Need Some Help Please: Brake Issue

[zkahle]

So, on my ride home I noticed that something was off with my brake feel when coming to a complete stop from city driving speeds. When decelerating the car feels normal but once that car hits exactly 10-11mph the breaks feel as if they are pulsating or grabbing which causes a slight jerking motion in the car. Below or above the 10mph mark the car is completely fine when decelerating. I checked the tires and all of them have the correct tire pressure, and they are new tires so wear is not a issue. I was also driving the car for the majority of the day with no issues at all. However after dinner I began to drive home and immediately noticed the issue. Asides from lowering springs and 18x9.5 rims which were installed over 5k miles ago I have no other modifications to the suspension or to the braking mechanism on the car. My FRS is a 2013 and has 18k miles with no track time what-so-ever.

Any and all ideas are welcome as I have absolutely no clue what could be wrong with the car. PLEASE and THANKS for the help!

[strat61caster]

When you are off throttle decelerating the injectors shut off to save fuel, around 10 mph in 2nd gear the engine is spinning too slow to be smooth so the injectors turn on again to maintain idle and prevent a stall. In my car this feels as you describe, a small lurch as the engine starts generating power again to stay spinning often resulting in a bit of oscillation as the idle tries to stabilize. An automatic transmission will smooth all these out but the MT is a direct mechanical connection so the effect is amplified, my old pickup did the same thing.

Dead giveaway: watch the instant mpg meter, you'll see it reading real high, probably even max at 127mpg and around 20 mph it'll start dropping and will continue to drop until it stabilizes around 10 mph if you allow the car to coast there.

At least my two cents, best of luck.

[Ultramaroon]

What strat said. My thoughts exactly as I was reading your description, OP.

I suggest experimenting with disengaging the clutch. If it's as we suspect, your lurch will magically disappear.

[CSG Mike]

any noises? Grinding? Pulsing in the brake pedal?

Are your tires inflated correctly?

[zkahle]

Originally Posted by CSG Mike

any noises? Grinding? Pulsing in the brake pedal?

Are your tires inflated correctly?

I should have mentioned I drive an auto... Sorry guys.

[zkahle]

Originally Posted by CSG Mike

any noises? Grinding? Pulsing in the brake pedal?

Are your tires inflated correctly?

No noise at all, no pulling to any direction, and no difference in brake pedal feel.

[roddy]

It could be slightly warped rotors, though you would usually feel that at higher speeds as well.

[swarb]

Jack the car up. Try to rotate the front wheel and see if the brake pad/caliper is dragging. If not, pull the wheel to see if the inside of the wheel/tire is damaged from rubbing.
Also what is you alignment settings? Camber settings? Camber bolts?
Are you lowered? Which springs?
Wheel offset?

[zkahle]

Originally Posted by swarb

Jack the car up. Try to rotate the front wheel and see if the brake pad/caliper is dragging. If not, pull the wheel to see if the inside of the wheel/tire is damaged from rubbing.
Also what is you alignment settings? Camber settings? Camber bolts?
Are you lowered? Which springs?
Wheel offset?

Thanks for the input, I'll put the car on a jack later. My wheels are 18x9.5 +38 ambit re08.2s the springs are Eibach sportline. I have yet to get a alignment, camber is fine but the toe is a little out of both of the front tires, I have no camber bolts or plates. I have periodically checked for rubbing on the inter part of the rim and tire for the front tires as there is only 3mm of clearence between the wheel and the spring assembly, but to date there is no rubing what so ever.

[zkahle]

The only reason I haven't gotten a alignment is I've been swamped with classes and I mainly only do city driving. Thus I though I could get away with not getting a alignment till I had some time. Is it that bad that I haven't gotten one yet, the wear or my tires seems absolutely normal.

[zkahle]

Just drove back from my volunteering shift, and noticed something important. The car experiences this jerking motion when deceleration regardless if i am using the brake pedal or not. I wanted to see what would happen if I was going 15mph and just let off the gas and let the car decelerate on its own. Well... when the car hit 10mph on the dot, the car jerked a bit as if the brakes were being applied. Also I checked and there is no sign of damage from rubbing on the tires or wheels. This is indeed very weird. Any more thoughts or ideas?

[Ultramaroon]

Originally Posted by zkahle

Just drove back from my volunteering shift, and noticed something important. The car experiences this jerking motion when deceleration regardless if i am using the brake pedal or not. I wanted to see what would happen if I was going 15mph and just let off the gas and let the car decelerate on its own. Well... when the car hit 10mph on the dot, the car jerked a bit as if the brakes were being applied. Also I checked and there is no sign of damage from rubbing on the tires or wheels. This is indeed very weird. Any more thoughts or ideas?

I'll guess your toe is off and you're scrubbing pretty badly. Tires start to hook up below a certain speed and that's what you feel. Total armchair quarterbacking here.

[zkahle]

Ok great, Ill schedule a appointment for a alignment at a local shop this week. Will update you guys to see if this fixes the issue. Hope that it is indeed the case.

[zkahle]

Originally Posted by Ultramaroon

I'll guess your toe is off and you're scrubbing pretty badly. Tires start to hook up below a certain speed and that's what you feel. Total armchair quarterbacking here.

I was talking to my buddy and he thinks the issue is related to my Tomei catless headers that I installed earlier in the week. He think the car is running lean, which may explain the "jerking" motion. What do you guys think? Being a car noob, I had no clue that the headers could have been contributing to the issue.