Replace vacuum lines...diy? - PeachParts Mercedes-Benz Forum

exsailor66 · #1 06-29-2018, 04:21 PM

My 85 w123 turbo has a vacuum problem. The engine won’t shut off and the auto door locks don’t work. This problem started a week ago, and I’m taking it into the shop for repair on Tuesday.
If I’m correct, the symptoms indicate that the vacuum pump has failed. If that is true, I’ll have them replace it.
After 33 years, I suspect that the rubber vacuum hoses also need to be replaced. I”m thinking of doing it myself by replacing them one at a time over the next week or so.
My question is: Is this something I can do myself, or is it recommended that I get the shop to do it? It must be time consuming, so if it is a do-able diy project, I work cheap.
Please advise.
Thank you.

tyl604 · #2 06-29-2018, 04:29 PM

It might be your vac pump. However, first I would put the Mityvac on the shutoff valve which is located in the rear of your injection pump. Just remove the vac hose from the nipple and stick on the Mityvac hose. Pump it and see if it will hold a vac. If not, the engine will not shut off and you need a new shutoff valve. Pretty easy replacement.

All of my vac hoses on my 1981 300SD are original and still do not leak. Even the rubber connectors are original. So I am guessing your vac hoses, in general, do not need to be replaced. They are hard plastic and last just about forever. Maybe one or two but not all.

You may have a bad door actuator causing the lock problem.

All of these are a lot cheaper than replacing the vac pump which also can be checked with the Mityvac; might try that. (However my experience is with a SD so I could be off base.)

kuene · #3 06-29-2018, 04:45 PM

if your vac pump was broken I don't think your brakes would work very well at all. easy to check if the pump is producing any vacuum by just feeling with your finger on one of the branches that comes off the main line to the booster.

there are like 50 places you could have a leak, but its probably only leaking in one or 2. I've been through my whole system and it was doable but quite a bit of work. For me, I had leak in master door lock element, trunk lock actuator, and a cracked line going to transmission modulator. if you start testing the door locks from the firewall yellow splitter/check valve, you just have to go down the various trees/branches until you fine the leak.

exsailor66 · #4 06-29-2018, 05:46 PM

Tanks. The shop I’m taking it to has been good, so I hope they diagnose it and it will be a $200 problem, vice the vacuum pump.
All in all, from my experience with cars (this is my first diesel), this is better than a electrical problem...I hope.

1983/300CD · #5 06-29-2018, 07:23 PM

A $40 vacuum pump with gauge could save you a lot of money (if you had the time, patience, and place to do it). I think letting the mechanic chase a vacuum leak was the straw that broke the camel's back for the previous owner of my car.

exsailor66 · #6 06-30-2018, 09:32 AM

An expensive mechanic, chasing a leak for hours, is my biggest fear. That is why I had the idea of changing out hoses myself. Getting a vacuum tester seems to be a common refrain, so I’m going to do that. I just want the shop to diagnose the pump and shutoff valve to eliminate them as a cause.
Thanks again.

Diseasel300 · #7 06-30-2018, 10:21 AM

Testing both the pump and the SOV are DIY jobs with a hand vacuum tester. With the engine running, hand pump into the main vacuum line from the vacuum pump and measure the vacuum shown on the gauge. It should be >18", preferably >20"

With the engine still running, disconnect the vacuum line from the SOV, connect your hand pump and pump the handle. It should stop the engine and the valve should hold vacuum.

$20 and 5 minutes of your time for a mightyvac diagnosis, or $200 diagnostic fees + labor for a mechanic to do it. Your money, your time... If you intend to keep the car for a long time, that little $20 hand pump will get used again, and again, and again.

Clemson88 · #8 06-30-2018, 10:37 AM

If you choose to have someone else do the testing you'll need to find someone who understands 30+ year old Mercedes and honest. In my 10+ years of experience with these cars and a variety of problems that is impossible.

shertex · #9 06-30-2018, 10:50 AM

When I owned an 84 300D for a brief while it struck me has having a very complex vacuum system....lots and lots of vacuum routing, which translates into lots of places for potential leaks. There are ways to simplify the vacuum system that can in turn make diagnosis simpler.

vstech · #10 06-30-2018, 10:51 AM

Totally diy.

First, take pictures of the vacuum connections.

Next, up by the dash is a yellow plastic octopus... pull the yellow hard line out of it, and plug the hose with a golf tee.

Now go start your car and see if it shuts off.
If not, pull the green line and plug it. Repeat.
Itll be one of those.
If green, your climate control is wonky.
If yellow, your door locks are wonky.
Next start chasing leaks in the affected system.

vstech · #11 06-30-2018, 10:53 AM

If none of that helps, there are two brown lines entering the dash... one goes to the key from the pump, one goes to the shutoff from the key.
Look the lines over carefully and see if something is off.

If you see any oil leaking from the lines, you need a shutoff diaphram.

BillGrissom · #12 06-30-2018, 12:42 PM

Pretty simple diagnostics. Disconnect the main vacuum tube from the pump (upper metal fitting) and connect a vacuum gage directly to it. Can buy either a single gage (used to work on 1960's gas cars) or use the gage on a Mity-Mite hand pump if you insure the pump doesn't leak. If you measure good vacuum (more than 15" Hg), your car's pump is good. If not, a new pump is >$300, rebuild kit ~$100 if you can find, or could change to the earlier 2-tube diaphragm pump since rebuild kit is $10 (has anyone?). A few have replaced w/ an electric "brake booster" pump from a modern VW or Audi.

If pump is OK, start connecting branches one at a time, while monitoring on the gage and see which connection loses the vacuum. In my experience, #1 culprit is the shutdown valve on the injection pump. When you re-hose, install a small clear filter there and when you see oil, you will know the replacement failed. Most people wait until oil is sucked up to the ignition switch and drips on their right foot, but an under-dash mess by then. The other culprit is the trunk lock actuator and 3rd is the fuel door actuator. Both can be capped off since non-essential and 95% of my other vehicles don't have those silly options. To cap both, look under the carpet on passenger side outside. You can test hoses there for holding vacuum w/ a hand pump. If a Mighty-Mite is too expensive, harbor sells a cheaper one which works fine (buy brake bleed kit). I use 1/8" silicone hose (ebay), in purty blue but also comes in black if averse to bling.

exsailor66 · #13 07-01-2018, 03:11 PM

Thank you, everyone. This diesel thing is new tome, but I love my car.
BillGrissom: I see that you have California models...like mine. Do you renew the air filter element with each oil change? I am thinking of using a vacuum cleaner next time. The filter element for the California model on the turbo version seems to be an enigma to many techs...even the old guys.
FUI for those with non-California cars, they added a scrubber beneath the turbo, so they reconfigured the air filter. It is taller and smaller in dismeter and is located at the front of the engine on the passenger side.

Diseasel300 · #14 07-01-2018, 05:11 PM

Why on earth would you replace the air filter at every oil change? Unless you live down a dirt road, you should be good for several tens of thousands of miles. Replace it when it needs it, not by mileage or time.

exsailor66 · #15 07-01-2018, 10:56 PM

Be gentle. My first diesel. I’m accustomed to paper filter elements, which I did ****-can at each oil change.