Difficult start of 300SD (W126)

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Ozmodiar - 02 Nov 2007 06:40 GMT

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

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The Spanish Inquisition - 02 Nov 2007 07:18 GMT

> 1984 300SD 224Kmi
> normally engine springs to life instantly, but it recently began missing
[quoted text clipped - 14 lines]
> what need to be replaced to automatically release the vacuum when i turn the
> starter? (if that is indeed the solution)

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

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trader4@optonline.net - 02 Nov 2007 09:40 GMT

On Nov 2, 2:18 am, The Spanish Inquisition <ximi...@myown.mailcan.com>
wrote:

> > 1984 300SD 224Kmi
> > normally engine springs to life instantly, but it recently began missing
[quoted text clipped - 23 lines]
>
> Ximinez

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.

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-->> T.G. Lambach <<-- - 02 Nov 2007 18:26 GMT

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.

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Karl - 02 Nov 2007 18:34 GMT

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=123193&M=617%2E952&GA=722%2E120315&C
T=F&cat=318&SID=46&SGR=105&SGN=05

> 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.

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trader4@optonline.net - 02 Nov 2007 22:30 GMT

> 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
[quoted text clipped - 16 lines]
>
> - Show quoted text -

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.

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