need to stay within about 10% or so range of factory to avoid having issues if I recall correctly.

thread depth is unlikely to be the cause if not more of us would be feeling this. PSM that I've experienced will pull you in in one swift motion rather than behave like ABS pulsing/ratcheting the following comments are w/out use of sports chrono 1. even though the fronts had plenty of thread depth left, they can understeer due to age of rubber (are your fronts still from 2013?) 2. did you do a full alignment after changing the tyres? (since you seem to be feeling this on one side more than the other) 3. have you checked for air in the brake lines? it may be wise to check/re-bleed them

I am having this same issue. Did you have to change the front tires to remedy the situation?

I have a similar issue in certain tight corners where I am slowing from a high speed. What I've done so far is go to better tires (Bridgestone Potenza RE71R) and Pagid Black brake pads. Yesterday I took the car out on the Porsche Owners Experience here in LA and did not encounter any issues. One thing to consider for me is am I braking too late where to slow the car enough I have to really hit the brakes that would cause a lock-up - thus engaging ABS.

When I am braking into the apex of a turn the brake pedal gives bounces back and feels loose for a second and disengages the brake. No abs lights or anything like that, but feels like there is air in the system and a light metallic rattle. then it goes away. I have only experienced this effect in left turns. It was fine until I replaced the rear tires and kept the fronts as there is plenty of tread left on them.