Brake booster check valve question

In the continuing saga that is my wife's 1993 245, one that is recurrent
is a terrible surging idle in cold weather. Being that we live in
Minnesota, this is a 6 month a year problem, especially in the past
couple of months with lots of below 0F stuff.

Anyway, the surging idle. Start the car, the idle surges. It will
sometimes die when the RPMs drop. Lousy starter in the cold as a
result. My wife mentioned to me that "the idle smoothes out if I push
the brake pedal down hard." And I have noticed when driving the car
that there is very little power assist when braking. Hmmm. Vacuum
boost problems? Could a vacuum leak in the brake booster system cause a
surging idle?*

This morning I tried a couple of tests on the brakes. Pumped the brakes
several times to eliminate the vacuum, pushed the brake down and stared
the car. The brake pedal sank a good inch or so. Standing on the pedal
it stayed firm. Pumping the pedal successive times did not result in a
noticeable rise to the pedal. I didn't notice much change when
depressing the pedal and turning the engine off. No hissing noises or
obvious air leak sounds in the cabin or under the hood.

Looking under the hood, the vacuum hose from the manifold to the check
valve looked fine to cursory visual inspection. I couldn't figure out
how to get those stupid damned clips to release so I could pull it off
and see if it held vacuum. The white plastic check valve, however, was
noted to rotate freely in the rubber grommet. I've never checked this
before- is that as it should be or should the check valve be tight and
not easily rotatable? Common sense suggests that the latter should be
the case. Also- removal of the check valve from the brake booster: is
that just a pull or does it unscrew? I can't tell from photos I have
seen. At 15 F I really didn't feel like standing outside experimenting
very long...


* There are also electrical problems in this car, e.g. the passenger
side power windows work very sporadically from either the controls in
the door or the driver's door; there's a weird flicker to all the lights
when the engine is running; etc. It doesn't blow fuses, though.

I could probably do 40 hours of work on this car and still not have all
the problems fixed. I think it was built late on a Friday afternoon.
Or the previous owner(s) were utterly neglectful.

 

Yes the check valve is easy to rotate. If you're unsure, cap off the
nipple on the intake manifold and see if the engine runs differently.
You can drive the car like this if you need to for testing, but the
brakes will be MUCH heavier so be careful.


Power window issue is probably either one of the switches (power goes
through both) or the motor.

Flickering lights is likely a bad regulator/brush pack in the alternator.

 

Yes the check valve is easy to rotate. If you're unsure, cap off the
nipple on the intake manifold and see if the engine runs differently.
You can drive the car like this if you need to for testing, but the
brakes will be MUCH heavier so be careful.
Thanks!

Power window issue is probably either one of the switches (power goes
through both) or the motor.[/QUOTE]

This problem affects both the front and rear doors on the passenger
side. I checked all the switches on the driver's door with a continuity
tester and they seemed to be fine. I haven't yet done that on the
passenger doors. Is there some other component to check, too?

Ah. I wondered about the alternator, although the flicker rate doesn't
seem to change with RPMs.

 

I assume you know how the booster works? Just in case you or others don't...

The vacuum booster has a front and rear section, divided by a rubber diaphragm
attached to a moving piston, which contains a 2-stage valve mechanism.
With your foot off the brake, the valve is open between the two sections, allowing
them to equalize pressure (or vacuum), and an internal spring keeps the diaphragm
in the "at rest" position. When you step on the pedal, the valve first closes the
passage between the front and rear sections, then opens another passage at the
rear where the pedal pushrod attaches, allowing air from the cabin to fill the rear
section of the booster. The resulting pressure difference in the booster pushes
the diaphragm and piston forward, helping you apply the brakes to avoid that
cow that just wandered onto the road in front of you.

When you take you foot off the brake, the rear passage closes, cutting off the
supply of external air, and the middle passage re-opens, allowing the vacuum
to again equalize in the two sections, returning everything to "at rest".

There is an air filter where the pedal attaches, normally hidden by the rubber boot
around the pushrod, which is supposed to keep any dirt or burger crumbs from
getting into the booster and jamming the valves, as well as a seal around the rear
of the internal piston's "snout" where it moves against the booster's outer body.
If the rear snout seal fails, or the filter (often foam rubber) leaks or breaks apart
and lets chunks of itself or other crap into the booster (where it can prevent the valve
from closing the rear passage), you will have air constantly leaking into the rear
section (sometimes too slow to hear), and since the passage to the front section is
open with your foot off the pedal, you get what we like to call an "unmetered air leak"
through the booster and into the intake, possibly enough to cause your idle surging.
With your foot on the pedal, the front passage is (theoretically) closed, cutting off the
air leak, and restoring the idle to normal.

 

MasterBlaster skrev:

I must thank you for a very clear and humourus explanation! I did'nt
know for myself how it worked, but I surely do now. And it gave me a
good laugh too, which is not to be dismissed!
/ Johan