1982 300SD Vac Bleed valve?

[BigJohnJB]

I am working on an intermit vacuum problem. Can someone explain how the "Vacuum bleed valve" works. I found 15" of vac at idle. When I unplug and cap the line to the valve the vac goes to 20. The lines on the output side of the valve test good. Is the valve working normal if there is a 5" drop through the valve at idle or is the valve bad. This is the valve at the back of the injection pump. It is white on top and sits ontop of the shut off valve.

Thanks

[sixto]

Sounds like you're describing the vacuum control valve or VCV for the transmission. Its purpose is to emulate a gasser manifold vacuum signal to the transmission modulator based on throttle position. The vacuum signal to the transmission tapers linearly from max at idle to no vacuum at WOT, or maybe sooner than WOT. I don't know what the idle vacuum spec is for your car but 15"Hg should give you smooth shifts with low throttle input.

Is there a problem you're investigating or are you looking for a can of worms to open?

Sixto
87 300D

[toomany MBZ]

Yes, 15inHg is normal.

[BigJohnJB]

I am trying to find the problem with the vac droping to zero at times. While looking around I found that the valve I mentioned lowered the system vac from 17-18 to 13-15. If I unplug the input (and cap) to the valve and watch the vac, it goes up almost 5". I dont know if thats normal. The output side of the valve doesnt have any leaks so I believe the valve is causing the drop. It seems like alot of drop at idle.

I have replace the check valve in the pump and fixed many small leaks. I am leaning towards the valve inside the pump but found this oddity while looking around.

[sixto]

There might be dry and brittle hoses or connectors in the system.

Sixto
87 300D

[tyl604]

Your system is supposed to produce 21" of vacuum. At the VCV the throttle stop is supposed to be set at .05mm gap at WOT and the vac is supposed to be between 10-15" - this feeds the transmission. This vac setting is accomplished by pulling out the plastic dome on the VCV and turning the nut just slightly - however, yours sounds like it is probably OK.

Mine is an 81 300SD. The system vac checked out at 20.5" but at the VCV it was only 8" and I was getting bad shifts. I adjusted it up from 8" to 15" and the shifts smoothed out.

Also you might consider putting golf tees in the two rubber Y fittings where the hose goes to the EGR. Do not need the EGR circuit and it might be leaking vac.

[tangofox007]

Originally Posted by BigJohnJB

I am trying to find the problem with the vac droping to zero at times.

The number one suspect for intermittent vacuum loss is the check valve in the nipple on the top of the vacuum pump.

[tyl604]

Tango - are you talking about the green damper just above the VCV? See pic. If not, can you point it out?

[tangofox007]

Originally Posted by tyl604

Tango - are you talking about the green damper just above the VCV? See pic. If not, can you point it out?

No. The check valve is in the nipple where the metal vacuum line attaches to the vacuum pump.

http://catalog.peachparts.com/ShopByVehicle.epc?q=1982-Mercedes--Benz-300d-Diesel--Injection&yearid=1982%40%401982&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6190%3AED%7C10000135%3AMBC%7C1504%40%40300D&catid=240706%40%40Diesel+Injection&subcatid=240735@@Vacuum+Pump+Check+Valve&mode=PD

The plate that retains the valve tends to rust and break. That causes the valve spring and plunger to drop into the pump intake port. The parts then rattle around, intermittently blocking air flow.

[tyl604]

New info.

Thx