OK, 'no brakes' is an exaggeration!

 

But on the last two near-empty tanks of fuel, the brakes on my 2008 2.0l CR TDI vRS have suddenly gone funny:

 

When braking gently, there is NO feel to the pedal, no bite on the brakes but the pedal drops steadily.  The car will brake if I stamp on the pedal, but it's really difficult to modulate braking.  It doesn't feel like there is no servo assistance - the brakes don't bite at the same point but don't feel hard either.  However, if I brake sharply, they bite as normal at first.

 

The first time I noticed it, it was on low fuel with ~30 miles left.  Filled up and they were almost immediately better.

The second time, today, it was on 10 miles remaining but felt fine.  Filled up and then the brakes felt like this for the next 5 miles.  On the same trip, after a few stamps and pumps of the pedal at lights, they recovered and have been absolutely fine for a 200 mile journey since.

 

Incidentally, I did a fault code scan on the first occasion, which threw up the following.

 

Engine:  P0191 - Fuel Pressure Sensor (G247), Implausible Signal

 

Are the two related?  Any ideas why low fuel might lead to duff brakes?

 

Cheers

 

I can't think of any reason why these would be related but if I was you I would just keep more fuel in the tank and see how it goes.

 

If you keep more fuel in and it still does this then its just been a coincidence. You do need to investigate the fuel pressure sensor though.

I couldn't fathom how they were related too, but it's quite coincidental that the only time the brakes have gone bad is on low fuel.

 

I cleared the P0191 fault on the first occurrence and as far as I'm aware it hasn't come back.  Scanned again yesterday and just now and it's not there, but that was on a full tank.

 

Any advice on how to start investigating the fuel pressure sensor?

 

Thanks

44 minutes ago, mrhappy said:

Any advice on how to start investigating the fuel pressure sensor?

 

I would check the connector and see how it goes for now, could of just been a sporadic blip. 

 

Unless somebody has heard of this strange occurrence before I think its best to just keep driving it and see if any patterns emerge as to when the brakes go weird and if this matches up with low fuel every time and whether you get the fuel pressure fault come in at the same times also.

Thanks SuperbTWM, I'll keep an eye on things

 

Update:  so my brakes are permanently pants now.  No feel/modulation of pedal, brake fade but servo is OK.

 

I've had my local mechanic look at it today thinking that the master cylinder is at fault.  He's tested this (somehow - clamped all brakes lines at wheels) and says it's solid and fine.  We're both wondering what else could cause the symptoms described above?

 

Again, no faults are shown using the Carista app I have at my disposal, and he will scan using VCDS this evening. 

 

Could the ABS pump be at fault?  The car has had the ABS/ESP recall, could this be to blame?  Are there any other things to try??  All suggestions welcome, thanks!

Have you checked all the vacuum lines to the brake servo? You could also test its integrity with a vac pump although you don't see many issues with master cylinders or brake servos.

 

If there are no leaks then it does seem like a master cylinder or servo issue

You're driving a car with 'duff brakes'!? 

If Carlsberg Duff made brakes....

3 hours ago, mrhappy said:

Could the ABS pump be at fault?  The car has had the ABS/ESP recall, could this be to blame?  Are there any other things to try??  All suggestions welcome, thanks!

 

Latest thinking is the ABS pump leaking internally.  Apparently this happens.  Also on the mk60 pump there's a pressure sensor that's known to fail.

 

I don't suppose I can blame a software recall for a mechanical failure, so I'll start the hunt for a reasonably priced hydraulic unit...

I have no idea if its the same on the CR engine, but on the PD the fuel pump/servo pump is all one unit. Could it be its wearing out and so at low fuel theres some kind of air leak passing into the wrong chamber (fuel to air or air to fuel)??

I've just had the car serviced and the fuel filter was filthy.  That might explain the P0191 fuel sensor error, which hasn't come back since

22 hours ago, robt100 said:

I have no idea if its the same on the CR engine, but on the PD the fuel pump/servo pump is all one unit. Could it be its wearing out and so at low fuel theres some kind of air leak passing into the wrong chamber (fuel to air or air to fuel)??

 

CR engine does not have a tandem pump 

Just to wrap this up: the fault was with the abs pump, which can fail internally and lead to a spongey pedal. I bought a second hand hydraulic unit and module on eBay and had it fitted and bled properly by my garage. 

 

The unit I bought had part numbers 1K0 907 379 BL (controller) and 1K0 614 517 DP (HU/pump), which was different from my old one. This was fine, my garage coded it in and I gained hill hold and XDS electronic diff lock, which is nice.

 

All together it cost me about £165 fitted and feels great to have good brakes back. Unfortunately now the second abs ring of the year has failed at the rear, so it'll have to go back to the garage in the new year...

Edited 28 December, 20187 yr by mrhappy

Teeves MK60 issue?

 

 

20 hours ago, silver1011 said:

Teeves MK60 issue?

 

It's probably unrelated to the MK60 issues as the part number they have a MK60EC1. Not common to hear of a MK60EC1 failure though :/