The Basics of Tracking Down Vacuum Leaks - PeachParts Mercedes-Benz Forum

Squabble · #1 01-28-2012, 11:54 AM

at the risk of sounding like an idiot (how else am i going to learn?), i would like to ask you guys about tracking down vacuum leaks in the w123. the issue is that i need to really understand the very basics of what exactly it is you do with the tools and the car. i've read the basics from the diesel giant page, i've done searching here and over at benzworld, and it seems that every discussion assumes that the reader already knows a bit about "how to" when it comes to vacuum systems/leaks. trouble is, i don't.

i have a mighty vac, i have a w123 with door lock leaks. the symptoms are: the doors do not lock or unlock when the car is shut off. not even a little movement. when the car has idled for about a full minute and the drivers door is locked or unlocked, the other doors sometimes lock and unlock with it and they all move at a different pace than the others. the back drivers side door will lock very slowly, the rear passenger side will lock and unlock quickly, the passenger side will not respond without a delay, etc. and they all seem to do different things each time.

i have replaced the drivers side master switch as well as the yellow check valve in front of the firewall. no changes.

so, where do i begin? do i need to buy a vacuum gauge as well? what does it mean when you "isolate" a section of the vacuum system, what do you do? how do you use the mighty vac to test a section? do you do the testing with the car running or not? what does a good working check valve look like when you hook a mighty vac up to it?

i feel sorta stupid but i'm a visual learner and i'm doing this on my own here. it's hard to read a description that may leave common sense holes in the story that i'm having trouble filling.

thanks for the help and the education!

Stevo · #2 01-28-2012, 12:08 PM

The three most important basics for Tracking Down Vacuum Leaks are Mity Vac, Mity Vac, and Mity Vac.

sixto · #3 01-28-2012, 12:17 PM

Smart people ask questions too

I don't know of proper instructions. I isolate systems to identify those that work and those that don't. In your case there's engine shut-off, ACC and central locking. IIRC those systems are fed from the check valve area of the brake booster vacuum line. Work your way south from there.

Sixto
87 300D

Biodiesel300TD · #4 01-28-2012, 12:21 PM

The vac system is like a tree. The main trunk being the main vac line from the vac pump to the brake booster. Off that line is a few to several other lines(depending on model and year), which then branch more. You'll find that each system has its own set of colored lines. To isolate a system you remove the hose supplying that system and plug the side that goes to the vac pump. Doing this will allow you to see if the other systems are working properly. The systems in the 123 that use vacuum are the door locks, climate control, engine shut off, transmission shift quality, and the EGR. On some models the cruise control is vac operated as well. You Mity vac will work as a vac pump and simple as a gauge. You can pull vac on something, or T it into a system and read what the vac is. Your main vac line should be reading above 20"hg. Nearly everything in the vac system should hold vac if you pull vac on it with the Mity vac. Actuators should actuate when you pull vac and then maintain the vac. If it's not doing both then it's leaking.

I've attached a document that I wrote for another forum about troubleshooting a 123 vac system. I need to get it posted here since the other forum has been closed.

Squabble · #5 01-28-2012, 12:27 PM

Quote:

Originally Posted by sixto

I isolate systems to identify those that work and those that don't. In your case there's engine shut-off, ACC and central locking. IIRC those systems are fed from the check valve area of the brake booster vacuum line. Work your way south from there.
Sixto 87 300D

okay, this is a good place to ask questions: when you say "isolate" what exactly do you mean?

it's the central locking that's having the problems. my vacuum adjust headlights are working perfectly as is my ACC (it is the manual kind if that matters). the headlights have their own check valve but are connected to the same 4 way vacuum line near the firewall as the locking system. there also appears to be a single gray vacuum line going through the firewall that i have no idea what it goes to.

if i wanted to "isolate" the rear passenger door to see if that's where the leak was, what would i do? take all the door panels off and block off all the other door diaphragms? unhook the lines to the diaphragm and hook up the might vac to it? what would i be looking for, a number?

would i then use this technique for all the other doors, plus the trunk, plus the gas flap? what about the vac tank?

thanks!!!

ah-kay · #6 01-28-2012, 12:28 PM

A Mity Vac is all you need. You do it by divide and conquer. The vacuum hoses is a just a network of hoses. Isolate those further back, like trunk, gas cap and back door and work on the front doors or back first. It looks like you have a leak on the lock and unlock circuit. The yellow hose with red marking lock the car, green marking unlock the car.

1) Lift up the carpet by the driver door and you will see some vac lines. Break off the T and put a Mity Vac on the back circuit and put a mity vac on it. The back door/cap/trunk should lock or unlock depending on which line ( green/red ) you attach the Mity Vac.

2) Repeat on the passenger side.

3) You should be able to isolate which door has issue.

4) You can isolate at each individual pod at the door by blocking it but it entails removal the panels which I normally do not do.

You will get a hang of it after some try and error.

Good luck.

funola · #7 01-28-2012, 12:35 PM

MityVac lesson #1:

Put your finger tip over the MityVac nipple to plug the opening and squeeze the handle a few times. The vacuum gauge goes up and stays there because there is no leak.

Pull your finger away slightly to create a leak, the vacuum gauge drops.

Now figure a way to plug an item you want to test while the MityVac is connected to it.

Junkman · #8 01-28-2012, 03:42 PM

I like having a vacuum gauge also. Measure vacuum with engine running before you start. It will increase every time you plug a leak.

The parts store sells packages of fittings to connect vacuum lines. Grab a couple of assortment packs along with some vacuum tube of various sizes. One of the most useful is a stepped straight piece that will connect hose sizes from <1/4" to ~ 1/2". Golf tees (plastic) work best for plugging rubber lines.

A shop replaced a friends vacuum pump & wants $700 but hasn't been paid yet. They misdiagnosed a nonperforming vacuum pump when the actual problem was a leaking brake booster. The complaint was that the car wouldn't shut off with the key.

Bottom line: make sure you have isolated the leak before replacing parts.

shertex · #9 01-28-2012, 05:28 PM

Thanks for the write-up. Would you guess that this would be basically applicable to 124 cars as well?

Quote:

Originally Posted by Biodiesel300TD

The vac system is like a tree. The main trunk being the main vac line from the vac pump to the brake booster. Off that line is a few to several other lines(depending on model and year), which then branch more. You'll find that each system has its own set of colored lines. To isolate a system you remove the hose supplying that system and plug the side that goes to the vac pump. Doing this will allow you to see if the other systems are working properly. The systems in the 123 that use vacuum are the door locks, climate control, engine shut off, transmission shift quality, and the EGR. On some models the cruise control is vac operated as well. You Mity vac will work as a vac pump and simple as a gauge. You can pull vac on something, or T it into a system and read what the vac is. Your main vac line should be reading above 20"hg. Nearly everything in the vac system should hold vac if you pull vac on it with the Mity vac. Actuators should actuate when you pull vac and then maintain the vac. If it's not doing both then it's leaking.

I've attached a document that I wrote for another forum about troubleshooting a 123 vac system. I need to get it posted here since the other forum has been closed.

sixto · #10 01-28-2012, 07:15 PM

Quote:

Originally Posted by shertex

Thanks for the write-up. Would you guess that this would be basically applicable to 124 cars as well?

Generally, except that in the 124 the brake booster and engine shut-off are fed off the big line at the vacuum pump and everything else (ACC, VCV/BFS, EGR and ARV) is fed off the small line. And central locking has its own electric pump under the rear seat. And the ACC line splits under the dash to feed the rear headrests.

[edit] And 124 (and 126 and 201 and...) central locking has single chamber pods that react to vacuum and pressure through the same line, vs 123 (and 116 and...) central locking which has separate lines and pod chambers for locking and unlocking.

Sixto
87 300D

Yak · #11 01-28-2012, 09:31 PM

You've already started a lot of troubleshooting by observing and describing the behavior of various systems, identifying things that are unique about your system, etc.

I would postulate that you have a leak (at least one, there may be others) in the reservoir system. You describe a generally functional vacuum system when the engine is running, but it doesn't work when off. The locks move, which implies that they're not completely bad.

To isolate a vacuum subsystem, you would generally remove the appropriate colored line and cap or plug that branch. That lets you focus on a remaining system. If you have multiple unknowns it lets you remove them one by one.

Take a hypothetical example in the W123 coupe. It has central locks, auto A/C, locking seat backs and something else with vacuum. If one or more of these things aren't working, then one or more of them could be bad. I could try to fix all of them, or try to find out which one is leaking. I know the seats don't lock, and the A/C controls are iffy, and it's sort of slow to shut off (diesel), so I've got a leak. My central locks seem to work okay, but the PO had the fuel flap disconnected and plugged with a screw, so the locks might be a factor.

To troubleshoot I would disconnect the lines from the distributor prior to the fire wall. From research, I know the seat backs are the blue line and they need the doors to be closed and the seats back in order to work. I'd set those conditions, then put the Mity-Vac (preferably with gauge) onto the blue line and the Mity-Vac now serves as the source of vacuum for the seats ONLY. I pump, and pump, and pump. I see a rise in vacuum on the gauge as the pump evacuates the seatback branch. Eventually the seat back locks move. I note the level of vacuum at which the seatback locks move for reference. I've now eliminated the seat back system as a cause of the leak and also identified how much vacuum they need to move.

I could try this with the yellow line(s) to isolate the central locks but I also know there's a big egg-carton looking reservoir under the hat shelf in the trunk. To evacuate that thing with a hand pump I'd need forearms the size of Popeye, so I choose to go to the A/C system next. This one needs the engine running and some temp inputs to be tested as a system, so I opt to check it on a component basis. I remove the PBU in the console and expose the vacuum control solenoids. This lets me check each solenoid and the pod. I pull the line, attach to the pump and check each. In my case, I found a leaking defrost pod.

Since I now know I have at least one leak in the A/C system, I put the system back together, except I leave the blue-green line disconnected and plug the open distributor port for that line. I've now "isolated" the leaking A/C branch from the system.

I start the car and everything works. It shuts down quickly (diesel). The seat backs lock, the central locks still work.

I make the conclusion that I need to replace the defrost pod. I double check the diagrams to see if that conlcusion makes sense and it does. I replace the pod, hook everything up and voila.

In your case you should get the diagrams for the W123 sedan. A gasser will be a bit different, but not too much.

For your specific questions, check valves are sort of "traffic cops" of air pressure. In a gasser, the intake manifold is the ultimate source of vacuum or the lowest of the low pressure locations. Air flows out of the (higher air pressure) system (and the reservoir) through the valves into the (lower air pressure) manifold. When the engine shuts off, it's no longer the lowest pressure. You've got a checkvalve between the reservoir and the central locks that keeps the now higher pressure air in the manifold from filling the lower pressure reservoir. The check valve would open again when the manifold is the lower pressure region when the engine is restarted.

When you move the central locks without the engine running, you're creating a small leak that shifts the locks due to a pressure differential. Eventually enough air leaks into the reservoir that no differential can be generated.

What I think you're seeing in your system that as vacuum is created as the engine starts and runs (the air is being sucked out of the system) the locks start moving at each one's respective limit. If the reservoir or its lines are leaking, then there's no no "buffer" of vacuum for smooth operation. As each lock moves there's a slight leak then reestablishment of vacuum.

Bottom line: I'd look at your reservoir, the connections, and the check valve for that line.

Squabble · #12 01-29-2012, 07:10 AM

thanks everyone, all of your posts have really made this issue make so much more sense! i feel pretty confident about tackling this now, i really appreciate the help!!!

Squabble · #13 02-02-2012, 05:27 PM

Alright, over the last two days I've torn the central locking system apart! Here are my findings:

When isolated:

the rear drivers side door locks and unlocks with no problems and holds vacuum. The passenger side front door locks but has a slow leak, and unlocks fine. The passenger side rear door locks and unlocks fine. The trunk locks and unlocks fine as well as the fuel door.

When tested with all the lines connected:

The fuel door and trunk will not function or hold vacuum. This is when you plumb into the lines on the passenger side door that go back to the rear of the car. There is a problem somewhere between the rubber connector on the passenger side floor and the connection to the actual elements.

The Reservoir Line:

The actual line is not holding vacuum. When I plug one end of the line at the firewall and pull the other end out of the tank and put the mity-vac up to it, it will not hold any pressure. The tank itself seems fine as well as the rubber grommet but the line itself seems to have a hole somewhere.

Questions:

Where do I access the fuel door and trunk lines? They seem to go under/behind the rear seats? I replaced the 3 way rubber connectors in the trunk but that didn't change anything. Is there a place where they typically break?

What should I do about the reservoir line? Replace the whole thing? Try to find the break? Any typical places this guy breaks? I had a theory that all lines might be broken in the same place? Do they run to the rear together? Anyone know how many feet of line this is and how to snake it through the car and back to the tank?

Thanks!

ah-kay · #14 02-02-2012, 07:41 PM

You seem to have a handle on it. It has been a while that I tackled vacuum problem. It is just a matter of divide and conquer and follow the lines. Check everything at the back of the car then move forwards. Or try to block a vacuum branch further back and check whether the front part works. It is really very simple.

All the vacuum line runs underneath the carpet on either side of the car. Some lines underneath the back seat. 2 lines behind the panel on the right side in the trunk.

Good luck.

charmalu · #15 02-02-2012, 09:32 PM

This thread might help. a good diagram of the door lock system

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

Charlie