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1981 380 SLC
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20 Posts
Discussion Starter #1
This topic, I am well aware has been discussed infinitum, however I am still trying to track down a believed vacuum leak in my Euro 1981 380 SLC, the two questions I have are:

1 / Are all the vac systems interconnected? i.e. if I have issues with the seats / doors not locking would this effect idle circuit as well?
2/ I would guess that the fuel tank needs to be under pressure, so I should get an intake of air when I take the fuel cap off – True or False? (I don’t by the way)

The symptoms for those that care to read on:
  • Poor idle on start up
  • White smoke (esp. if it’s been raining)
  • Vacuum Gauge only goes to middle (never pinned to the left)
  • Sounds like it’s running on 7 cylinders – this improves once warm (temp gauge reaches normal point)
  • Car runs ok once warm and the vac gauge improves (3/4) still never pinned to the left.
  • The tube (large diameter) that connects to the air box has mayo in it (oil and water mix)
What I have done so far for those that are still reading:

  • Purchased a hand vac pump and checked the visible vac lines in engine bay
  • New rotor on the dissy
  • Checked leads, all have spark
  • Pulled the vac line that goes to the vac store under the fender, heard the air being sucked in
  • Checked the line that goes into the intake manifold ‘I think’ based on the pictures below
I have looked at the following vac diagram (below) and find them hard to follow:

  • I only have two lines from the dissy not four as pictured (no air pump on my car)
  • Could find 43 switch over valve
  • Or 17 frequency valve
  • So gave up with that and tested what I could see which all held pressure
 

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1980 450SL & 1988 560SL
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1,667 Posts
Check the check valves at the firewall / drivers side where the tubes go into the cabin. This is where the systems are seperated, with the check valves and they could be stuck open. They are one-way units that can be tested by sucking or blowing on them removed.
 

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1980 380SL
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65 Posts
Been having similar problems with my 1980 380SL, poor running when first started but OK once warmed up, central locking not working.
Not had much time to investigate yet and I'm away from home for a few weeks so hopefully someone will come up with some suggestions in between now and when I get back home to help us out here.
 

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1981 380 SLC
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20 Posts
Discussion Starter #4
Graham, does your economy gauge perform in a sililar maor to mine? I will update this tread after the weekend once I have had another stab at this.
 

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2008 E320 Bluetec, 1988 560SL totalled unto death). 1997 Dodge Ram 1500, 1983 300CD
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1,308 Posts
Picture 2 above

Looks like it is trying to show the vac connection at the rear of the intake manifold. If so, that is usually the problem since it is so hard to see and service. If not, check it. The hose from that fitting runs along the firewall to a 4-way connector. Three vac hoses go from that connector through the firewall. One hose goes to the "economy" gauge, one to the door lock and one to the climate control. A leak in any of those connections will give you a vac leak that will affect engine starting and idling.

At idle and at no-load cruising your "economy" gauge should be pegged to the laft (black) side. Anything else indicates a vac leak.

One of the first rules of fault isolation is (or should be), fix one thing at a time so that you know what caused the problem. There is also NO rule that says you can only have one fault at a time. My advice is to fix the vacuum problem first then go on from there.
 

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1980 380SL
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65 Posts
My vacuum gauge shows hard left going downhill with my foot off the gas and all the way in the red when I boot it but at idle is somewhere around the middle but when cold goes a bit into the red.
I suspect my non functioning central locking and poor running may be connected(or not )if you see what I mean! so hopfully it will have stopped raining by the time I get home and the temperature will be above freezing and I can investigate further!
 

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1980 380SL
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65 Posts
Had to get a new alarm/immobiliser fitted last week and when we opened up the drivers door found all the vacuum lines disconnected and just lying loose in the bottom of the door!!!
Reconnected the lines still no central locking, opened the other door and found the same, connected them and the system worked, except the boot which locked when the car was opened and unlocked when the car was locked, swapping the hoses fixed that so now it all works, sort of as the vacuum valve attached to the drivers door lock leaks a bit so the car locks ok but doesn't unlock after standing for a bit as all the vacuum has leaked away.
Anyone know the part no. for the door switch?
 
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