Fluctuating brake pedal pressure affecting car performance

[Richie]

Desperately looking for some help ......

I'm having some issues with my car for some time - the pressure in the brake pedal keeps fluctuating:

Pedal goes from being fairly hard with little room for movement - when pedal is like this steering feels heavy , accelerator pedal not as responsive & car feels like it's under more pressure to perform, it requires higher revs to build up speed & gear change is slow.

On the other side the pedal goes to being softer & bouncier with greater room for movement - when in this mode the cards performance is far superior - car is really responsive , steering feels so much lighter as does the accelerator pedal. When in this mode when u hit the accelerator the car reacts immediately & will race up through the gears & not under any great pressure.

A few different garages / mechanics can't seem to get to the root of problem - master cylinder has been replaced but still has not corrected the problem.......

So hoping someone in this forum has had similar problem or if not recognise the symptons .......

It doesn't matter if car is hot or cold - this problem can occur @ any time & can literally change from one mode to the other if u were stopped @ the traffic lights .........

It seems very strange & is really annoying - feels like I'm driving two different cars & it's horrible to drive in the first mode described above ..........

Lastly (& this could be a pure coincidence as hard to see a relation between the two) but when the car is low on petrol it perform well & generally doesn't have the "harder inflexible " pedal ......

Any help would really be most appreciated ......

[KiDo_Tuning]

Desperately looking for some help ......

I'm having some issues with my car for some time - the pressure in the brake pedal keeps fluctuating:

Pedal goes from being fairly hard with little room for movement - when pedal is like this steering feels heavy , accelerator pedal not as responsive & car feels like it's under more pressure to perform, it requires higher revs to build up speed & gear change is slow.

On the other side the pedal goes to being softer & bouncier with greater room for movement - when in this mode the cards performance is far superior - car is really responsive , steering feels so much lighter as does the accelerator pedal. When in this mode when u hit the accelerator the car reacts immediately & will race up through the gears & not under any great pressure.

A few different garages / mechanics can't seem to get to the root of problem - master cylinder has been replaced but still has not corrected the problem.......

So hoping someone in this forum has had similar problem or if not recognise the symptons .......

It doesn't matter if car is hot or cold - this problem can occur @ any time & can literally change from one mode to the other if u were stopped @ the traffic lights .........

It seems very strange & is really annoying - feels like I'm driving two different cars & it's horrible to drive in the first mode described above ..........

Lastly (& this could be a pure coincidence as hard to see a relation between the two) but when the car is low on petrol it perform well & generally doesn't have the "harder inflexible " pedal ......

Any help would really be most appreciated ......

2004 LGT or non turbo?

LGT will be the MAP sensor vacuum hose near the alternator.

[Richie]

Hi Mate ..Cheers for reply

...standard 2.4 non turbo.

Would your theory not stack up for non turbo's ...?!

Thanks

[bigBADbenny]

2.5L auto or manual, abs or vdc?

[Richie]

Automatic - ABS (never heard of VDC)......

3 garages have all failed me ......when they hook up to computer no faults come up so they say no problem ........but as problem is intermittent it never plays up when it's @ garage ......

[Richie]

In addition to that - subaru replaced the master cylinder (charged a fortune) but it made no difference - problem re occurred

they showed the car when up on the hoist & all the wheels had locked up..

I've had the transmission deleted and reloaded as well as they thought it was a gear box problem but again this still didn't resolve.....

problem can occur at any time .....can go weeks without happening or can happen daily - no pattern

[bigBADbenny]

Maybe look at the vac hose that goes to the master sylinder as Matt suggested...

[KiDo_Tuning]

Will be an intermittent MAP sensor issue Seen it in the 2.5i 2007 model

Test procedure: Remove the vacuum hose from the MAP sensor and put a small screw in the Vac hose to stop any air leaks. Take car for a drive. Should replicate your issue.

[Richie]

Cheers guys for the info - will swing past mechanic on way home tomorrow to see if they checked for this in past ......

If this is the case would it have come up previously on diagnostics ?!?

Again if MAP is the problem - How much should it cost to fix ?!?

Thanks once again for assistance ........

[bigBADbenny]

I just googled this:
The ECM has a sensor value which it compares to the MAP Sensor. When the readings are different, the code is triggered. So you either have a bad MAP or a bad ECM. Try checking the wiring and contacts with a DVOM and make sure there is continuity and no resistance between the two. Most common problem is the MAP.

Are you getting a check engine light or just poor performance?

[Richie]

Poor performance - on way to work now & it's v poor ......so dead - no acceleration @ all - just a slow / gradual build up of speed with rev counter behaving in same fashion & car sounding under pressure in the process .......put too much weight on the accelerator & car will automatically hit 6 or 700 Rpms ?.....

Silly question but running low on fuel could not have an impact on MAP sensor ?!? Just a coincidence that it happens not to occur @ this time ?!?

[KiDo_Tuning]

I just googled this:
The ECM has a sensor value which it compares to the MAP Sensor. When the readings are different, the code is triggered. So you either have a bad MAP or a bad ECM. Try checking the wiring and contacts with a DVOM and make sure there is continuity and no resistance between the two. Most common problem is the MAP.

Are you getting a check engine light or just poor performance?

Actually if the MAP sensor value is 1bar absolute(aka 0psi) with the Vacuum hose off, no codes are thrown. We actually had Raph's car in WA do it in during a 3 port install. Saw it straight away in the logs, car drove horribly for the first 30 seconds... pulled over, connected Vacuum hose and suddenly the car was alive again. Throttle tries to make target by shutting the throttle to meet vacuum targets etc.

[bigBADbenny]

Awesome explanation.

Is the MAP aka Sensor assembly-intake air, 22634KA071 ?

Or on the throttle body?
Eg:
Sensor assembly-pressure
22627AA200
alternate: 22627AA330
03, May — '04, May 22627AA220

http://opposedforces.com/parts/legacy/e ... tration_5/

[Richie]

Stopped by garage last PM - regular chap that works on car is on holiday & another lad there .....he is familiar with the issues I'm experiencing .....he said that @ last service the MAP & ECM were checked but not "the wiring" .......so booked in Thursday to check this.

Interestingly again driving the car today & fuel is after cOming under half tank mark & the performance is excellent ..........

[Richie]

Hi ,

So garage have checked out the MAP etc & the wiring .....

Still doesn't feel great - feels like it's still restricted .....& being held back ...

Any ideas as where too next ??? Brake booster was mentioned - could the brake booster cause car to behave in this fashion ?!?