I've found the final problem of my vac leak. It's the VCV. It leaks with the engine off. I know its supposed to leak but should it do it with the engine off? Is there a provision to keep the VCV from draining all the vac reserve when the car sits?

Thanks,

Willson
85 300CD

The vacuum reservoir is isolated by check valves. I suspect that your leak is unrelated to the VCV.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Hey Tango,

Car off, with VCV line removed and plugged, my system holds vac perfectly, but as soon as i plug the vcv line back in, the entire system begins losing vac at about 1hg/2 minutes.

You have performed the diagnostic test.

Your results prove the check valves are bad.

Typically the check valves are yellow/white or yellow/Grey, in the engine compartment by the bulkhead.

.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

Thanks for the advice Whunter.
I have a new yellow check valve for the doorlocks (the one with a barb on the vac end and two barbs on the other side for the yellow vac lines). My main vac line was rebuilt and I only have one line off the main vac line that feeds everything.

Vac holds fine with the VCV disconnected but when its connected and i shut the engine off, i get a buzzing sound and when i touch the VCV, i can tell thats where the sound is coming from and its accompanied by a vac leakdown. It stops humming when it drops below 17hg but continues to bleed the whole system of vac.

At this point, I'm going to remove and try to clean the VCV this weekend.

I suspect that you are barking up the wrong tree. The VCV leaks by design; it is not intended to hold vacuum. If the reservoir is bleeding through the VCV, you have some other issue. Either you have a bad check valve or your jury-rigged plumbing repair has a design fault.

"New" doesn't mean problem free. Have you checked that valve for proper operation? I don't see how it could possibly be leak free if it's allowing the situation that you describe.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Is the lever/arm of your VCV actually connected to the throttle linkage right now?

Can you possibly snap a pic of the vacuum setup, around the VCV and bulkhead?

__________________


Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

You've got a point Tango, I tested the new valve before, found a leak in the diesel door and plugged it and it didn't leak but I will test it again after dinner.

There are TWO vacuum check valves that isolate the reservoir and passenger compartment from the engine / transmission.

If either check valve leaks, it supplies vacuum to the engine / transmission while the engine is off, until the vacuum supply is exhausted.

http://www.peachparts.com/shopforum/diesel-discussion/160169-w123-vacuum-interlock-diagram.html



http://www.peachparts.com/shopforum/diesel-discussion/356372-83-300d-vacuum-line-vcv-question.html

https://www.ifixit.com/Guide/Mercedes+W123+Vacuum+Lock+System+Diagnosis+Technique/20201

.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

Thanks Whunter, that makes more sense. I removed the yellow check valve with 3 barbs and pumped up the vac reservoir line to 15hg(mityvac on reservoir line only, no check valve). At first i couldn't see the needle move much but after letting it sit over night, the truth was revealed. It leaked down completely so I'll dive into a fix for the reservoir. I tested the other yellow line to the master switch and the needle didn't budge.

It's interesting though, I tested again with the check valve in place on both yellow lines and pumped it up to 10hg, and as soon as i removed the mighty vac, it hissed hard and the vac reserve was depleted in about 15 seconds. I'm wondering if the check valve, though new, is questionable too?

As always, I appreciate the knowledge,
Willson

Now, there's a thought! You might be on to something.
You might want to ask whunter if he thinks that testing the check valve would be a good idea.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

YES.
Cap it and test over night if possible.

Through the years, I have junked thousands of NEW defective vacuum check valves in prototype R&D.

Chuckle; There are many W123 around Detroit metro area with FORD or GM vacuum check valves replacing the MB = a cheap local replacement.

Good luck.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

Hey I wanted to let you know of my fix.

1. Before starting any diagnosis with your Mityvac, make sure the mityvac itself holds vac first.

2. Check your new check-valve for proper operation.

3. Understand how the system you are checking is supposed to work. (door locks are linked to the vac reservoir and are both isolated from all other vac users by the yellow check valve).

My problem was the door lock master vac switch. I ended up making the vac problem worse by applying silicone grease on top of the switch cylinder (black body with silver piston), thinking it would lube it up and create a seal.

Three months later I pull the door panel again, remove the master switch, test the middle main vac socket with other two sockets plugged and find the leak. I then followed the steps for drilling out the bottom and removing the piston. Boy was it dirty. After a good cleaning, my locks are holding vac like never before.

Thanks for everybody's help. Yes I was barking up the wrong tree at first but thanks to the great minds on this forum, I isolated and solved the problem.

Willson
85 300CD

The reason the vac reservoir leaked down overnight was because the MITYVAC LEAKED DOWN.

The Mityvac needs to be taken apart and cleaned and lubed with vegetable oil if you use it regularly. Or at least mine does.

If it's a 'real' Mityvac (there are copies), last time I checked you can get reseal kits as well.

__________________


Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22