Quite possibly a stupid vacuum testing question - PeachParts Mercedes-Benz Forum

JHZR2 · #1 07-08-2019, 12:51 AM

Ive generally been lucky with my w123 cars, to not have many major vacuum issues. Really not much that a lock actuator or tubing section couldn’t fix.

For my 300cd, since Ive put it back in service, Ive noticed that my center vents no longer work, one of my four seat back locks doesn’t work, and the transmission clunks when going back into 1st when coming to a stop, then clunk when going back to 2nd slowly (best shifting AT Ive had otherwise).

So this all points to vacuum for me. Get out my mityvac and vacuum gauge and start testing just the parts. The gauge and mityvac tubing holds vacuum well. So then I start diagnosing - beautiful behavior of the vcv when the car is on. But when off, more or less nothing holds vacuum. But should it?

If I pump down to 25in Hg, within 30 seconds or so most things will leak down to 20. Leaking down slows substantially down when at 20 and especially 15”.

Is that fine? Should everything hold significantly? Should I be able to lay down a connected mityvac for minutes/hours, and come back to the same values?

optimusprime · #2 07-08-2019, 05:20 AM

i would say you should have no leak whatsoever .

JHZR2 · #3 07-08-2019, 07:45 AM

Quote:

Originally Posted by optimusprime

i would say you should have no leak whatsoever .

That’s my gut feel, and for the cabin consumers, it’s my assumption since there’s the need for the reservoir to hold some lock actuation for a few days. For engine/transmission, wasn’t as sure.

Diseasel300 · #4 07-08-2019, 09:18 AM

Quote:

Originally Posted by optimusprime

i would say you should have no leak whatsoever .

So how does the vacuum leak down from 25 to 15"? There's a small leak somewhere, but probably not bad enough to cause major issues.

You may have a check valve failed somewhere or a vacuum connector rotted out that leaks when under high vacuum.

The critical troubleshooting element....how's your vacuum with everything hooked up and the engine running? Lousy vacuum from the vacuum pump or a leaking brake booster will cause system-wide vacuum issues.

5cylinder · #5 07-08-2019, 09:51 AM

Quote:

Originally Posted by JHZR2

If I pump down to 25in Hg, within 30 seconds or so most things will leak down to 20. Leaking down slows substantially down when at 20 and especially 15”.

Is that fine?

If your objective is to locate and stop a vacuum leak(s), then the answer is "No."

Quote:

Originally Posted by JHZR2

Should everything hold significantly?

No. Everything should hold fully.

Quote:

Originally Posted by JHZR2

Should I be able to lay down a connected mityvac for minutes/hours, and come back to the same values?

Yes.

JHZR2 · #6 07-08-2019, 10:49 AM

Quote:

Originally Posted by Diseasel300

So how does the vacuum leak down from 25 to 15"? There's a small leak somewhere, but probably not bad enough to cause major issues.

You may have a check valve failed somewhere or a vacuum connector rotted out that leaks when under high vacuum.

The critical troubleshooting element....how's your vacuum with everything hooked up and the engine running? Lousy vacuum from the vacuum pump or a leaking brake booster will cause system-wide vacuum issues.

Nothing is a big issue, more just stuff that I can notice during operation.

There’s lots of ways it could leak. Question really is if some of them will naturally do so regardless. For example I read elsewhere that the AT modulator can leak down so long as it is slower than 15s. Not sure if that’s the truth or not, but I’d assume that would mean a persistent leak when operating. Obviously depending upon throttle, the VCV causes a leak/vent as well. Stuff like that.

Wasn’t sure if everything absolutely is supposed to hold tight at >20, if it is anticipated that it leaks down to a more shallow vacuum then holds longer, etc.

Diseasel300 · #7 07-08-2019, 01:29 PM

My "how does it leak down" comment was aimed at Optimus who I quoted earlier. If there's no leak, the vacuum will never leak down.

A healthy vacuum pump will pull 20-22" of vacuum. If you have no significant leaks, it has enough volume to maintain 18-20" of vacuum on the rest of the system. To determine if you have bad enough leaks to cause problems, you need to test the vacuum with everything connected. In your photo above, you're tied into the brake booster with everything else isolated. That only tells you the vac pump is working, not if it has enough capacity to overpower whatever leak you have.

As a rule of thumb, a "good" vacuum pod will hold vacuum for hours or days without any noticeable leak down. an "acceptable" vacuum pod may have a very slow leak over a matter of minutes. If it bleeds down 50% in under a minute, it's junk, it may still run for now, but it's going to fail sooner rather than later. If you have original rubber connectors in the engine bay and elsewhere, replace them first. They're cheap to replace and all of the leakage added together can become significant.

JHZR2 · #8 07-08-2019, 05:54 PM

Agree with all this. I was short some tees but now have them. Looking forward to more info.

Diesel911 · #9 07-08-2019, 06:28 PM

Your symptoms are similar to what I had when I accidently disconnected the vacuum line to the Transmission modulator.
The Modulator is the only thing that is supposed to hold vacuum in the circuit of vacuum from the main line through the vacuum valve and down to the Modulator. Note that the Modulator itself has a Cap that can leak vacuum, crack or even turn up missing.

I am not sure where you would find it in the Factory Service Manual but it has that the restricted orifice is supposed to reduce the vacuum between the orifice and the Vacuum Valve to X amount then as you depress the Accelerator the vacuum is supposed to get less and less as you get closer to full throttle again I cannot remember how much less but I think it was all the way down zero vacuum in some cases.

So the vacuum into the vacuum valve is not as high as the main line vacuum and the vacuum between the modulator decreases as you depress the Accelerator pedal.

There is also an adjustment of the linkage to the vacuum valve in the FSM.

Note that you need to T into those lines.

Diesel911 · #10 07-08-2019, 06:50 PM

I just remembered there is a screw on the vacuum valve that provides some re-adjustment. I have never messed with that.

JHZR2 · #11 07-09-2019, 11:18 PM

Quote:

Originally Posted by Diesel911

I just remembered there is a screw on the vacuum valve that provides some re-adjustment. I have never messed with that.

Do you (or anyone) know the effect of too much vacuum at the VCV and modulator?

My gauge shows >15” at idle, and the vcv works beautifully taking it to zero.

But it’s >15.

So I sort of see two situations here... one where there’s too much vac, and the other where I know the modulator is leaky (have a genuine cap on order, as changing the rubber fitting didn’t help and the modulator looks good...).

The issue is that I still have a clunk in the final downshift from 2-1 when coming to a stop. My reading is that this is a sign of vacuum issues to the transmission, but it’s not clear which issue it might be for me.

Diseasel300 · #12 07-09-2019, 11:27 PM

Vacuum to the modulator softens the shift. Typically the modulator vacuum is somewhere around 12" at idle. I'd expect more vacuum to make the shift even softer, or if the modulator is maxed out at that point, make no difference at all. Try replacing the cap and see where it gets you before going nuts trying to fix a problem that may not exist.

vwnate1 · #13 07-10-2019, 01:53 AM

Um,

"four seat locks" ? .

5cylinder · #14 07-10-2019, 09:33 AM

Quote:

Originally Posted by vwnate1

Um, "four seat locks" ? .

The thread starter's vehicle is a CD.
In a CD, each front seat is designed to utilize two vacuum elements for the purpose of locking the seat back in place when the vehicle is in operation.
Hence, two seats utilizing two vacuum elements equals four so-called "seat locks."

vwnate1 · #15 07-10-2019, 10:28 AM

Thank you ! .

I can always trust the brain trust here to teach me new things ~ I thought there was only one vacuum element in the Coupe's seat back....

For the OP : the seat back locking vacuum circuit is all in blue and almost always leaks, it's a simple thing to cap it off under the hood for testing .

Vacuum leak testing is simple if tedious .