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Old 01-16-2015, 09:00 PM
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Charlie
 
Join Date: Sep 2007
Location: DFW, Texas
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vacuum leak - transmission modulator?

I'm chasing vacuum leaks on an '83 300SD. Gray on gray, in case that matters; about 269k on the clock and I've had it for a year now. The shut-off is always a bit slow; and sometimes recently I have to manually press the stop lever under the hood.

I noticed oil leaking out of the start switch so I replaced the shutoff vacuum solenoid on the ip (not quite sure whether that was the source of the oil but I've heard it can be).

Usually on those occasions when the motor keeps running after the key is turned to the off position, I can get the motor to shut off by shifting into one of the lower forward gears (with foot on the brake). Does this indicate where I am losing vacuum? Is the modulator bad?

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Old 01-16-2015, 09:54 PM
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I replaced the shutoff vacuum solenoid on the ip (not quite sure whether that was the source of the oil but I've heard it can be).

Yes the above is the source of the Oil.

If the 2 Hoses that go in the Vacuum Shutoff Switch on the Steering Colum Lock are not not connected on the correct Nipple it will not shut off properly.
The solid Brown Vacuum Tube goes to the top Nipple and the Brown Vacuum line with the Blue Stripe goes to the bottom nipple.
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Old 01-16-2015, 10:25 PM
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To trouble shoot for other Vacuum leaks you disconnect and plug off one system at a time. If then one you disconnected and plugged off improves the Vacuum then that system is where the leake is.

As an example you have only so many Vacuum Tubes going through the Fire Wall. The one with the Check Valves is the locking System. To see if there is a leak in the Locking System; under the Hood disconnect the Vacuum Line goint into the Check Valves and plug that (you are disconnecting and plugging the line closest to the Vacuum Pump). If your shutoff starts working OK then that would be an indication that the Locking System has a leak.
The Solid Black Vacuum Tubing is a vent line and is connected to nothing inside.

If you have checked all the Vacuum Tubes going through the Fire wall what is left is the Tubing that goes to the little valves on the Valve Cover, the Transmission line and the Trans moduel and the EGR line if you have one.

On the Transmission Moduel the Cap the covers the Adjustment has been know to Crack or even fall off.

Inspect all of the Rubber connectors under the Hood especially the 3 and or 4 way connectors.
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Old 01-16-2015, 10:29 PM
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If there is an advanced auto near you, they rent out hand held vacuum pumps. Extremely useful in testing the vacuum system. I think it was $75 deposit, when you return the pump you get your money back.

Also MB vacuum hose tends to be the best priced.
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Old 01-16-2015, 10:56 PM
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Charlie
 
Join Date: Sep 2007
Location: DFW, Texas
Posts: 205
Thanks for the quick replies! I have a mighty vac and it seems to be a good little vacuum pump. I've already diconnected the lines going to the EGR and replaced their 3 way connectors with straight lines. I do think I should order all new rubber connectors. What I was wondering, though, is why would shifting the transmission from park into low or 2 (not drive) some how seal off a vacuum leak?
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Old 01-18-2015, 03:00 PM
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I cannot imagine any way the vacuum system would know what gear the car is in. So I cannot answer why this would happen in L and 2, unless the vacuum line to the transmission modulator is somehow caught in the shift linkage. I can't visualize it, tho.

I would take a shot at an answer by saying that your motor mounts and/or transmission mount are worn enough that when you put it into gear the motor is moving enough to seal off whatever is leaking. That suggests to me a rubber connector or possibly even your main vacuum line to the brake booster.

Have you checked how much vacuum you are getting from the pump through the main line? That is a useful thing to do before anything else.

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