Hard Break Pedal And Stalling Engine?

I took my 98 S40 with 160,000 miles (I know its a lot!) into my local
garage yesterday as my break pedal went hard when driving.

Volvo (who I took it to first) thought it may be a locked break
caliper but the garage checked it over and told me there was an air
leak from the vacuum hose and that was causing the pedal to get hard
and making the idle on my car drop down to around 500 whenever I
slowed down for traffic or lights.

So they changed it and I picked my car up and drove off and the pedal
was soft for a few minutes then went hard and then within 15 minutes
my revs were dropping to 300-400 RPM and then I started stalling!

I must have stalled 10 times - everytime I slowed down - although when
I got on the main road with no traffic the revs seems to be idling at
800ish.

So what is going on? my car was NEVER stalling before they changed the
vacuum hose!

and when I have been driving fast for 20 miles it seems to be idleing
normally? but if I was sitting in traffic I am sure it would start
stalling!

The garage didnt know what it was and said they think there is
probably another air leak somewhere causing it!

Does anyone have any ideas what it may be? thanks.

 

Sounds like you have more than one thing that has failed. My first
thought was that your vacuum booster had failed causing a heave brake
pedal and letting extra air into the intake. I think the hose replaced
probably needed to be replaced but so does the brake booster.

Chuck Fiedler
Nothing but Volvo since 1974

 

Yes, it may be the fitting where the hose goes into the brake booster,
or the check valve (sorry, dont' have a 40 so I'm not sure if it's an
in-hose or integrated with the booster fitting). The other possibility
is the booster itself.

Bill

 

ok the break booster or check valve for the break possibly? thanks!
but what about the stalling engine?

obviously I want working breaks but if my car keeps stalling I won't
get very far? what is causing that?

and why has it started happening since the hose was changed?

 

Actually, it started happening when the brakes went hard. This is just the
way it progressed as teh tear in the diaphragm got larger. Here's the big
clue: how does the engine know when the brake pedal is pressed?

When I read the original post, it screamed "brake booster" to me. Not the
check valve, either :-(
What is happening is that the booster is not using the vacuum to maintain
pressure across the diaphragm, but instead is allowing a bunch of air to
leak into the intake manifold through the hose that was replaced. When you
step on the brake your foot is also off the accelerator, so even a modest
amount of air - introduced through the booster when you stepped on the
brake - is enough to cause some serious stumbling. If you pull that now
useless hose off the booster and tape the end closed the brakes will feel
just as they do now and the engine will not react any more.

The booster has to be replaced. The Volvo price for boosters is usually
frightening, so you may want to explore aftermarket and wrecking yard
sources. RockAuto.com doesn't list a '98 S40, but their price for a 2000 S40
http://tinyurl.com/2nj7ts is $292. The master cylinder has to come off to
replace the booster, and I'd consider it likely the end of the MC that bolts
to the booster is leaking fluid. If that isn't fixed the new booster won't
last long, either.

 

Thanks for that advice I am slightly confused though.

what do you mean by the pressure against the diaphram? what diaphram
do you mean? where is it?

The vacuum hose is a new hose so why would that make the problem
worse? remmeber they changed the hose and it seemed to solve the hard
pedal problem, but shortly after I collected the car
the pedal went hard and the car started stalling.

How would I explain this to the garage in a manner that they would
understand? as they seem pretty clueless! thanks

 

Inside the brake booster, across the large cylindrical part, there is an
elastic diaphragm. Vacuum from the engine's intake manifold is applied
through that hose and the check valve to hold a vacuum on the front side of
the diaphragm. When the brake pedal is released, a valve inside the booster
closes off a port between the rear of the diaphragm and the outside air, and
allows the vacuum to enter the rear side of the diaphragm so there is no
pressure across it. No pressure, no push on the master cylinder, no brakes
on.

When the brake pedal is depressed the valve inside changes things around.
The front is still on engine vacuum, but the rear is closed off from vacuum
and the valve meters the amount of outside air into the chamber on the rear
of the diaphragm. The higher pressure pushes on the diaphragm, which pushes
on the master cylinder pushrod (harder than you push on the pedal, which
provides the "boost") and the brakes are applied. You feel the spring in the
valve when you press the brakes, while the pressure difference across the
diaphragm pushes on the brakes.

Your problems started when the diaphragm developed a tiny tear. It doesn't
take much to ruin the vacuum, and you felt that as a hard pedal. At that
stage there was already air being sucked through the tear into the intake
manifold, but not enough that it upset the engine. You took the car in for
repair, and they replaced the hose. Didn't hurt, may have helped
momentarily... but the tear grew after that. Now when you press the brake
pedal that valve still lets air into the rear side of the diaphragm.
Unfortunately, the air now goes through the tear in enough volume it is
affecting the engine.

The garage was hoping for the best - they found the hose in bad shape and
gave it a go. You'd have needed the hose anyway, and the symptoms at that
point were also consistent with a bad hose or bad check valve. It wasn't
until the engine started stumbling that the booster could be declared bad.

Mike

 

I know this may seem too simple but have you lifted the hood and made sure
the hoses are still connected? : )

 

The car is actually with the garage so I would like to think they
checked that it was attached!

The damaged break booster makes sense to me although it is as you say
expensive so its a shame if it is that!

Just two quick questions which will seem stupid now - but from what I
said is it therefore inpossible that the heavy break isnt a stuck
caliper which is just coming unstuck now and then? and that the
stalling is being caused by a dirty "butterfly" which is causing the
revs to drop so low?

I realise it would be an awfully big coincidence and I havent asked
the garage if thats the case but I am wondering? thanks.

 

The car is actually with the garage so I would like to think they
checked that it was attached!

The damaged break booster makes sense to me although it is as you say
expensive so its a shame if it is that!

Just two quick questions which will seem stupid now - but from what I
said is it therefore inpossible that the heavy break IS a stuck
caliper which is just coming unstuck now and then? and that the
stalling is being caused by a dirty "butterfly" which is causing the
revs to drop so low?

I realise it would be an awfully big coincidence and I havent asked
the garage if thats the case but I am wondering? thanks.

 

The critical part is: does stepping on the brake cause the engine to change
a lot? If not, there could still be other explanations. Otherwise....

The garage will probably do the definitive test on the brake booster -
connect a hand vacuum pump to the hose fitting on the booster and verify it
will hold vacuum. Next, repeat the test with a helper's foot on the brake.
If the booster diaphragm is okay, it will still pump down. If the diaphragm
is torn, the vacuum will leak off as fast as it can be pumped up.

Mike

 

I got the car back from the garage today - they say they tracked down
another air leak somewhere else and that caused the pedal
problem and stalling.

The car seems to be idleing better although I am going to take it for
a longer drive tomo and see if it doesnt stall.

But the break still feels hard - not as hard as before but firmer than
it did since I have had the car! the garage say this is ok and normal
and my fathers friend who knows about cars says that it feels better
as its firm... but it never felt like that before so I am worried
about
the breaks!

I called out my breakdown service tonight as I wanted an unbiased
opinion of the problem from someone who can look at the car but does
what the garage told me sound like it makes sense? do volvo breaks
feel firm normally or soft? surely if my breaks were too soft before
then
when the car has been in the garage and checked many times (including
gearbox and engine issues) then the garage would have said my
breaks feel too soft and are not normal?

what is going on? who is right? someone clarify this for me please!

 

Ok I had the RAC here (breakdown people) they checked the vacuum on
the engine and the guy (who was very good and helpful) said he was
happy with it
and it felt fine to him - we then went for a drive and he said the
breaks were firm but that was what they should feel like!

I asked him why they were soft before and he told me he didn't know
but it sounded like there was a problem with them before and now they
are good... I trust what
he said but thats just wierd that they would be soft and now they are
firm and work well?

anyway I will see if the stalling is sorted by driving around a bit
more tomorrow - if that is ok and they said the breaks were ok then I
will assume all is well but
its just strange that they would feel so different now than before!

 

Air in the system will make the *brakes* feel mushy, a good setup should
be firm, but 240 brakes are usually quite sensitive so you should not
have to depress the pedal very hard to stop the car.

 

I have an S40 - and I did have to depress it far to break before - it
did feel soft and mushy although I liked the fact that it didnt jerk
around when i breaked

the fact is the breaks are sensative now and not much pressure breaks
the car! the guy actually removed the break servo/booster vacuum tube
from the car to show me how hard it was to break without it and it
was very difficult!