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560SEL 560SEC 300SEL E320 Cab. MB Metris Van / 300TD 300TE 300SDL, 300D, Unimog 406 (Gone)
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Discussion Starter #1
My ultra reliable independent MB shop can't seem to solve this problem. Perhaps someone can comment.

When cold, <32F before the engine is warmed up, it hesitates on the highway Not enough to describ it a "bogging" but more than an engine "miss. Feels entirely like a fuel delivery problem as opposed to an ignition problem, but I could be wrong. Ideas? 101,000 miles, otherwise in perfect condition and service.
 

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560SEL 560SEC 300SEL E320 Cab. MB Metris Van / 300TD 300TE 300SDL, 300D, Unimog 406 (Gone)
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2,319 Posts
Discussion Starter #3
Yes, it starts right up.
 

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1985 500sel and 500sec 2012 E63 1989 Porsche 911
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5,234 Posts
fuel accumulator?
 

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Premium Member
1989 560 SEC - V-8 Kompressor - 95 E320 Estate
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1,910 Posts
Maybe take a close look at the distributor cap. Possibly condensation or a slight crack. If you choose to replace the cap I have been using Bremi brand on M119 engines with great results.
 

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Premium Member
1991 560SEC / 2013 GL550
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1,372 Posts
I had my fuel injectors removed and thoroughly cleaned out (the brass ones can be cleaned and re-used) which really seemed to help with the engine running smooth. Could definitely be something in there gumming it up. I did mine at 140k miles, first time it had been done

I had a fuel accumulator problem before, but that was happening right after the car starts. It's supposed to hold the fuel pressure after you turn the car off and if the part is bad, it'll have a bad time as the fuel goes through the line and re-pressurizes
 

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1986/1990 W126
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13,659 Posts
All the above make sense, but I would check threads by @H.D. and proceed from there. If there's a chance your adjustment screw on the tower hasn't been messed with (quite possible on yours because I think you usually buy lower mileage original examples to run?) Then you will have spot on diagnostic information there by checking the duty cycle. Don't let the shop mess with it, it's often the go-to thing and then this info is lost.

Decent fuel is worth buying for these preferably without ethanol. And occasional good quality fuel system cleaner. The MB one is made by Wurth so I use that periodically. I believe MB recommend it because of deposits on the backs of valves or something?
 

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1988 560 sec
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685 Posts
Problems like this are the most vexing, could be almost anything. My bet would be one of the fuel pumps is on the way out or the oxygen sensor is not doing it’s job properly..

Does the hesitation disappear suddenly at a specific engine temperature or do the symptoms gradually decrease as the car warms up?
 

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560SEL 560SEC 300SEL E320 Cab. MB Metris Van / 300TD 300TE 300SDL, 300D, Unimog 406 (Gone)
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2,319 Posts
Discussion Starter #9
Thanks so much, all. These suggestions are all consistent with the symptoms. Hopefully, I can get this solved soon.
 

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1983 380-SEL, '67 220D (sold), '65 200D (sold), '75 L-206D (sold)
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298 Posts
Bog down - I think it's fuel pump related, possibly clogged fuel filter. There is also a fuel tank filter. Both of those get worse when the car is run under 1/4 tank a lot. If the fuel filter has never been changes, try that. Also consider checking the fuel pumps.
 

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1990 420SEL, white ; 2002 VW Eurovan GLS
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101 Posts
My 420 does the same thing: when it is cold it starts fine, but it has a "miss" like thing going until the engine temp reaches about 75 degrees (during this time the vacuum gauge is deflected more to the right than normal), and then it's as if a switch is thrown: no more miss (and good indication on the vac gauge). I replaced both thermo switches without change.
 

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560SEL 560SEC 300SEL E320 Cab. MB Metris Van / 300TD 300TE 300SDL, 300D, Unimog 406 (Gone)
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2,319 Posts
Discussion Starter #12
Thanks, both.
 

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1990 420 SEC
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189 Posts
Below written is in case it is fuel injection related.
As engine warms up injection controls goes to closed loop and fuel amount is controlled based on O2 sensor readings instead of preprogramed chart on injection controller. That preprogramed chart does not work satisfactory if sensors, mechanics of injection system and EHA do not work as intended. When warm the controller is able to correct things based on O2 sensors output. The transition form open to closed loop can be felt if there is something wrong with the system. There is a kind of easy fix to mask that but member H.D. tells it is not the right thing to do. Kind of waiting him to tell what to do instead.
 

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560SEL 560SEC 300SEL E320 Cab. MB Metris Van / 300TD 300TE 300SDL, 300D, Unimog 406 (Gone)
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Discussion Starter #15
OK, my ordinarily flawless mechanics who are stumped look like they are gonna throw some parts at the problem. Not my favorite way to proceed but, what the heck. I end up with some new parts. Never a bad thing on a 30 year old car.

Top two likely fixes from the mechanics are EHA and O2 Sensors.

Any guesses on which you would do first for the mystery problem described above? (Please note: it won't do anything wrong when fully warmed up.)
 

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1990 420 SEC
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189 Posts
It most probably isn't O2 sensor as the car works ok when warm. When warm computer gets feedback from O2 sensor and corrects air/fuel ratio. Could be EHA. There would be ways to test its function. There are instructions at manuals which are at stickies at top of the page.
 

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1991 560sec. 1969 280SL
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1,077 Posts
Maybe the heated o2 sensor heater is inop causing a delay in o2 sensor output. It’s worth checking out at the sensor wiring to see if power and ground are being supplied to the o2 heater. If no power I think the fuel pump relay is where the sensor gets its 12v. Check for ground and if necessary clean up ground.
 

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1986/1990 W126
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13,659 Posts
Do you use ethanol fuel? It's good at killing EHA units.

The other problem with replacement parts is they may not be of the same standard, even from MB. Another reason to test what you have.

O2 sensors are easy to test aren't they? I've never had a car with one so I've not done it.
 

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560SEL 560SEC 300SEL E320 Cab. MB Metris Van / 300TD 300TE 300SDL, 300D, Unimog 406 (Gone)
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Discussion Starter #19
Thanks again. We are gonna test the [email protected] sensors and replace any questionable units. Then on to the EHA. I guess Bosch is it, right?
 

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1986/1990 W126
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MB Bosch is better than just Bosch. But yeah. It was the cure for my SEC, and I didn't think it was expensive from MB really.
 
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