Runaway diesel after installing check valve - om617 w123 Turbo Diesel - PeachParts Mercedes-Benz Forum

Sev · #1 04-15-2017, 07:06 PM

Gents, thanks for looking at this thread. This is regarding the yellow check valve that brings/sends vacuum to the engine/cabin, located near the firewall above the injection pump The original part number for this valve (the one originally on my car that I thought was bad) is part #123 800 02 78. From day one of owning the car, I had runaway diesel, the car would not shut off without pressing STOP. I took it to my mech, who placed a golf tee in the 3-way rubber vacuum elbow. This essentially solved the runaway problem. Later on down the road, the shutoff valve went bad, so I replaced it with a brand new one. Now today, I decided to put what I thought was a replacement part for the aforementioned check valve. However the part number I installed was 126 800 00 78/36. I originally bought one for my w124, but the compatibility chart listed my 82 300d in there, so I bought another one. Visually they are different, but after doing the 'suck test', I noticed both allowed air one way but not the other, so I assume they are functionally the same. After installing this part and connecting the rubber vacuum elbows for the two vacuum tubes going into the cabin, the car does not shut off again. Is this a common problem? What would be the solution? By the way, apparently the original check valve does pass the suck test and is only letting air in one way, so my assumption is that the old valve is still good and something else is at fault. Thanks, -Sev

barry12345 · #2 04-15-2017, 09:05 PM

Runaway or just will not shut down? If just will not shut down. You may have blocked the source of vacuum to the shutoff is a high probability. . This post is just what I suspect at the moment.

I think your expression of runaway. Occurs when the engine takes of and runs at very high RPM.

I wanted to add that a check valve used in a vacuum line is different than one in a pressure line. In a pressure situation you are trying to block pressure buildup from escaping. In the other you are trying to stop vacuum from declining. I think you just reverse the check valve to enable this. If you installed the check valve with pressure in mind it would not work in a vacuum situation.

tyl604 · #3 04-15-2017, 09:13 PM

Agree; runaway is not the right term.

BWhitmore · #4 04-16-2017, 02:06 PM

The shut down or off system on a W123 diesel also involves the vacuum control valve which is part of the ignition switch. This switch has two brown vacuum lines going to it. One is a vacuum source the other leads to the shut down valve on the injection pump. To check the valve on the ignition switch, start the engine and apply vacuum using a Mityvac to the brown plastic tube that supply's vacuum to the vacuum control valve on the ignition switch. If the engine shuts down then the switch is good, if not the switch is bad.

ROLLGUY · #5 04-16-2017, 02:55 PM

You have a vacuum leak either in the door locks (yellow tubes), climate control (green tube) that is stealing vacuum, not allowing enough vacuum to operate the shutoff valve. If the golf tee was put in the connection going to the yellow lines, and the engine shut down fine, then there is your problem. What I do to test is start the car with everything hooked up as it should and then turn the key to the off position. Then pinch off the rubber portion of each hose (rubber tee or hose joint) till the engine shuts down. You will then know the vacuum thief, and what to fix.

BillGrissom · #6 04-16-2017, 06:45 PM

I 2nd Rollguy. The cabin check valve is for when the engine is off, to isolate cabin devices from consumers in the engine, since some of the later operate as "controlled leaks" (ex. VCV) which would consume the stored vacuum (reservoir in top of trunk). You should get ~4 door unlock cycles before the stored vacuum is used up.

If you cap off the cabin supply and your shutdown valve now works, you have a leak in the cabin. Most likely culprit is the door lock actuators, specifically the trunk locker. Another culprit is the fuel door lock. Those are 2 rubber tees right at the fuel door (inside trunk). Reach inside the plastic trunk liner and you will feel them. A vacuum gage makes trouble-shooting easier.

ROLLGUY · #7 04-16-2017, 08:09 PM

I have seen a few valves go bad (drivers door) that control all the door lock vacuum pods. Either way, a Mighty Vac is your best friend if you own one of these cars.

Sev · #8 04-21-2017, 11:20 AM

Gents, thank you. I do have a known problem in the climate control. When you press the 0 button the tempomatic climate control does not turn off. I have to press and hold the 0 for some time in order to have it turn off. The climate control not turning off problem seems to be worse when the vacuum check valve is installed, better when it's not (easier to turn off climate control). If the climate control is the cause of the vacuum leak, is it a bad switch, or is it the inline set of five vacuum connectors that go behind the switch?