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Old 12-16-2021, 06:05 PM
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OM616 Hunting Idle Pneumatic Governor

I have a 1976 240D with a Pneumatic type governor. The car was running great until 3 days ago. It was about 30 degrees outside that morning and the car started but had an uneven hunting idle. It saws back and forth high and low. It has continued to do this for 3 days even after the car is at normal operation temperature. It is especial bad when in gear under load. The idle oscillates back and forth no matter what idle speed I set it at. I cleaned the throttle body and venturi with brake cleaner to no affect. I made sure the hose coming from the manifold was not cracked or loose. The pneumatic diaphragm was replaced about 2 years ago and even before then the idle has never been so bad. Anyone have this happen to them before or know a solution? Thanks in advance.

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Old 12-16-2021, 08:01 PM
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Air in the fuel inlet line. If the governor, it could be the O-rings on the shaft causing a vacuum leak.
I don't know if the old method used on gassers would work to find a vacuum leak. You spray something like starting fluid around the suspected leak area and if the rpm changes you know it got sucked in through a leak.

If the vacuum pump diaphragm goes bad oil vapor gets sucked into the intake manifold. I don't remember the symptoms.

You could do the injector cut out test and see if one injector or cylinder is having an issue.

The fuel pressure relief/overflow valve springs becomes compressed over time and effects the low-speed fuel supply pressure. If you have one that can be taken apart the spring can be stretched to a 27mm free length and reused.
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Old 12-16-2021, 08:33 PM
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My '74's engine had some hunting idle before I repaired the leaky governor, but it wasn't all the time, only when cold. Another symptom is lots of grey smoke, especially when the engine is cold. Something else mine did was under moderate power the engine's output kinda surged and bucked, too, but it was subtle.

The governor should hold a vacuum, for sure. You can undo the banjo bolt at the throttle body and pull vacuum with your mouth. That's not very "by the book", I know, but you'll know whether or not it holds vac. If you pressurize it you can listen for the "hiss" of escaping air to help locate the leak. I also used soapy water sprayed on the rear of the injection pump to help locate the leak.

On mine the leak was at the inner/engine end of the shaft that runs through the rear most portion of the IP. The shaft and bushings were worn so bad that even fresh o-rings would not hold vacuum. The diaphragm was good.

But there could be other causes for hunting idle, like air in the fuel system, and others as mentioned by Diesel911.
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Old 12-16-2021, 11:29 PM
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