M104.980 high idle again

[Herrick Yuen]

Hi, my Merc is a 92 300CE-24 with the M104.980 engine. One of the haunting problems is the high idle and would like to seek your advice.
Three years ago (2019), the idle all of sudden stuck at 1500 rpm. My mechanic tried adjusting the duty cycle without success. It turned out the air potentiometer is faulty. I replaced it with a new Bosch one, the idle was back to normal. After 2 years of poise and quiet idle, the idle problem occurred again and this time it is like this:
Current problem

M104.980 idle problem

Throttle hesitation

It starts at around 1000 rpm, seems normal. However, it rises to 1300-1500 rpm after 1-2 minutes. This symptom happens every time. In case I depress the accelerator and release it, the idle does not return to 1500 rpm but stays at 2000 rpm.
Recently, when I press the accelerator, it hesitates between 2000-3000 rpm, you can see the problem from the video clips, then it goes to higher rpm with no problem
Diagnosis findings
Finding of the diagnosis are as follows:

• Voltage of the air potentiometer is not stable. Every time I restore it to 0.7v, it goes to 1.x v after engine restart
• Duty cycle sticks at 10%, no response to the adjustment regardless CW or CCW turn
• OBD I always indicates Crank Shaft Sensor error
• Throttle value sensor - ok
• Decelerator switch - ok

Parts have been replaced
Since i bought the car in 2014, I replaced the following parts with new with the purpose to fix other issues and to do preventive maintenance

• ICV (I installed another one for testing, no improvement)
• Air potentiometer (To fix the 1st high idle problem. I installed another one for testing this time, no improvement)
• OVP (I installed another one for testing, no improvement)
• EHA valve
• Ignition and HT cable
• Fuel pressure regulator
• Fuel pump and filter
• Crank shaft position senor

Right before the problem reocurred, the following parts were replaced

• Cam shaft adjuster
• Cam shaft position sensor
• Alternator + OAD
• Distributor cap and rotor

Since the problem has come back, the following were done

• Fuel distributor rebuilt
• Clean the throttle valve
• Replaced the air hoses
• Replaced the Decelerator switch (it
• Replaced the coolant temp sensors (The ohm of the one going to the ECU seems much higher than what it should be according to the info I found in this forum, however, I tried both 2 new sensors, they report the same ohm reading)

Kind of running out of ideas, any thoughts are appreciated

 

[eugenqose3d]

Hi, i have exactly the same probllem and i replaced almost everything just like you, can't solve the problem. did you solve your problem with high idle?

 

[rayhennig]

• Voltage of the air potentiometer is not stable. Every time I restore it to 0.7v, it goes to 1.x v after engine restart

Is the air meter sticking? How does it feel when you slowly press it down? Nice and smooth and returning to the top?

Duty Cycle is best used for diagnostics, not for "tuning". But you've already moved yours so you have nothing to lose. People often twiddle this to correct a symptom, rather than a cause.

Have you measured the duty cycle as the air meter is moved?

I shall try to find the duty cycle fact and figures and return later.

>>> Found some stuff, now attached. <<< Drat, I could only upload the small file but this one contains specific 300-24 stuff. Tried compressing the other PDF but it didn't reduce enough. If I had an email address for you, I could send it.

Is the idle control valve sticking open?

"OBD I always indicates Crank Shaft Sensor error" - They often do. Well, mine always does. I think it's a red herring as the sensor has nothing to do when the engine's not turning so it can't return a result.

What are your duty cycle values? When:
Ignition on,
Engine idling cold,
Engine idling 70+ C
Engine at 2500 rpm 70+C.

Have you checked/changed the rubber boot under the air meter? If that's leaking you will have a vacuum leak.

Just a few ideas from someone who's far from expert but has had the 300-24 for 27 years.

RayH

 

[DREAMER NO2]

Any fault codes ???/

 

[eugenqose3d]

Thank you , let me send you my email: eugenqose3d@yahoo.it

 

[rayhennig]

Thank you , let me send you my email: eugenqose3d@yahoo.it

Email sent. Check your SPAM box if you don't see it as some filters see me as a website (which I am) and therefore possibly a rogue (which I am not).

R

 

[rayhennig]

Thank you , let me send you my email:

Did you get all that stuff?

R

 

[szymek10rad]

Hello, I ahave exactly the same problem as you had. Did you manage to find solution?

 

[rayhennig]

Hello, I ahave exactly the same problem as you had. Did you manage to find solution?

Another this with these is not to touch Duty Cycle until you are sure that all sensors are working correctly. And then this:

Connect the meter to the diagnostic coupling X11 port 3 and 2 (or ground).
With ignition switched on (engine not running) the duty cycle should be:

• ~70% (California: 85%) in case of M103 engine.
• In case of M104 & M119 it should be
• ~30%, if the coolant temperature is below 70°C,
• ~50%, if the coolant temperature is above 70°C.

Take off the air filter lid and check two other duty cycle values with ignition switched on (engine not running).

I can say more on this if you wish.

RayH