I have noticed on cold mornings I am getting an Oscillating Idle. When i back up the car it lunges backwards then hesitates then lunges backwards again at regular intervals. The same would happen going foward. I can also hear the engine speed up and slow down at about 1 second intervals. Any thoughts on why i am getting this uneven idle rhythm? Thanks in advance for your comments. I value this forum and the help i receive here.

Since it's OM616, when was the last times the valves where adjusted?

Does the car smoke?

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

I adjusted the valves 3 months ago and replaced the injectors with rebuilt ones using monarch nozzles. The car does not smoke at idle but it does some under acceleration especially when from a stop going up a hill. It does not burn any oil as i check the level every week.

Once the Engine reaches operating temperature does it smooth out?

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84 300D, 82 Volvo 244Gl Diesel

Yes. It smooths out after the car is at normal temperature. Sometimes at a stoplight i can here a slight oscillating noise at idle but if i put the car in neutral it gose away immediately. Of course in neutral it is no longer under any load with the torque converter. I dont always here the oscillating noise at idle.

It could be several things happening at once - the worst I can think of I won't mention at this stage!

I'd look at the condition of the motor mounts plus the third one under the transmission and change the fuel filters.

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I changed all the filters and the fluids 3 months ago. The nitor mounts are in good condition. The engine provides good power and acceleration. It so far has only got to 32 degrees Fahrenheit here in North Carolina but the engine starts ok with some nailing for 30 seconds or so on the coldest mornings. I am getting on average ablout 25 mpg in fule economy. I drive 21 miles one way to work. Half of the miles are at highway speeds. Since i bought the car 3 months ago i have notice my fuel economy has been going up. I started with about 22 mpg.

I had an oscillating idle on my w115 220d which improved substantially by adjusting all of the linkages that control the injection pump and butterfly valve on the intake manifold. The factory service manual has specifications for all of them. I did my best to follow their instructions, and it really helped out. I can still notice a very subtle loping idle on rare occasions, but it is 90% better.

Mercedes-Benz Model 114/115

It is section 30-300 that you could try.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

Could be a number of things,.. as Stretch said.


1) Change your fuel filters, you could be getting waxy buildup clogging the filter from
late summer fuel reaching its cloud point in the cooler weather.

2) Re-adjust you valves again...they may have tightened up again.

3) After re-adjusting your valves, adjust your idle speed according to the FSM.
A photo tach (Harbor Freight) will give the engine rpm, and your the temp
gauge in the instrument cluster should give you the approximate oil temp.

http://eva2.homeip.net/W115/program/Engine/615/07-100.pdf

4) You may just need to use the cold weather start procedure in the owner's manual.

IIRC, before inserting the key, depress the accelerator pedal (to give slack on the idle adustment Bowden cable),
rotate the the idle adjustment knob on the dash clockwise, release the accelerator pedal, insert key, glow
(or double glow if very cold) and start the engine.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Well the lack of burning base oil means a lot overall. Especially when using the 616 type engine on the highway.

I suspect most posters viewing this site go right to thinking the unthinkable. I guess that includes me. Yet there is a small chance it just might be a change in the fuel supply pressure to the injection pump. Something like the lift pump piston is sticking for example for those that think I am off the wall. Round the bend perhaps though I suspect.

Close off the return line to see if there is any change is a test I might do. Really though I would want to put a gauge on the system to make sure the fuel pressure to the base of the injection pump is remaining constant.

If it is some kind of flakey governor problem the injection pump might benefit from a proper solvent soak. To have a 115 that burns no noticeable oil indicates a really good engine as it is one yardstick I use to determine their probable overall condition. These engine rev pretty high on the highway and engines with real miles seem to have some ring issues. At back scraping base oil off the walls really well.

Both my later 616 engines do use some base oil and are still very good engines. One very little and another about a quart every 1200 miles or so on the highway. If this is a real lower overall milage example it would have had to have sat unused for long periods. Or usage might have been very small short local trips only. This could account perhaps for your improvement with use. You are burning the accumulated carbon out.

Then again I am pretty much totally unfamiliar with your injection pump type.

Other than it is a little different than those that are in the 1977 and later models. You do have to change the injection pump oil sump out periodically on yours for example. Actually make sure to check the current oil level in there as well. If you have not already.

Try to include as much as you can of any observations. Even if you feel they are not applicable to your issue. Many times what is left out is the most signifigant clue on what may be going on.

There was a time when people would post their issues for example. Omitting the car was or had been on waste vegetable oil. This lack of mentioning it made guessing what may be the persons issue present almost impossible.

Plus there are a few other odd things that differentiate your type of injection pump. That may be common faults other members hopefully will mention with your type. With some luck your issue on your system may be more common to some because of this.

You are to me describing either a car with low miles or one that has a rebuilt engine. If it is an automatic transmission model. There was another thing I might consider. I would check the fluid level both properly and carefully. As the automatics used in these models including the 123 series seem to be really sensitive to even relatively minor low fluid levels.

Possibly even fluid higher than desired levels. Yet that has never really seemed to be a reported issue. I have limited experience in life but find their automatics of that era to be far too sensitive to automatic transmission fluid levels than any other brands I have owned.

I want to say thanks to all the great input and advice given to me on this topic. I will adjust the valves again and adjust the idle speed and throttle linkage as suggested. It may take me a while because I am working 12 hours a day 7 days a week but when I finish I will report back with the results. This car is completely rust free but was rarely driven in the last 20 years but the owner did keep up maintenance with regular oil changes. It starts easily and stops abruptly kicking back on it self when I push in the stop knob. The cam shaft looked brand new with the original paint marking that looked like thay had just been applied. I think I have a good engine I just want to take good care of the car. I really appreciate all of your help.

is your coolant temperature sensor on the engine block working?

Coolant temperature seams to be working fine. It stays steady at 175 when the car has warmed up. It may go slightly higher in the summer with high speed driving on the interstate.

Not sure why the coolant temp sensor entered this discussion.....you have a mechanical governor on a mechanical injection pump...

Back to the car at hand.....this vehicle has the pneumatic governor right? If that's the case, since the issue seems to be related to cold mornings, it's possible you've got some sticky linkage bushings causing your hunt. As Shortsguy suggested above, check your linkages, but take it a step further and pop the ball joints apart on the linkages and clean out the goo, then re-lubricate them with a couple drops of ATF.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Subscribed as I too have and love dearly an OM616 powered Mercedes albeit newer .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better