Engine Shutoff Valve Times - OM 617
 

I am investigating my 1984 300SD's shutoff time. At one time, the engine was taking 12 seconds to stop with the key. In 12 seconds, I could already be in the house if I hurried. Something had to be done. I decided to follow the directions for replacing the shutoff valve at Dieselgaint. The directions are great. The link is: http://www.dieselgiant.com/repairyourshutoffvalve.htm

I ended up buying two vacuum shutoff valves at the JY for $9. Instead of taking the shutoff valve off of my car, I decided to test the used ones directly on the vacuum line coming off of the vacuum pump. The arms of both of the shutoff valves take 7 seconds to fully retract. I added two small vacuum reservoirs to the shutoff valve lines. I managed to get the engine to shut off in 7 seconds.

This is what I think is happening in the 7 seconds. For the 5 seconds, it seems the engines idles normally, but in the 6th second, the engine begins to slow down. In the last second, the engine shakes and dies like it is starving to death.

Is the 7 second shut off time normal? The engine will shutdown instantly as quickly as I push the stop lever. I am suspecting that there is not enough vacuum being produced by the pump. However, the car's brakes work great. The HVAC doors open and close as they should. The transmission shifts nicely. The vacuum system will even hold vacuum overnight. Any ideas?

No. Should be more like 0.7 seconds. You have a vacuum problem somewhere.

As determined by...?

tangofox007,

I agree that .7 seconds would be more like it. Interesting you should ask how I know the system holds vacuum. I was having problems with the temperature gauge, so I replaced the instrument cluster with one from a 380SE. I use the economy gauge as my vacuum gauge, and I was surprised to see that the system held vacuum overnight.

Do you think the cause of the problem is the vacuum pump?

Answer
 

YES, there is an issue with the vacuum system.

The shutdown time is .5 - .7 second with a good vacuum system.

A good system will generate 28 - 30 inches mercury of vacuum.

FYI:
Any vacuum pump reading of 13 inches mercury of vacuum or less = serious issues.



.

How do you know there's an issue with the vacuum pump? He hasn't posted any vacuum readings that I can see. It could be a bad shut off diaphragm. You need to put a Mity Vac on the system and tell us how many inches of vacuum you have and your elevation.

I just checked the vacuum--19 in Hg at 2000 ft. Seems to be low. We expect around 26" at this altitude for evacuating air conditioning systems. I am considering visiting the JY to pick up a new pump.




Otherwise, these are the instructions I found for rebuilding the vacuum pump from Stevo:


1) Loosen the power steering pump belt and remove it to get it out of the way.
2) Remove the top radiator hose.
3) Remove the metal vacuum hose from the pump.
4) Clean the gunk out of the Allen head bolts holding the pump to the engine.
5) Remove the six bolts and pull the pump off and out.
6) Clean the gunk off it
7) Remove the seven slotted screws that hold the pump housing together. Have a hammer and chisel ready. Save one for the impending trip to the hardware store. M5x16, .80 thread.
8) Note the orientation of the three valves. The replacements must go in the same way.
9) Loosen the nut in the center of the piston. Provide pressure on the arm on the back to help with this.
10) Push the piston out of the bore from the slots in the back using a Popsicle stick.
11) Remove the weird cardboard-like band from around the piston and the o-ring underneath it. Clean the piston up and smear some oil on the new o-ring and band before fitting them onto the piston.
12) Clean out the piston bore, and smear some clean oil in it. Carefully push the piston into the bore (you remembered to put that washer back in there first, right?) and struggle to get the spring loaded threaded rod back through the center of the piston. Replace the outer washer and new captive nut. This is the hard part.
13) Fit the new seals for the three valves, then drop in the valves in the original orientation.
14) Fit the new cover seal and put the cover on, using the new screws from the hardware store.
15) Loosen the fitting on the top where the vacuum line connects. Inspect the check valve in there and make sure it works. Replace if needed.
16) New gasket (or sealer, if you've forgotten to order a new gasket) and re-install the pump, vacuum line, power steering belt, radiator hose.

No problem with 19 inches at 2000ft. You're good to go.
Does the shut off valve hold vacuum when you pull a vacuum on it alone?

Correct
 

Oops.
I typed pump, and was thinking system.
Thanks for catching that, now corrected.

The shut off valve holds 22 in with the Mity Vac and drops to 21 in after a minute.

Shut off valve is good.

Put Mity Vac in line with the shut off vac line and turn the ignition switch off to determine how much vacuum is being sent to the valve by the switch.

Your not getting enough vacuum out of the main vacuum line it sounds like. Sometimes they get clogged.

When you attach your gauge, does it rise up quickly, if not then there is a problem.

Kerry,

I got 21" also on the second test.

79Mercy,

I was thinking about clogged lines when I was doing the second test. I will think of a way to test for this.

Thanks.

Trip to junkyard canceled!!!!
 

Looking to see if there were any clogs, I removed the vacuum lines coming off of the main vacuum line. When I bought this car, someone had reworked the vacuum lines with their own parts--hard to match the vacuum routing with diagram stick. I believe this work was done at the Mercedes dealership, since the main vacuum line was marked Daimler-Chrysler. One of the t's had a small outlet (black), and the other t had a larger outlet (white). I switched the lines around, and engine instantly shuts down. Transmission shifts smoother since there is more vacuum.

Fixed!!!!!

Thanks.

Good news!