So my 300se with 68.000km on the clock is acting up. The problem is that when the car is idling, It will run OK, but not 100% stable, and will hesitate more and stumble more the warmer the engine is. When cold, the problem is small. But when warm, the car is undrivable.
The symptoms are worst at operating temperature, if you accelerate hard, it will hesitate and stumble, almost like it has some problem ingniting the fuel. That will also happens when you drive, if you keep a steady speed, you can feel the power is being lost slowly. The solution is to let go of the throttle, and press I down again, sometimes that will help to get it up to speed.

I've done a full tune up if the ignition system, replaced and tuned EHA, new O2 sensor, and did the fuel filter a year ago, will change it now, but I don't think the filter is the problem.

After reading online, i narrowed it down to be either the fuel pump, or the lamda adjustment (fuel mixture) or maybe the coil, altough I think that too is unlikely the culprit. Or an electronic problem somewhere.

One thing I have noticed, is when I unplug my temperature sensor at operating temperature, nothing changes to the engine, so I have replaced that too. But it did not help. Can the temperature sensor make my engine act like it does?

What do you guys think can be the culprit?

This thread over at BW has some ideas (if you can stand all the ads that pop up...:

How do I clean my 300E MAF sensor? - Mercedes-Benz Forum

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Your issue is likely ignition related. The coolant temp sensor won't have any effect because more than likely you are not running in closed loop. (Have you checked your duty cycle?).

I understand you said you did a "full tune up". When? What parts did you change? Did you use genuine MB parts? Bosch? Other? People tend to cheap out on the distributed cap. Bosch is $100. Pay it. The other ones last five minutes. If you take off your distributer cap and see some black charred carbon on the points, maybe your coil is running a bit too hot.

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2

Nothing cheap goes on my 300se, it's in mint collector condition Except for this problem though By full tune up I mean

-All filters
-Cap, wires and spark plugs
-Oil and fluid change
Did all of that a about year ago, the car is garage kept most of the year, but driven at least 2 -3 times a week.

I will buy a new fuel pump and filter tomorrow, and a new coil. Stuff is old so will need replacing soon anyway. Hoping that will fix it, but I am not sure. I will also do a double check my distributor cap.
By the way, the car has no problem starting, when cold. But when warm, it might not start at all. I have to wait for the engine to cool down a bit.

Bosch is fine, genuine is better. Cap and rotor are typically the culprit for your symptoms. It would be a good idea to change that suppressor plate that is just under the rotor.

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2

could be your eha is failing

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Peach Parts W124.128 User Group.

80 280SL
85 300SD
87 300TD
92 300D 2.5 Turbo
92 300TE 4Matic

Check throttle body sensor and possible leaky injectors. Although I would check the throttle body sensor first.

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1990 190E 3.0L

Remove this ICV valve clean it out, with white spirits , or car cleaner.. And look for splits in the connecting hoses both of them that come off it.,And any splits in vacuum hoses you can find in the engine depot The fuel distributor sits on a large rubber gasket or boot more like it .These are a known problem ,and split along the base . Do you have a econometer in the console [speedo] if so what is the econometer needle sitting at when ticking over?At engine tickover the needle should be bang over to the left . If its sitting in another possition ,you have a vacuum leak.

sondrusen this is the icv or acv valve in centre.I should have said clean you fuses and check each one .

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Did you install a new EHA?

If so, it may have been better to get everything running well before any "tuning". Did you note how many turns you made to tune it? If so, I would go back to original and then check the lambda duty cycle. Assuming you have no vacuum leaks and your HT is 100% that might give you a good starting point.

Good luck.

RayH

I checked the distributor cap today, and found that the brush that sits on the spring was gone :O So much for Bosch. Went and bought a Beru cap, so hope it will cure it One thing though, the Beru cap has that brush and spring, but the brush is pointing inward, so only the spring is at the top. Is it suppose to be that way? The brush sure is there.

The carbon center contact can be dislodged when installing the cap. It can get caught on the ridges on the rotor if the cap isn't slid straight in.

The carbon brush should be pointing out so it is what rides on the center of the rotor. The spring should be behind it to give it pressure.

OK, guess its something Beru did to make sure it doesn't fall off during handling. So will get the spring out and put in in the right way

Your car is going to run great for like 20 minutes with that beru cap

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2

OK, run and return then