A Dumb Vacuum Question - PeachParts Mercedes-Benz Forum

EricSilver · #1 07-25-2004, 07:18 PM

1989 260E

I want to check for vacuum leaks which could affect my shifting and my brake booster. The problem is, I don't know where to start looking.

When I open the hood, what am I looking for? Where do I start?

bwheitman · #2 07-25-2004, 08:21 PM

Are you having braking problems? Usually the vauum booter has a large line that goes directly into the intake manifold. Sometimes it is clear, if it is amplified by a vacuum pump, in that case it will go into the pump and then there will be a line from the pump to the manifold. The reason they are clear if you have a mechanical pump is so that you will know if there is a diaphram failure, it will turn dark. What is the braking problem you are having?
Brian

EricSilver · #3 07-25-2004, 08:39 PM

It seems that more force is needed to fully engage the brakes. Normally, a toe tap is enough, but now I have to use the ball of my foot to achieve what my toes used to do.

I just replaced my pads and rotors yesterday, so that could be a small factor. Still, the braking issue, rough shifting, and also the defogger "activating" on acceleration are known symptoms of vacuum loss. These conditions have been present for about a year.

The rough shifting occurs when I don't "modulate" or feather the accelerator pedal; if I do not lift off too much, or depress it too far when accelerating or decelerating, shift are smooth. If, however, I lift off the accelerator when slowing, or depress it hard when accelerating, shifting is hard.

I assume this means that when engine RPMs are constant, so is vacuum pressure. When RPMs are not constant, vacuum pressure fluctuates. Again, this is my assumption. I don't know much about the engine vacuum system.

bwheitman · #4 07-25-2004, 09:57 PM

Does your 260 E have an auxilliary vacuum pump? I had a 280 that has one mounted on the front of the engine connected to the cam to drive it. Does your car have one of those? Does your engine smoke erratically?

Vacuum is the greatest from the engine at idle, so you are correct when accelerating you lose vacuum, that is why there is usually a resevoir or assited pump in the system. My 280 had an extra pump that helped with the brake booster. The first thing I would do is find the line to your brake booster, if you did your brakes you know where your fluid resevoir is, the booster is behind it, there you will see a line about 3/8 in size, trace that line. If it goes directly into the manifold make sure it is connected and not leaking. if it is ok, then start your engine and at idle, listen for any hissing noise, if you are losing that much vacuum you will hear it. If you can not find a leak, your booster may be failing, at that point hook up a vacuum pump to it and test it. Just pump it up and see if it hold a vacuum. If not it has probably failed and this is likely where you are losing vacuum.
If you have a vacuum assist pump make sure it has not failed by checking disconecting the line between the booster and the pump, hook a vacuum gauge on the nipple and start the car. So you could either have a bad pump, a broken vacuum line, or a bad booster. If the vacuum pump is failed it is usually the diaphram inside that only costs about $20
Let me know what you find out! Good luck!
Brian

EricSilver · #5 07-25-2004, 10:15 PM

Brian,

Thanks for the info.

I have no aux vacuum pump, nor any engine smoke.

I will check the brake booster tomorrow when there is daylight. It is possible a connection could be loose, or perhaps some rubber has deteriorated. Would such a vacuum leak on the Booster hose also explain hard shifting?

By "hard" I mean there is a clunk from the differential when I lift off the accelerator. The hard upshifting is abrupt and jerky at times, unless I am feathering the accelerator, at which times shifting can be imperceptible.

I was reading in other threads that a leaky or failing brake booster will make a hissing noise when the brake pedal is pumped. When I pump my brakes, the sound I hear could be hissing, but then again it could be just the normal sound. I can't quite tell.

bwheitman · #6 07-25-2004, 10:18 PM

The brake booster is the largest user of vacuum, so if it is leaking or malfunctioning it could cause many other problems in your vacuum system. Good luck!
Brian

EricSilver · #7 07-26-2004, 01:36 PM

This morning I performed the test, copied below. It saw it on USENET (alt.mercedes)

Quote:

You need to determine if the load of being in gear causes the engine to
idle roughly or if there is another cause of the rough idle.

You can easily test this by idling the engine in D with the parking, not the foot brake, applied. How does it run in D vs. N?

The parking brake is entirely mechanical and uses no vacuum so the engine should run the same except for the load of being in or out of gear.

Now, with the transmission in D and the parking brake on, step on the foot brake. If the foot brake booster is defective you will see the difference in the engine idle as soon as you step on the foot brake.

A vacuum leak in the brake booster will dilute the air / fuel ratio and cause the engine to idle slower and roughly hence the lower oil pressure.

My brake booster is OK, if this test is a good measure. When I shifted from D to N with the parking brake on, there was a rise in RPM when the car was cold. When I repeated that when the car was hot, there was no variance.

When shifting from D to N when using the foot brake, there has always been a brief rise in idle RPMs, before it settles back down. Am I correct in assuming that is normal?

So my task appears to be to look for loose, cracked or leaking vacuum lines.