just pushed down hard on the accelerator while at around 50 mph to about 70, and then started to move out of the fast lane, and as i did so backed of the accelerator but she kept accelerating, i touched the brake and it was rock hard , i pushed down and may be twice more before there was that felt through the brake pedal like a metallic clunk and the brakes responded and the power
dropped back off, fritt the living daylights out of me , so have just been looking over the various threads and can see that this seems very similar to what others have experienced but in various different models, but cant find an explanation ,, any one have any ideas what just happened

 

Maybe a loss of vacuum - thus the hard brakes - in which case go hunting for a leak & maybe to do with brake servo.

 

Hi JAGV8, its been fine since yesterday, I took her out this morning and just pottered around up and down the dual carriage way, and first thing I did was set my cruise control at 25mph, did it a few times just to be sure , then when there was no traffic tried to recreate, sounds stupid to try but had to know, but nothing everything seems 100%
6 weeks ago I had both fronts off and replaced the discs and pad and gave the callipers as good service and they also spot on after having done them, its the combination of the acceleration and no brake that did my head in, but as said nothing similar , can you think of any combination that would firstly cause the power to keep increasing even when no throttle being applied and to simultaneously cause the break pedal to harden, the only time when the brake is hard is without the servo on , IE before you start the engine, IE with no power, ignition off no key in, nothing I can think off makes any sense !!

 

Loss of vacuum.

 

+1 on that, or is it +2 since JagV8 has already had to repeat himself?

Also +1 on the possibility of a vacuum leak at the power brake booster thingy.

Keep in mind the engine computer is very smart but simultaneously very stupid. Should there be a vacuum leak, the suddenly lean mixture will be sensed by the O2 sensor. The computer responds in full stupid mode by adding more fuel to compensate.

At the same time, the vacuum leak reduces the boost of the power brakes, and you feel a hard pedal. Normally the power brakes maintain a couple of boost applications even after a loss of vacuum "input". Since you lost the power brakes at the same time as the increase in RPM, the power brake booster is the most likely source of the vacuum leak.

 

Just to mention it adds the fuel in a desperate attempt:
1. to keep the engine running (so as no to leave you stranded)
2. not to let it destroy itself by actually running lean

A loose/perished/etc hose/joint is the most likely I suspect.

 

Here's a quick check you can do to make sure your power brake booster is holding a vacuum "charge". Remember, the whole idea of this reserve is to make sure you still have power brakes (generally about 2 applications) after any loss of vacuum, such as an external leak or an engine stall. This lets you get stopped safely.

Let the car sit several hours. Before starting the engine, step on the brakes. The first application should feel normal, as the booster has a check valve to maintain a vacuum charge even with no vacuum supply from the engine.

With the engine still off, press the brake pedal again. The brakes may feel a little harder, as you are using up the vacuum charge and may have limited assist.

Try a third time and the brake pedal should feel hard, as you've used up all the vacuum reserve.

Now start the engine. Immediately vacuum should return to the booster and the pedal should feel normal with power assist available again.

If this test passes, there could still be other issues with the brake booster, so this is not conclusive it's good. If the test fails, however, and you can confirm the booster has a good vacuum source, then the booster is bad.

 

+1
good idea

If it's not that, go hunting for a hose/joint that can move around and make a leak.

 

Another possibility, although somewhat remote, is the presence of two faults:

1) Some unexplained (non-vacuum related) fault caused the RPM surge.

2) The check valve in the brake booster is not holding a vacuum, but this has gone unnoticed for who knows how long. Normally when you apply the brakes, the engine is at idle, so plenty of vacuum is available and the brakes would feel normal. You're not likely to realize this fault is present unless you specifically look for it. This isn't actually a leak, so the computer wouldn't inadvertently overcompensate. When your RPM surged, manifold vacuum would have dropped and you'd have lost power brakes if the booster check valve was stuck open.

I still think a vacuum issue is the one root cause of both problems. Just keep the two fault theory in the back of your mind if you don't find a vacuum leak that could have caused the engine surge.

 

KR98 / JAG V8
Thank you both for the Input, much appreciated and very informative, I will test as advised & report findings

cheers guys
Steve

 

Hi, KR98 / JAG V8
undertook your suggested check 4 times now and everything is just as you said, no problem at all....
but there was one thing that you mentioned that caught my attention, running lean ,
the one thing that I did not mention is that I dont run with standard fit airbox, I run with
cone filter only, but full connection to the MAF sensor, could that have been the cause, as I say it was and has been the only time it has happened since owning her in the last
3 months, just a thought,

 

Go back to the standard one as you now have a hot air intake with known faults instead of the good OE cold air intake without faults.