617: Probability of fuel shutoff valve failure? - PeachParts Mercedes-Benz Forum

phasmatisnox · #1 05-17-2006, 02:08 AM

I bought my '84 300D Turbo with some problems. One is that it does not shut off with the key. I know it has a weak vacuum system, so my first suspect is the fuel shutoff valve actuator on the side of the IP, as seen on DieselGiant. I'd link, but the site isn't responding right now. I traced its vacuum line back through the firewall. What's the best way to isolate the problem? Can I just try to shut off the car by hooking up a hand vacuum pump to it? How can I find out if my vacuum is so leaky that it won't actuate this valve? Thanks!

rhodes2010 · #2 05-17-2006, 02:29 AM

Sounds like you have the hand vacuum tool like mighty vac - good.

You need to determine if vacuum will kill the engine.
Hook the mighty vac style tool to the connector on the back of the IP and apply vacuum with the engine running.
This should shut off the engine.
If it does not - replace the rear of IP shut-off. (I bet it is fine).

The vacuum shut off is really simple, - two lines - the one going into
the interior that has vacuum constantly which goes to the keyswitch.
The other line comes from the keyswitch - that is the one you disconnected to test if engine shuts off. Locate the one into the keyswitch and attach the vacuum guage/tool and look at the vaccum while running.

If you get vacuum here, then attach the gauge to the line coming from the keyswitch that should go to shutoff.
Turn off the key while running and check the vacuum reading. It should jump up when switched to off.

If all those things worked out - you just need to snug the fittings. Shove them in tighter.

If you get a low vacuum on the vacuum source line going into the key then at worst you need vacuum pump rebuild (cheap and easy), or maybe just a leak somewhere.
If you have good vac on ythe source line, but low after the key, then it may be the key switch. I have never had to replace this - others here can assist.

phasmatisnox · #3 05-17-2006, 08:38 AM

Wow, great info! Thanks!

Quote:

Originally Posted by rhodes2010

Sounds like you have the hand vacuum tool like mighty vac - good.

You need to determine if vacuum will kill the engine.
Hook the mighty vac style tool to the connector on the back of the IP and apply vacuum with the engine running.
This should shut off the engine.
If it does not - replace the rear of IP shut-off. (I bet it is fine).

The vacuum shut off is really simple, - two lines - the one going into
the interior that has vacuum constantly which goes to the keyswitch.
The other line comes from the keyswitch - that is the one you disconnected to test if engine shuts off. Locate the one into the keyswitch and attach the vacuum guage/tool and look at the vaccum while running.

If you get vacuum here, then attach the gauge to the line coming from the keyswitch that should go to shutoff.
Turn off the key while running and check the vacuum reading. It should jump up when switched to off.

If all those things worked out - you just need to snug the fittings. Shove them in tighter.

If you get a low vacuum on the vacuum source line going into the key then at worst you need vacuum pump rebuild (cheap and easy), or maybe just a leak somewhere.
If you have good vac on ythe source line, but low after the key, then it may be the key switch. I have never had to replace this - others here can assist.

phasmatisnox · #4 05-17-2006, 01:16 PM

Okay, now that I'm awake and my four-hour class is done for the day...
I don't know a Mityvac, but I can borrow one. Should I buy some vacuum hose to do these tests? For some, there's no break or junction where I can hook up the vac tool. How much is some cheapo stuff just for testing?

Also, if my valve ends up being bad, can it possibly be refurbished? It looks fairly straightforward for a $50 part, and I've got facilities and resources here that could come in handy. Is it likely just a leaky diaphragm? I'm startlingly cheap sometimes, and I might attempt to repair it with skilled application of RTV.

Your input is highly valued. Thank you!

Quote:

Originally Posted by rhodes2010

Sounds like you have the hand vacuum tool like mighty vac - good.

You need to determine if vacuum will kill the engine.
Hook the mighty vac style tool to the connector on the back of the IP and apply vacuum with the engine running.
This should shut off the engine.
If it does not - replace the rear of IP shut-off. (I bet it is fine).

The vacuum shut off is really simple, - two lines - the one going into
the interior that has vacuum constantly which goes to the keyswitch.
The other line comes from the keyswitch - that is the one you disconnected to test if engine shuts off. Locate the one into the keyswitch and attach the vacuum guage/tool and look at the vaccum while running.

If you get vacuum here, then attach the gauge to the line coming from the keyswitch that should go to shutoff.
Turn off the key while running and check the vacuum reading. It should jump up when switched to off.

If all those things worked out - you just need to snug the fittings. Shove them in tighter.

If you get a low vacuum on the vacuum source line going into the key then at worst you need vacuum pump rebuild (cheap and easy), or maybe just a leak somewhere.
If you have good vac on ythe source line, but low after the key, then it may be the key switch. I have never had to replace this - others here can assist.

rhodes2010 · #5 05-17-2006, 01:50 PM

I can say that I too have used RTV silicone to fix diaphragms and even speakers that tore.

You won't have any trouble connecting into the system.
A foot of cheap rubber line of small size would not hurt, but may not be needed, the vac tool should have some attached.

phasmatisnox · #6 05-17-2006, 03:34 PM

Quote:

Originally Posted by rhodes2010

I can say that I too have used RTV silicone to fix diaphragms and even speakers that tore.

You won't have any trouble connecting into the system.
A foot of cheap rubber line of small size would not hurt, but may not be needed, the vac tool should have some attached.

Okay. Do you know what size the hose is, should I need to buy some?

phasmatisnox · #7 05-17-2006, 05:06 PM

I just took a look at all the fittings that I could see, and they look soft and fairly new- I suspect someone might have changed them out before, because it seems like there are some substitutions- two tees connected together to make a four, for example. I suspect that a previous owner tried to fix it, and reconnected something wrong. Maybe.

I still don't have a Mityvac- I might end up buying a cheapo $20 one from HF.