Hard Break Pedal And Stalling Engine?

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

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!