Vacuum Shut Off Valves - Mercedes-Benz Forum
 
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post #1 of 5 (permalink) Old 10-02-2017, 03:58 PM Thread Starter
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Vacuum Shut Off Valves

Having an issue with 1982 300CD taking 30-60 seconds to shut down on its own after key is turned to the off position. Car has 180,000 miles.

Broke the MityVac out today and measured 15HG from the brown vac line which connects to the steering column shut off valve. However, the valve would not hold vacuum and lost about 1HG every 1-1.5 seconds. The car was not running as the valve should hold no vacuum in that situation.

Next, I moved to the VCV shutoff valve and in two pumps of the MityVac with car running the gauge shot to 15HG but then immediately came down to 5HG or less and did not shut the engine off. The vac line by the VCV shutoff looks to be ok. It does not look original as in old/degraded and may have been replaced somewhere along the way. I have extra vac line and will replace before ordering the valve.

Vac pump itself is showing 20HG. Also have all service records on the car from 112,000 miles to current. The prior owner was not a DIY'er and took the car to an indy for service. The records do not show either shut off valve ever being replaced. Everything inside the cabin related to vacuum functions properly except for the defrost dash pod.

The tests indicate that both valves require replacement. However, wouldn't the motor simply not shut down especially if both valves are bad?

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post #2 of 5 (permalink) Old 10-02-2017, 06:33 PM
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I am not sure, but I think you might be a little confused. There is not such thing as a “VCV shutoff valve”. There is the shutoff valve and the VCV valve, and they are different things. The VCV valve controls the amount of vacuum that goes to the vacuum modulator according to the throttle position.

As far as I understand, the VCV valve doesn’t hold vacuum, in fact it was designed to leak a certain amount of vacuum depending of the throttle position and requires a specific procedure to test it.

By your description, the shutoff valve is partially shot, it should hold vacuum. If you are going to replace the shutoff valve, please carefully read the instructions on how to do it because if you install it in the wrong position, the engine will run away.

Also, that faulty defrost pod or any other leaky pod in the dash can make engine shutoff slowly if the climate control is running.
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post #3 of 5 (permalink) Old 10-03-2017, 06:35 AM Thread Starter
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Originally Posted by luisg92 View Post
I am not sure, but I think you might be a little confused. There is not such thing as a “VCV shutoff valve”. There is the shutoff valve and the VCV valve, and they are different things. The VCV valve controls the amount of vacuum that goes to the vacuum modulator according to the throttle position.

As far as I understand, the VCV valve doesn’t hold vacuum, in fact it was designed to leak a certain amount of vacuum depending of the throttle position and requires a specific procedure to test it.

By your description, the shutoff valve is partially shot, it should hold vacuum. If you are going to replace the shutoff valve, please carefully read the instructions on how to do it because if you install it in the wrong position, the engine will run away.

Also, that faulty defrost pod or any other leaky pod in the dash can make engine shutoff slowly if the climate control is running.
On the same page now. Thanks

And to clarify the CCU was off when referencing the 30-60 second delayed engine shut off.

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post #4 of 5 (permalink) Old 10-06-2017, 06:18 AM
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Why don't you connect your mitty vac directly to shutoff valve and test it on non-running vehicle. It is located behind injection pump towards passenger compartment. It has brown with black or white strip (dont remember) vacuum line running from ignition switch to shutoff valve. Pull the line and connect your mitty vac directly. If your shutoff valve doesn't hold vacuum on then you need to replace it. If it does then you need to clean vacuum line and replace all rubber connectors. Sometimes engine oil from leaky shutoff valve travels up the vacuum lines to ignition switch and kills all rubber connectors along the way -- softens and bloats them. Also you may have vacuum leak somewhere else that prevents shutoff valve from working properly.

Hope this helps.
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post #5 of 5 (permalink) Old 11-18-2017, 02:35 PM Thread Starter
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Finally replaced the steering column shut off valve yesterday which reduced delayed shut off to 3-4 seconds. Removed glove box today to test the vacuum switch over valve. Low and behold the blue vac line was plugged with a screw!!!

When I purchased the car one actuator under each seat was bad. Removed both seats and replaced all four actuators a year ago but seat backs still didn't lock. Tested seat back locking valves and actuators with Mighty Vac at that time- all was good. Once screw was removed from vac line and line connected to switch over valve the seat back locks started working. Delayed shut off was reduced to 1-2 seconds.

Tested shut off valve on engine and it's fine. Tested all locks a year ago and found fuel flap actuator bad. Replaced it at that time. Most rubber connections under hood have been replaced. Will recheck door/trunk lock actuators. This is the same thing that happened on the 85 wagon. Replaced one actuator and as the vac pressure increased it put more strain on the other actuators. The locks functioned properly or rather a hair slower than normal due to small leaks popping that delayed engine shut off.

I worked on the car on/off today, tired of it this afternoon, directed a few expletives its way and went inside to make a Bourbon.

'85 300TDT Nautical Blue 126K
'87 300TDT Midnight Blue 188K
'87 560SL Anthracite Grey 65K
'92 600SEL Project in waiting 128K
'10 Ford Expedition 86K
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