Tracing Shutdown problem - PeachParts Mercedes-Benz Forum

bgoodeve · #1 05-16-2003, 06:48 AM

My 93 300D 2.5T is fast developing the inability to shut down. I'm looking for a little guidance in isolating the problem. There are no obvious vacuum lines loose or leaking. Originally the lag in shutdown was barely noticeable. Over the past few days it has progressed to about 5 seconds. There seems to be vacuum at the shutdown valve since when I pull the line after shutting down you can hear it and the manual lever pops back up. Does what I'm describing indicate that the valve itself is failing? How much vacuum should be pulling at the valve? Are there issues in servicing this valve that I should be aware of? Other simpler things I should explore first. Thanks.

sixto · #2 05-16-2003, 10:59 AM

The spec is a source vacuum or 400mbar with a max leak of 6mbar/min (12inHG, 0.2inHg).

If your brakes and AC are working properly I'd check downstream of the vacuum source first.

With the engine running, disconnect the line that goes to the key switch and hook it directly to the stop valve. If the engine shuts down quickly, the problem is between the key switch and stop valve. If the engine shuts down as it does with the key, the problem is upstream of the key switch.

I think the key switch sources off a tee fitting in the big vacuum line to the brake booster.

Sixto
91 300SE
87 300SDL
83 300SD ... any takers at $1100?

slowmoe · #3 05-16-2003, 12:25 PM

I had the exact indications a few months ago and ultimately discover that my shut-off valve was bad. I don't remember the cost being too high. Although, it was not the easiest part of the car to access.

The manual shut off lever should remain down and not "pop back up." I believe this indicates a bad valve - it did on mine. I too had vaccum sound at the shut off with a disconected hose but not enough to keep the valve closed.

Let me know if you get to R and R and I will help as much as possible as it is still fresh in my mind.

bgoodeve · #4 05-17-2003, 07:49 AM

Thanks to both of you. I'll try the diagnostic hose swapping over the weekend. This "feels" like a valve problem since all othe vacuum systems are fine. Sloemoe, access looks tight. I didn't find a procedure for this in any of my references (then again I haven't looked real hard). Two questions; Is this a rebuildable valve (o-ring kit, diaphrams etc.) or just replace it? Either way, is it possible to remove the assembly without removing what looks to be most of the manifold assembly etc. A quick walk thorugh of how you did this would be appreciated. Thanks,

slowmoe · #5 05-17-2003, 05:59 PM

bgoodeve,

I do not know if this is a rebuildable part. For $28 (from Performance Products) I would not even consider rebuilding. I was able to do the job without removing the manifold. I think it all depends on how big your fingers are - it is definitely a tight fit.

To make it work you will need to first remove the ALDA - flat mettle piece above the shut off valve. I believe there are only two bolts holding the valve on. Remove both of them and then the valve. Take note of the position of the shaft coming down from the valve and its orientation with respect to the fuel pump. It is imparitive that the new valve be placed identically. On the bottom of this shaft you will see a small latch. This connects to a lever inside the fuel pump.

When replacing the new valve be sure that this latch is connected to the lever inside the pump. If it is not, the engine will run at max RPM until it blows. I remember someone on this site recently rebuilt an engine and improperly replaced the shut off valve – the engine was lost within a few minutes. It advisable to construct an emergency plan to cut off either fuel or air to the engine in the event that the valve was not latched properly – If you plan on cutting fuel lines, have a fire extingwisher nearby and remember there is quite a bit of fuel in the pump. I would recommend restricting airflow – make sure there is a tight fit.

You can test that the valve is operational by applying vaccum and seeing the stop lever drop. This confirms that the latch is seated properly.

Good luck. Let me know if you have further questions

bgoodeve · #6 05-19-2003, 06:02 AM

Thanks alot. Especially the advice about testing after the installation. I had a diesel on some power equipment run away from me once like that. The panic during screaming flat out high rpms was nothing I ever want to experience again. I'll probably deal with this next weekend. In the meantime I'll practice tying knots in a matchbox to get my fingers in shape.