Diesel Runaway - PeachParts Mercedes-Benz Forum

stuart750 · #1 02-04-2010, 10:03 PM

So, having just replaced my engine after some unfortunate circumstances with, well, old car things.

So, my new imperfections with the car are as such:

The power brakes are not powered. They are stiff, or rather, they require a lot of force to use them, and they do not work so well. I suspect something was not reconnected.

And, unfortunately, I have diesel runaway. When I turn the key, the car does not stop running. It runs just the same as before. I suspect the vaccuum pump that cuts off the fuel is stuck. I don't know how to get at it, so I'd like to learn how to get at that.

Right now, I stall the car to get it to turn off. Is this extremely bad if I do it twice a day for a few days before I get this assessed by my mechanic?

I have tried to press the emergency shutoff (the stop sticker on the throttle lever, and it slows the engine, but does not stop it)

I have also tried clogging the air intake, even taking the cover off and trying to clog it (with my hand, not a rag that could be sucked into the engine)

, which also only slows the revs, but does not stop them. (I try doing both, and it does not stop).

What should I do?

Skippy · #2 02-04-2010, 10:08 PM

What you have isn't a runaway. A runaway is when the engine receives fuel uncontrolled by the governor. Without intervention, this will generally result in engine speed increasing until catastrophic engine damage occurs.

You issue seems to be a combination of lack of vacuum to the shutoff solenoid, or lack of solenoid function when supplied with vacuum, combined with a binding or misadjusted accelerator linkage.

bustedbenz · #3 02-04-2010, 10:08 PM

That is not "diesel runaway", which is an engine rushing to beyond redline using uncontrolled fuel.

Your vacuum system is completely disconnected somewhere, or else leaking. The brake booster which makes it easier to brake, and the mechanism which shuts the engine off, both receive vacuum from the same vacuum pump. If this pump is working, you have a leak in the system that is absolutely massive, or else you have some lines routed improperly.

Skippy · #4 02-04-2010, 10:10 PM

The stop lever should still stop the engine regardless of vacuum issues, so there has to be something more going on. However, the lack of power assist on the brakes confirms there is a big vacuum problem.

What year and model are we talking about anyway?

elchivito · #5 02-04-2010, 11:03 PM

Thank god. I'd just gotten over reading the last two long runaway threads and having the beejeezus scared out of me, worrying that my 616 was going to go monster on me at any moment and destroy everything within a city block with shrapnel.

stuart750 · #6 02-04-2010, 11:21 PM

It's a 1980 240D.

Skippy · #7 02-04-2010, 11:37 PM

If you plan on paying someone else to fix it (which I don't recommend), using the clutch to shut off for a few days isn't going to do anything major. I suggest you get in the habit of doing most of your own mechanical work. If not, an old car is going to suck all your money out of the bank.

I would start with the accelerator linkage and get that squared away before I started vacuum troubleshooting. The linkage should be able to move the little arm on the injector pump all the way to the full fuel stop when floored, idle normally when released, and pressure on the stop lever should move the arm on the IP enough in the "less fuel" direction to stop the engine. Have a look at things and see if you can identify anything broken, bound, loose, or missing that might be preventing that.

Vacuum troubleshooting can be even more fun.

samboyellowsub · #8 02-05-2010, 01:51 AM

make note of what everything does with your throttle linkage when you go in the "less fuel" direction and you will notice what is not going enough when you puss the stop sticker on the throttle. you can disconnect whichever thing you think it is that the linkage is not allowing to bottom out far enough and then bottom that out piece out separately from the rest of the linkage system. while you're at it, clean and grease all the connections.

quite honestly, if its giving you that much trouble to cut it off, potentially it could be a nice strong motor that you have there! you just need to learn some rudimentary things about the fuel system and vacuum system. I suspect that your power locks don't work either. how many miles are on your 1980 240D?

dadette123 · #9 02-05-2010, 12:17 PM

Do a quick, visual check on the following for vacuum issues before proceeding to the next troubleshooting steps: Check your brake servo vacuum line (see pic). The fittings can get brittle and break and/or easily broken when working on the engine. Check all rubber connector vacuum fittings for fine cracks and tight fit to the hard plastic lines (slightly bend each soft rubber fitting to check for cracks and lightly tug to check for tightness). Make sure your hard plastic lines aren't cracked but, be careful as they can be brittle.

Which brake servo line is correct for your car depends on the VIN number. I think 1980 saw a lot of variations(?).

dadette123 · #10 02-05-2010, 12:28 PM

I know he has vacuum issues (i.e. hard pedal) that could fix the stop engine problem but, also...

Question: doesn't this engine (616.912, M-injection pump) have a start and stop cable control (idle speed adjuster cable)? If it is properly adjusted, turning the knob completely counterclockwise will stop the engine.

Skippy · #11 02-05-2010, 09:42 PM

If it's like my 77, counterclockwise turns the idle up, clockwise turns it down. I don't think turning the idle adjust all the way down is supposed to stop the engine.

Codifex Maximus · #12 02-05-2010, 10:19 PM

Quote:

Originally Posted by elchivito

Thank god.

I was thinking the same thing; thankfully not a runaway.

I agree with the getting the mechanical throttle linkage working properly first. That way if a problem occurs, the stop should work right.

Then, check that ceegar hose from the vacuum pump (on front of engine) to the brake booster. Temporarily plug any exposed junctions on the cigar hose with caps (preferably) or gently inserted golf tees (if you have no other choice). See if the brakes are easier to push. If not, you need to diagnose lack of vacuum. Is it the pump, the hose, the plugs or your brake booster.

MightyVac is a good tool for diagnosis because it provides a vacuum scale and a pump in one tool - lots of nifty fittings too.

dadette123 · #13 02-05-2010, 11:15 PM

Quote:

Originally Posted by Skippy

I don't think turning the idle adjust all the way down is supposed to stop the engine.

Perhaps I am remember it wrong but, I seem to recall the manual saying it would. But, then again, my memory is not what it used to be...

stuart750 · #14 02-07-2010, 12:38 PM

Okay, I've sent it to my mechanic, and he says I need to salvage the old vaccuum pump from my other engine.

Would someone happen to have a picture from their W123 service manual on how to do this?

Skippy · #15 02-07-2010, 08:44 PM

No picture, but the vacuum pump is on the front of the engine, towards the right side (left as you look at it). It's held on to the engine with six (or is it eight?) allen head fasteners, 5mm I believe. The vacuum hose or hoses (one for piston pumps, two for diaphragm pumps) will also help you ID the pump.