Serious Brake issue: ABS? Stiff pedal, engine at high rpm?
07-30-2005, 01:49 PM
#1
Serious Brake issue: ABS? Stiff pedal, engine at high rpm? Here's the situation: At the autocross last weekend, beautiful weather, clean track. Running on Kumho 710s, standard size 17x9s. No suspension or brake mods on my car. I have the engine fully up to temp before the first run. Traction control off.
On the first run out, we have a couple slow speed turns, then a high speed sweeper with a pretty sharp decreasing turn at the end. As I come through the sweeper/straightish area, I think I'm pretty far up in the revs in 1st gear, maybe yellow line for a couple seconds. I mash on the brakes hard for the turn, and the pedal is EXTREMELY stiff. I mean the sort of stiff that it took all my effort and concentration (and there was hardly any pedal travel). It also seemed that the engine wasn't spooling down like I expected it would. It seemed that maybe the engine was keeping itself running faster than it normally would when I was not on throttle. I don't left foot brake and I do not think that I had my foot somehow still on the gas (but things were happening fast so I couldn't register everything that might be going on). In another turn the brake still felt extremely stiff. At the end of the run I had no ABS lights on nor any other engine oddities occur.
On my second run I had a similar event occur at the same hard turn, but the pedal was not quite as stiff.
On my third run I decided to try shifting to second to not go so high in the revs, and that seemed to make the stiff brake/engine overzealousness subside. I am not sure if I was driving significantly smoother aside from shifting to 2nd and down to 1st again for another turn. By 3rd run the tires were getting pretty warm. My times got better and braking was more normalized each run. 3rd and 4th runs the brakes were fine and 'normal'.
My questions are: Was I just trying too desperately to dive on the brakes? Does the ABS system actually increase pedal pressure that much in a turn? (I know it sort of does a alternating wheel grab in straight line sometimes but the pedal is not that crazy stiff.) Is there some sort of engine programming that keeps the revs up in a 'panic' situation to keep vacuum on the brake system, or also does the ABS system behave differently in hard turns compared to straight lines? Do the brakes behave so much differently when nearly cold that it could account for this type of behaviour?
Any thoughts, theories, speculation appreciated. -Chris
On the first run out, we have a couple slow speed turns, then a high speed sweeper with a pretty sharp decreasing turn at the end. As I come through the sweeper/straightish area, I think I'm pretty far up in the revs in 1st gear, maybe yellow line for a couple seconds. I mash on the brakes hard for the turn, and the pedal is EXTREMELY stiff. I mean the sort of stiff that it took all my effort and concentration (and there was hardly any pedal travel). It also seemed that the engine wasn't spooling down like I expected it would. It seemed that maybe the engine was keeping itself running faster than it normally would when I was not on throttle. I don't left foot brake and I do not think that I had my foot somehow still on the gas (but things were happening fast so I couldn't register everything that might be going on). In another turn the brake still felt extremely stiff. At the end of the run I had no ABS lights on nor any other engine oddities occur.
On my second run I had a similar event occur at the same hard turn, but the pedal was not quite as stiff.
On my third run I decided to try shifting to second to not go so high in the revs, and that seemed to make the stiff brake/engine overzealousness subside. I am not sure if I was driving significantly smoother aside from shifting to 2nd and down to 1st again for another turn. By 3rd run the tires were getting pretty warm. My times got better and braking was more normalized each run. 3rd and 4th runs the brakes were fine and 'normal'.
My questions are: Was I just trying too desperately to dive on the brakes? Does the ABS system actually increase pedal pressure that much in a turn? (I know it sort of does a alternating wheel grab in straight line sometimes but the pedal is not that crazy stiff.) Is there some sort of engine programming that keeps the revs up in a 'panic' situation to keep vacuum on the brake system, or also does the ABS system behave differently in hard turns compared to straight lines? Do the brakes behave so much differently when nearly cold that it could account for this type of behaviour?
Any thoughts, theories, speculation appreciated. -Chris
07-31-2005, 02:23 PM
#2
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Seems like maybe the booster had a loose hose and
got bled down (up), the makeup vacuum acts like a
vacuum leak? Might check that the vac booster hose
is all nice and tight, all the way back to the manifold
(maybe soft grommets need a dab of grease, etc.).
Might've been that a little more than usual cornering,
engine rolling on its mounts from engine-braking, or
whatever tugged things a little out of bed.
got bled down (up), the makeup vacuum acts like a
vacuum leak? Might check that the vac booster hose
is all nice and tight, all the way back to the manifold
(maybe soft grommets need a dab of grease, etc.).
Might've been that a little more than usual cornering,
engine rolling on its mounts from engine-braking, or
whatever tugged things a little out of bed.
