1974 W115 240D engine stopping problem (idle too high) - PeachParts Mercedes-Benz Forum

j-galap · #1 07-30-2020, 12:08 PM

Hi folks,

I've been having an issue with my '74 240D (automatic) and it's weird idle for quite a long time and need to finally sort it out.

The short version: when it's in park or neutral when cold, the idle is nice and low. When the engine is warm, the idle in park/neutral is ridiculously high. BUT if I put the car in drive, the idle drops to where it should be. I've since been turning off the engine with the brake pedal down and the car still in drive, otherwise I can't turn it off!

I made a video of the issue to demonstrate it better:

https://www.youtube.com/watch?v=GnyLXM9sW9s&t=7s

When the car is cold, there's no problem. But after driving for more than 5-10 minutes, it's an issue.

I have already measured the throttle linkages across the top of the engine to ensure they're the correct length (they are) and adjusted the idle by the screw on the throttle body, so I think the problem is elsewhere.

Any thoughts? Linkage problem or something more vacuum related?

Thanks

Shortsguy1 · #2 07-30-2020, 03:41 PM

That is a nice video. Thanks for putting that together. There is a very detailed procedure in the Factory Service Manual for adjusting all of the control linkages. Some of it requires a special measurement tool, but most of it you can do without anything special. I just checked and it does not appear like the w115 FSM is readily available online right now. Someone publishes it on occasion, but I could not find a copy quickly.

In your case, the issue is obviously temperature dependent, so I don't think it is linkage related. Somehow, the car appears to be getting more fuel when warm. So I guess it could be a problem with the injector pump, the air filter oil being consumed, the crankcase oil being consumed, or some sort of fuel like blockage that relaxes as it warms. The vacuum line from the intake manifold to the vacuum pump sometimes can carry oil up to the engine from the vacuum pump (front of engine, not the injection pump). Your line looks dark up top (oily?), but not obviously bad. There is a diaphragm in the vacuum pump that can tear and release oil where it is not supposed to go. Look at that line and see if oil could be traveling up it to the intake manifold.

That vacuum line from the back of the injection pump to the butterfly valve can also leak and cause problems with idle. Or a diaphragm inside the injection pump can do the same. I forget the details of how to check those, but they are here on PP if you dig around.

Good luck. Hopefully some of the experts will weigh in. I own a similar car, but am not super knowledgeable.

j-galap · #3 07-30-2020, 06:24 PM

Thanks for the detailed response. I'll go ahead and clean up all the hoses to begin with.

There's definitely an issue with fuel: if I'm going at highway speeds, throttle down and then lift off the car will produce a bunch of grey smoke, which I believe is unburnt fuel, correct? It sometimes produces the same on starting up too

resago2000 · #4 07-30-2020, 06:50 PM

Mine would smoke bad, The line that runs from the butterfly valve to the IP was cracked. replaced with standard vacuum line. no more smoke. ever.

j-galap · #5 07-30-2020, 07:01 PM

Good suggestion. Maybe as a piece of general maintenance I should replace all those vacuum lines? They're all intact from what I can see but pretty dry/inflexible now

Diesel911 · #6 07-30-2020, 07:04 PM

Quote:

Originally Posted by resago2000

Mine would smoke bad, The line that runs from the butterfly valve to the IP was cracked. replaced with standard vacuum line. no more smoke. ever.

As in the above. If your Fuel Injection Pump has a Pneumatic Governor vacuum leaks cause too much fuel. The mentioned line, the Governor Diaphragm and housing O-rings or wear in the housing can all cause vacuum leaks.

Shows old Pneumatic Governor and shows gravity feed drip timing also has pneumatic governor tests

http://www.benzworld.org/forums/vintage-mercedes-benz/1494188-75-240d-injection-pump-oil-consumption.html

Governor Diaphragm

http://www.peachparts.com/shopforum/showthread.php?t=223548

j-galap · #7 08-09-2020, 11:32 AM

Thanks for the insight!

Another thought (forgive me if this is way off track): Could the issue be made worse by vacuum leaks elsewhere on the car - away from the engine? Reason I ask: I had the door panels off a month or so ago and noticed that on two of the doors a bunch of the central locking parts/hoses were missing. No idea why they're gone (and I didn't even know the car originally had central locking until then!)

Or is this a whole separate vacuum circuit? Apologies, I'm a novice when it comes to vacuum stuff!

Thanks