How fast should vacuum build? - PeachParts Mercedes-Benz Forum

tankowner · #1 07-31-2011, 10:59 PM

I've been knocking off some projects on the 240D recently and now I have returned to the problem of not being able to shut the engine down with the key. In the past I had tested the SOV with the Mity Vac and it seemed to be fine . . . can shut it down easily with the hand pump hooked directly to the SOV. I had also taken some measurements to make sure the vac pumps was pulling enough vacuum and it will do about 21", which seems good. Those things led me to believe that perhaps the vacuum valve connected to the steering lock (operated by the key) could be bad. It seemed to test out okay, but I wanted to be certain, so I replaced it . . . turns out, no problem there.

So today I did some more testing - I placed a vacuum gauge in line between brown line that leads to the key operated valve and the main vac line coming from the pump. I removed/closed off all other line (HVAC, door locks, etc.). The only other line that was still attached was the main line to the brake booster. Note: door locks not operational, but I have never noticed a problem with the brakes.

So then I started up the car and let the vacuum build. My question is . . how long should it take vac to build up. It got up to 10-15" fairly quickly, but then it took some time (maybe 2 minutes) to reach 21".

Then I turned the key off . . . the vacuum drops down to next to nothing, but the engine doesn't shut off. Then the guage begins to slowly climb back up . . . after about a minute it makes it up to 5" and it kills the engine.

So it seems to me that the pump just isn't pulling like it should . . . but I also don't know how fast it should be pulling. Worn out pump? Bad seals? Other options? In my searching, I see that the check valve on the pump and can gummed up, I'll go check it out. Figured I would ask about the pump though and get some feedback. Thanks.

Diesel911 · #2 07-31-2011, 11:24 PM

The year of your 240D might give a clue if you have a Diaphragm or Pitson type Vacuum Pump.
I had low vacuum once and removing and spraying out the Check Valve that screws into the Vacuum Pump solved the problem.
Others have found the internal plastic parts of the Check Valve broken.

There is also little Disc Valves inside of the Vacuum Pump that can get cruddy.
Several years ago I also removed the Front Plate of my Piston Type Vacuum Pump and cleaned the Valves. I sheared off 2 of the Screws in the process and had to use penetrating Oil, Heat and a Hand type Impact Wrench (they type you hit with a Hammer) to get the rest of the Screws off without shearing any more of them.

Remember which direction the Disc Valves Face.

tankowner · #3 08-02-2011, 01:35 AM

Quote:

Originally Posted by Diesel911

The year of your 240D might give a clue if you have a Diaphragm or Pitson type Vacuum Pump.
I had low vacuum once and removing and spraying out the Check Valve that screws into the Vacuum Pump solved the problem.
Others have found the internal plastic parts of the Check Valve broken.

There is also little Disc Valves inside of the Vacuum Pump that can get cruddy.
Several years ago I also removed the Front Plate of my Piston Type Vacuum Pump and cleaned the Valves. I sheared off 2 of the Screws in the process and had to use penetrating Oil, Heat and a Hand type Impact Wrench (they type you hit with a Hammer) to get the rest of the Screws off without shearing any more of them.

Remember which direction the Disc Valves Face.

It's the piston type - I guess I will have to investigate. The check valve is right where the main line connects to the pump, right? Thanks for the tips.

tangofox007 · #4 08-01-2011, 09:55 AM

Quote:

Originally Posted by tankowner

Then I turned the key off . . . the vacuum drops down to next to nothing, but the engine doesn't shut off. Then the guage begins to slowly climb back up . . . after about a minute it makes it up to 5" and it kills the engine.

Sounds like a leak in the shut-off circuit.

tankowner · #5 08-02-2011, 01:16 AM

Quote:

Originally Posted by tangofox007

Sounds like a leak in the shut-off circuit.

Certainly could be, but where? I guess I can test the SOV again . . . I believe it was holding vac though. I guess what I don't get is why it is that when I turn the key, the vacuum is released but doesn't kill the engine. However, with the Mity Vac in line, all I have to do is give it one pump (producing about 5" of vac) and she shuts down. What happens to the 15-20" of vac when I turn the key off? Obviously it is going some place. I guess I will test the SOV again and then isolate everything but that line and go from there.

tangofox007 · #6 08-02-2011, 08:51 AM

Quote:

Originally Posted by tankowner

I guess what I don't get is why it is that when I turn the key, the vacuum is released but doesn't kill the engine.

There should not be any significant "release" of vacuum. Vacuum is "released" (vented) from the shutdown circuit when the key is is in the "run" position. When the key is moved from the "run" position, vacuum should be routed to the shut-off actuator, but not "released."

Quote:

Originally Posted by tankowner

I guess I will test the SOV again...

None of the symptoms seem to implicate the valve, itself. Look at the switch (valve) and associated vacuum lines.

winmutt · #7 08-01-2011, 10:35 AM

Got the vac lines backwards?

tankowner · #8 08-02-2011, 01:10 AM

Quote:

Originally Posted by winmutt

Got the vac lines backwards?

Good thought, but no . . . I checked that one already.

funola · #9 08-02-2011, 09:15 AM

You have a leak outside the shut off circuit. Disconnect and plug those leaking circuits one by one to find which one(s) is leaking.

Leaks are often at the Tees, take them out, clean and test w a Mityvac.

tangofox007 · #10 08-02-2011, 10:01 AM

Quote:

Originally Posted by funola

You have a leak outside the shut off circuit.

That is certainly possible, but how would a leak outside the shut-off circuit explain the sudden loss of vacuum when the key is turned off, but the engine continues to run?

funola · #11 08-02-2011, 11:04 AM

[QUOTE=tangofox007;2762488]That is certainly possible, but how would a leak outside the shut-off circuit explain the sudden loss of vacuum when the key is turned off, but the engine continues to run?[/QUOTE

The shut off valve needs a certain amount of vacuum to shut off. If the leak is faster than what the vacuum pump can keep up with for a shut off, then it won't shut off?

tangofox007 · #12 08-02-2011, 11:08 AM

Quote:

Originally Posted by funola

The shut off valve needs a certain amount of vacuum to shut off. If the leak is faster than what the vacuum pump can keep up with for a shut off, then it won't shut off?

Note that the OP reports 21" Hg vacuum with the key on.

funola · #13 08-02-2011, 11:16 AM

Quote:

Originally Posted by tangofox007

Note that the OP reports 21" Hg vacuum with the key on.

.Maybe the leaky circuit was taken out of the circuit when that 21" of HG was measured?

winmutt · #14 08-08-2011, 09:19 AM

I did mean the ignition switch... FWIW it might now hurt to put a paper filter inline with the shutoff diaphragm valve. This can prevent oil contamination when diaphragm blows.