My Volvo sucks in but can't hold it!

Hi all,

I've been lamenting ceaselessly about a vacuum leak but have been
unable to clearly identify where it is. I believe I am getting closer
based on observations from driving with the radio off and all the
windows rolled up so I'm calling on you all again.

I know there's a metal vacuum can in the engine compartment but is
there also a vacuum can of sorts in the cabin? If so, is it connected
to the accellerator?

Here are my observations (please forgive the mixed syntax as a vacuum
is an absence of matter and I treat it like it's the opposite):

1. When I step on the accellerator I can hear a short hiss as vacuum
builds up. [This indicates to me that the vacuum bottle is "empty" and
is being "filled" with vacuum.]

2. When I coast and brake I cannot hear any vacuum hiss, but when I
again step on the accellerator after a stop I hear the brief hiss
again. [This indicates to me that the rate of vacuum "leaking out" is
slow but constant.]

3. When I step gently on the accellerator I can keep the hiss
constant. ie. [rate of vacuum applied = rate of vacuum loss.]

4. If I release the accellerator before vacuum has built up (eg. part
way through step 3) the car will do one of two things:
4a) Stall.
4b) All the dummy lights will flash, the engine will falter but then
recover and idle normally. This is a near-stall with RPMs dropping to
just above zero and then recovering.

This problem is most evident in stop-and-go traffic or where gentle
accelleration is the norm. When I stomp the gas there's rarely a
problem (although sometimes RPMs take a short, sharp dive before
climbing normally).

There are probably a dozen feet of hoses that could be failing in this
scenario but, since I can hear it so clearly, I have to assume that
it's happening inside the cabin so I'll start with what's in there.

Any suggestions on where I should start checking first? It's a pretty
awkward place to work as I have trouble jamming my portly torax under
the wheel to see what's going on.

Suggestions are, as always, very much appreciated.

blurp
ps. I believe (hope) that resolving this will address my
cold-start/warm-start issues, my cruise control problem, and my rough
idling problem all in one shot. If not then hopefully it will get me
closer to a complete solution!

 

Right, forgot to mention the car is

1983
240 Turbo
330,000 km
Auto
Sedan
K-Jetronic
my baby

Thanks again,
blurp

 

sorry blurp - you are probably hearing the turbo boost
and not the vacuum. I think the cruise problem is electric,
at least on the newer models it is, ask someone local to
show you what the one way vacuum valves look like and
replace the faulty ones. When you accelerate - there is no
vacuum with a turbo. That is the reason for the one way valves,
they keep vacuum in the system when the turbo is in the
boost mode. Should be a $10.00 fix.

 

Hank,

I am 100% positive that I can distiguish the sound of the turbo from
the sound of a vacuum. The nail that secures this observation is the
hissing sound of vacuum leak that can be heard when you park in a
quiet place and shut the car off... with the engine not turning (ergo
turbo not turning) you can hear a short 'SSSsss...' In fact in all
instances the sound is evident when the turbo is doing little or
nothing.

I realize the turbo may not be vacuum dependant but the rest of the
system is. The other information you mentioned is, however, very
useful. Based on what you've written I can perhaps draw closer to
assuming it's one of these one-way valves that I hear leaking.

Again, the problem is not the turbo nor is it in accellerating. The
problem is idling and stalling when engine RPM are dropping to and
through the idle speed.

How do I determine if the one-way valves are faulty? I believe I have
some idea of how they look (like a little plastic pilgrim-hat in a
metal clip) but I may be way wrong. I think I have seen a red and
yellow one in the driver's footwell and there is a light-blue one
under the hood on the firewall. How many more are there?

Thanks for thinking this through with me.

Regards,
blurp

 

<snip>

Hi there,

Have you thought about the vacuum servo for the brakes?
The tail end of this item protrudes in to the cabin.

You could try your previous tests with your foot lightly pressing on the
brake pedal.

The only other vacuum in the cabin (depending on model) is for the heating
controls.

hope this helps,


Steve.


1989 740 GLE (UK)

 

Thanks Steve, I'll definitely check the vacuum servo for the brakes
(would some failure here not also affect braking?) but I believe there
are other vacuum components (or at least vacuum dependancies) in the
cabin.

I know the brake has a vacuum release switch associated with the
cruise control. I also know that the cruise operates a vacuum
connected to the accellerator and the throttle actuator under the
hood. I understand that there may not be relevant vacuum STORAGE in
the cabin (assuming the vacuum tank in the bottom-rear of the centre
console only affects environmental controls) but there are vacuum
hoses and, vacuum being what it is, a failure anywhere in one of these
hoses can cost you your stored vacuum.

One thing I did discover when testing the cruise control system
earlier is that the fatter of the two hoses that lead into the
throttle actuator does have some leak. I don't exactly know where that
hose leads but it's what I'll be tracing next.

Any idea how I can recognize weakness or failure in the one-way
valves?

Thanks again,
blurp

 

Hi Blurp

A leak in the vacuum servo would'nt cause a complete loss of brakes, just a
lack of braking assistance.
This lack of assistance might not be noticeable if it has happend over a
period of time.

You have lost me on the vacuum for the cruise control as unfortunatly I have
never had that option.

As for testing a one-way valve I would remove it and after wiping it down,
try sucking/blowing on it and see what happened!

The leak in the fatter hose sounds promising.

Good luck


Steve

 

What you're missing, though, is the fact that you have maximum vacuum
when the throttle is completely CLOSED, and less vacuum when the
throttle is open. The throttle lets air in to the manifold, which is
where your vacuum is...thus making it less of a vacuum.

More than that, on turbo cars, the air is _forced_ into the
manifold...so instead of just having less of a vacuum in the manifold
when your foot is down, you have boost. Positive pressure.
Anti-vacuum.

I didn't quite understand what you were saying about the vacuum can. I
assume this is a kind of vacuum reservoir, which is evacuated when the
engine is idling (ie maximum manifold vacuum), and has a non-return
valve so that it maintains vacuum when the throttle is opened and, in
our case, the turbo boosts up. That's a typical arrangement for a
brake booster, which is vacuum-powered. If the non-return valve is
leaking, then air might be hissing through it and into your
(supposedly) evacuuated vac can thing.

Anyway, if your hissing is proportional to throttle position, it's
more likely to be a boost leak than a vacuum leak. Air will be leaking
in the opposite direction to what you've been thinking.

fs

 

My 765T uses an electric pump to maintain vacuum, but I suspect your 240T
uses an engine driven pump. (It has to have *something*.)

Perhaps you have a giant vacuum leak - a torn hose - and you are hearing the
vacuum pump work its little heart out trying to pull a vacuum? That would
follow engine speed rather than throttle, but if manifold draw offsets the
demand on the vacuum pump it could account for what you hear.

I have used a hand bicycle pump to chase leaks before - I cut the valve stem
off an old bicycle inner tube and jam a rubber hose on it. Maybe that would
help. (Be sure to use minimum pressure, since you could damage things with
the pressure a hand pump can produce.)

Mike

 

This is a VERY useful piece of information. I had been under the
impression that the increased throttle and resulting increased air
flow somehow created vacuum for the other vacuum-based components. So
the vacuum that the engine naturally creates at the manifold (held
back by the throttle plate?) is not the same vacuum that drives all
the vacum-driven systems in the car? Does the engine then try to draw
air at a constant rate and, if given air at its maximum desired rate,
the throttle is wide open?

You see, things that are happening at the manifold (things that might
generate a quiet hiss) could not be heard inside the cabin. This also
does not explain hearing the hiss of air when I shut the engine off.
Unless it's a one-way valve somewhere in the system that's simply not
doing its job in keeping air from leaking into the system. I have
inspected all the major hoses from the rubber cowling coming off the
airbox to the aluminum pipe with rubber cuffs that runs from the turbo
across the top of the engine and into the manifold and all are intact.

So confused. Based on what you've written I have a new model in my
mind to explain the vacuum system but I cannot reconcile it with the
reality of the observations.

The hiss I'm hearing is similar to that heard when pressing the
"FLOOR" button on the heating control and you can hear the vacuum
closing the vents to redirect airflow.

This is making me so crazy.

Anyway, thanks for the useful info and the perspective correction. I
make assumptions that seem logical to me but appreciate it greatly
when someone points out wheat I'm doing wrong.

Cheers,
blurp
ps. I haven't noticed any change in my braking. The fat vacuum hose
from the Throttle Actuator is still suspicious but I haven't had a
chance to investigate!

 

My 240T had the brake booster hose crack once, it sounded just like a vacuum
leak, it was actually a boost leak, same deal, just air rushing the other
way.

 

You've got a boost guage right? When the guage is in the black you have
vacuum, when the guage is in the yellow you have positive pressure, simple.
There's check valves on the vacuum system to keep it from getting
pressurized and a vacuum resivoir to run vacuum accessories. Cars with
cruise controls had an electric vacuum pump to run them during prolonged
boost.