Difficult start of 300SD (W126)

1984 300SD 224Kmi normally engine springs to life instantly, but it recently began missing starts. it sometimes takes 2-3 tries to get it running. then, when it
starts, it runs well immediately.
it seems like the engine is not getting fuel on the failed starts.
there is a little brown(?) vacuum hose that connects to something near the fuel system(?) with a small rubber elbow. if i pull this off, i can hear a vacuum released and then the car will start, first try, every time. if i do not re-connect this vacuum hose, the engine will not shut off unless i press the stop lever.
i figure this vacuum needs to be released to start the car and enabled to shut it off.
what need to be replaced to automatically release the vacuum when i turn the starter? (if that is indeed the solution)
thanks very much, --rr
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ozmodiar wrote:

I'd say the valve in the ignition lock must be faulty and leaking vacuum. It's responsible for feeding vacuum to the thingy that stops the fuel from going to the engine and shuts it off.
Someone else will probably explain it much better with all the right terminology ;)
Ximinez
--
Our three weapons are fear, surprise, and ruthless efficiency...
and an almost fanatical devotion to the Pope....
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Yes, I had to think about this a bit, but I think you have it right. Vacuum is applied to STOP the engine. This is done via a vacuum valve that is part of the ignition switch that applies vacuum by connecting the vacuum source to the bellows on the injection pump when the key is switched off. Usually, the common problem is the engine won't turn off because of a vacuum leak from a broken hose, connector, etc. But in this case, the vacuum valve in the ignition could be leaking so that it applies some vacuum even when in the on position.
I guess the other possibility would be that the vacuum after being applied to stop the engine is not then vented when the ignition is in the on position. Not sure how this is done, but I would bet it's part of the vacuum valve on the ignition switch too.
You could hook a vacuum tester to the line going to the shutoff bellows, which apparently you have found. It should have no vac with the key in the on position and vac when you move key to off.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Vacuum is needed to stop the motor and is supplied to the shut down device via the brown vacuum line that you cite. Usually the shut down is the problem and the motor doesn't stop. I suspect the "ignition" switch's vacuum control is the problem here.
--
2007 T.G.Lambach. Publication in any form requires prior written
permission.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
And this usually happens after the shut-off ruptures and engine oil gets sucked into the vacuum switch. I always flush the vacuum system with brake clean spray by having the engine running, disconnect the brown w/blue stripe hose from the vacuum unit, turn off engine so vacuum is at the hose, and then spray brake clean into the hose. It will go thru the system and vent into the engine.
#14 is the shut-off switch:
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F 3193&Ma7%2E952&GAr2%2E120315&CT=F&cat18&SIDF&SGR5&SGN
"-->> T.G. Lambach <<--" <"T.G. Lambach at NoHamorSpamcomcast.net"> wrote inmessage

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I never thought about it before, but is there some recommended replacement interval for the injection bellows to avoid having it fail and oil getting sucked into the vac system? How hard is it to change? I've got an 80 300SD with 125,000 miles with the original.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.