My wife has had this problem twice in the past few days. It works fine for me so I suggested that she drives another car and leaves the XK for me. That did not go over well so I need to look into it.

She said that when she has the problem starting there is little brake pedal resistance and the pedal goes almost to the floor. When pressing the start button the display says "Press brake before starting" and the car won't start, even though the pedal is pressed. She pressed the start button to turn it off and tried again several times and it finally started, the first time she had the problem. Second time it would not start and she called me. I told her to shut it off, lock it, and retry. When she opened the door to get out she got pedal resistance and it started fine.

Battery was replaced in January of this year.

I have not looked in the service manual yet. Is the brake not mechanically connected to the master cylinder and it is a fly by wire instead? Any suggestions on this one?

thanks,

 

it is manually connected, sounds like you need
to change brake fluid and bleed your brakes.


I had a similar issue with soft brake pedal, bleed the brakes first.

 

1st concern is a soft brake peddle. Check the brake fluid and see if it is low and if so you may have a leak but I would have expected to see a brake light on in the dash.

 

Fluid was changed in January 2017. Reservoir is full.

"Pedal goes down fast" is the closest symptom in the manual.
-Air in brake system.
-Leak in hydraulic system.
-Brake booster/master cylinder.
-Brake pads
Above items are listed as possible source of problem. All of these should be persistent problems - not intermittent.

I am questioning the brake pedal symptom now.

 

Reverse the peddle/button order for starting.
1. Press and hold the button
2. Step on the brake

This is the most goof proof method for starting the car.

 

Please do not be quick to dismiss the following hypothesis:
The gas pedal is being confused for the brake!

 

With a female, the above hypothesis is quite possible. No, I'm not sexist.

 

Did a test:
Without sitting in the seat, applied pressure by hand to the brake.
Functions like a normal hydraulic brake.
Eliminating the other theory that she may not be tripping the occupant sensor.

What type of floor mats do you have?

 

Hey there buddy - this is the wife replying to you direct. I know the difference between the brake pedal and the gas pedal!

I would say something more offensive in reply, but my husband is restraining me since he has some weird obsession with this forum. Kelly

 

In order to add another year to our blissful marriage I will discount Q&C's suggestion and see if the problem persists.

 

 

Master cylinder is shot.

 

I deserve that.

 

I've noticed that sometimes my brake pedal feels much softer on cold start vs others. I can't really justify it or isolate it further (colder vs. warmer, dry vs. wet or length of waiting). Never had it fault out and not start for it though. If the master cylinder had bled vacuum off the pedal would be stiff. One thing I read about some ABS systems is they have return systems where when they detect lock up they first block the pressure from increasing going to the wheel and if it still locks they then release the pressure back to the reservoir. Possibly the release valve is leaking by? It's way out there but...

Maybe double tap the pedal (but I assume she'd do that already)?

 

I use the method described in #5 above by KJo7XK, and it works perfectly for me. just press the start button and kind of tap on the brake pedal with some authority. Do not release the start button until the engine is cranked.

 

... heh heh heh.......

 

The car will not start unless the ABS module detects sufficient pressure in the braking system. So, if the pedal's going to the floor, the non-starting is definitely some aspect of the brakes, not the starter.

Does pumping the pedal result in normal resistance? That might indicate air/vapour in the system.

 

That is possible. I would think air in the system would be a persistent problem, not intermittent. Also I would expect a brake or ABS system fault to be displayed if there is something wrong with those systems that prevent starting the car.

The car is my wife's daily driver and she has not had the problem again. I will wait and see if it occurs again before I do anything more.

 

Hmmmmmmmmm, and right after the discussion about "Brake and Accelerator Pedals"........................

 

I hope this is taken in the spirit it is intended, which I realize is quite difficult to do when it contradicts one's beliefs.

Psychology plays a big part in diagnosing auto problems or even driving. I have made the mistake many times myself, where the mind is so set that one cannot possibly be wrong, it obscures all other possibilities. Particularly if it marginalizes our driving skills or an obvious error. I believe the following statement will help you with mechanics and yourself. I have never met anyone who believed they were bad mechanics or bad drivers. If you need proof of that, just look at all the road rage, where clearly neither side believes they could have made an error.