Brake/inlet manifold vacuum issue

[Fabiafail]

Hi all, thought i'd start a new thread on this. 

Can anyone explain how the vacuum on a Fabia 2002 1.4MPi is created at the inlet manifold. i always thought it was created by the induction of the engine. However, i have heard talk of a vacuum pump on other threads & engine types.

 

My problem is listed on this thread:

 

http://www.briskoda.net/forums/topic/288904-engine-management-light-map-sensor/

 

Going by what has been happening to the car, I think there is an issue with the vacuum circuit. 

 

Before I removed the throttle body, to clean it, the ESP/TCS warning light had been coming on and going off intermittently for a few weeks and my girlfriend had mentioned that her brakes didn't operate/ feel right all the time. To me, this suggests a failure in the vacuum circuit. 

I'm not sure where to start. I checked (blew through either end.)the non return valve in the vacuum line to the servo and it seems ok. All the vacuum hoses looked in good condition, but I taped them up just to be sure.Is it possible that the brake servo has failed, causing all the vacuum to be lost? how would/could I test this theory?

 

Cheers in advance

 

 

[rum4mo]

In answer to the first part of your question, compression ignition engines - ie TDI etc, run without a throttle plate in the inlet side - they end up with an auxiliary vac pump for the brake servo. Spark ignition, ie petrol engines have a throttle on the inlet side and tend to use inlet manifold depression to provide a vacuum to the brake servo

[Fabiafail]

Thanks for clearing that up  . So, its quite a basic circuit. I'm still looking for a source of vacuum leak. one thing that occurred to me is: What does the ESP/TCS system measure to generate the warning light?

[Fabiafail]

Oh and how does vacuum effect its function?

[waool1988]

After you depress the breaks they need vacuum to travel away from the disks. When they don't have sufficient vacuum in the inlet manifold they don't.

 

Just find that leak and hopefully everything would be good again. Don't drive the car a lot though. The engine is running lean which isn't good.

[Fabiafail]

Right, think I may be getting somewhere.. without a doubt it's a vacuum issue.

1) I've removed all the hoses and throttle body (left the plug connected). 

2) Blew and sucked through all the vacuum hoses, they seem to sustain the vacuum created by sucking and the non returns work as they should.

3) taped up all the hoses with insulating tape (just to be sure & as a temporary measure)

4) put high vacuum grease on all the nipples to ensure a good seal with the hoses.

5) cleaned the mating faces of the throttle body and manifold and rebuilt using gasket and hylomar(PL32).

finished rebuilding and started the car. no fault light showing (no EML or TCS/ESP lights). After a few minutes the lights came back on.

 

I decided to test the brake servo.

1) switched off the engine and pumped the pedal a few times

2) kept the pressure on the brake pedal and started the engine.

3) brake pedal dropped a couple of inches (as it should with the vacuum assist)

4) did this a couple of times to double, triple check

5) after a few times i left my foot on the brake pedal, turned the engine off and on again.

6) after the second time doing this, keeping my foot on the pedal (applying pressure all the time), both the EML and the TCS light went out.

7) the TCS light came back on after about 20-25 seconds.

8) I repeated this about 10 times to check its concistency. Everytime the results were the same.

9) I also found if I did steps 5 and 6, then took my foot off the pedal straight away, the TCS light came on after only 4-6 seconds.

 

The only difference I can think of is the volume of air. In step 7, because the pedal is depressed there is a smaller amount of space on the vacuum side of the diaphragm so the total size of the vacuum circuit is smaller. whereas in step 9 the pedal is release, the diaphragm moves back and the volume increases, causing a faster collapse of the vacuum in the circuit.

 

having eliminated all the usual suspects, I am drawn to conclude that the diaphragm in the brake servo may have a split in it.

 

I've had enough, the car will be going to a garage to get vacuum checks carried out. initially on the brake servo and then on the other components.

 

If anyone has any comments, feel free.

i'm open to all/any suggestions

cheers

and thanks for listening to my constant bleating

[Breezy_Pete]

...

 

The only difference I can think of is the volume of air....

The signals received by the ECU from the brake pedal switch will differ too, no? Edit: Or at least the timing between signals.

[Fabiafail]

In step 7 the signal received from the brake pedal switch doesn't change because the brake pedal remains pressed (switch closed) and yet the fault returns. In step 9 the signal received changes because the brake pedal is released (closed to open) and the fault returns.

 

So with the switch either open or closed the fault returns.

 

cheers

[felicia16v]

have you tried blanking the pipe for the charcoal canister off, it has a valve attached to the coolant bottle and these sometimes stick open and cause this fault.

 

also unless you have had the throttlebody readapted with vcds or similar you will get the same fault and little vacuum as it is open too far.

to eliminate the servo try blanking the pipe off to it and see if the light still comes on after 20 seconds etc.