Fuel Shut Off Valve Vacuum Test - Mercedes-Benz Forum
 
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#1 (permalink) Old 03-18-2012, 12:21 PM
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Post Fuel Shut Off Valve Vacuum Test

My 1985 300D has a delayed shut off with the key. When the car is very warm, it takes longer to shut off. To test the fuel shut off vacuum valve, I applied vacuum to the fuel shut off vacuum line with a mityvac while the car was off and the line holds vacuum. I performed the same test with the car on and it does not hold vacuum. Is the fuel shut off vacuum valve behaving as it should?
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#2 (permalink) Old 03-18-2012, 02:41 PM
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Originally Posted by Rathgeber View Post
My 1985 300D has a delayed shut off with the key. When the car is very warm, it takes longer to shut off. To test the fuel shut off vacuum valve, I applied vacuum to the fuel shut off vacuum line with a mityvac while the car was off and the line holds vacuum. I performed the same test with the car on and it does not hold vacuum. Is the fuel shut off vacuum valve behaving as it should?
As long as the engine shuts off right away and the valve holds vacuum then it is fine, you have a small leak in another part of the vacuum system. Or a check valve is bad, they should only let vacuum pass through one way.. and the transmission vacuum dashpot should allow vacuum both ways. Sounds like a vacuum check valve might be slowly leaking causing a slow shut off. One at a time.. Unplug the vacuum locks, then the climate control system and see if the car shuts off when one system is not connected and you just plug the hole off.


UNDER NO CIRCUMSTANCES WHAT SO EVER UNLESS YOU KNOW WHAT YOU ARE DOING WILL YOU CHANGE THAT VACUUM SHUT OFF. IF YOU DO NOT KNOW WHAT YOU ARE DOING AND START YOU CAR AFTER INSTALLATION, YOU COULD END UP WITH WIDE OPEN THROTTLE THAT WILL DESTROY YOUR ENGINE. IF YOU HAVE NOT DONE THEM IN THE PAST, DO NOT ATTEMPT, FIND SOMEONE TO PUT IT IN FOR YOU.
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#3 (permalink) Old 03-18-2012, 03:16 PM
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Post Fuel Shut Off Valve

Thank you for your reply. Is the fuel shut off valve supposed to hold vacuum both when the car is on and when it is off?
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#4 (permalink) Old 03-18-2012, 03:17 PM
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I left some info in the below Thread:

http://www.benzworld.org/forums/w123...vers-door.html
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#5 (permalink) Old 03-18-2012, 03:44 PM
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Post Fuel Shut Off Valve

Thanks,

That thread does not specifically address the fuel shut off valve. It addresses things to look for if the car will not shut off correctly.

I have isolated the problem to a couple of components, the locking system has been repaired and works correctly. The climate control is not attached currently to the vacuum system and its port is t'd. The EGR is not connected and its port is t'd. The switchover valve to the transducer and the shut off valve itself are the only remaining potential culprits. When off, vacuum holds well. When on, vacuum is strong at first. As the car gets warm, vacuum to the locking system disappears, the transmission gets sloppy and the car labors to shut off with the key. When cold, the car shuts off with only a small delay (less than 1 second) the transmission is perfect and the locking system works correctly.
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#6 (permalink) Old 03-18-2012, 06:41 PM
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[quote=Rathgeber;5166776]Thanks,

I posted the other thread because it addresses the items that can cause the slow shutoff issue reguarless of the condition of the Vacuum Valve itself.

In the Manual they have a similar test on the Vacuum Shutoff Valve on the Fuel Injection Pump but only with the Engine not running. If you hooked your Mighty Vac to the Vacuum Shutoff and pumped it to obtain Vacuum and after you were done it continued to hold vacuum for a reasonable time (the Manual gave no time limit) the Valve itself is OK.

If you did the same test with the Engine running and it would not hold Vacuum the Manual does not comment on that.

Last edited by 300Dman; 03-18-2012 at 06:56 PM.
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