m103 Low idle
 

Hey guys,
Have an interesting problem with my 1988 300CE m103 idle.
it starts fine and idles around 1000 to 900 rpm in park when cold.
BUT...when fully warm the idle goes down to around 600 in park and 500 in drive
drives ok and idles nicely in drive but if i shift it back to park or neutral the idle stays around 500 as if nothings changed! (lots of shuddering and lumpy idle)
I have changed all the vacume lines including the boot for the air flow meter,
have tried a couple of well cleaned ICV valves (new vacume lines) the cap rotor and wires are around 2 years old. the coil is brand new. Temp sensor is brand new.
have checked the OVP relay and since cold start is fine i assume its ok.
Crank shaft sensor gives me a reading of 947ohms which is within range.
I'm at a complete loss as to what is causing the problem. Any ideas?
Thanks in advance.

I wouldn't say your idle is worrisome. My 103 idles in a range of 550 to about 700, depending on load, weather and other ambient conditions.

thanks for the reply. the thing is it used to idle around 800 but just changed to low idle without any other indication. and its pretty lumpy in park (smooth with a slight shudder occasionally in drive). Changed all the 'usual' things like idle valve temp sensor ect thinking it was some kind of sensor failure. The duty cycle fluctuates around 50% at idle and 45% at 2500 rpm. the engine mounts are about a year old.

When is the last time you changed your O2 Sensor? That and also check the hoses for cracks on each side of the IACV.

O2 was changed about 2 years ago and the two hoses along with the air flow sensor boot was changed last week. from what i gathered an air leak would give these engines high idle not low?

Is there power and ground to the ICV when this occurs? Is there any change in the idling, when this occurs and you disconnect the ICV?

yep have about 5.2v to icv in idle, disconnecting it increases the idle to around 1K. is there a auto transmission shift position sensor or switch on these cars? what tell teh ECU that the car is in PARK?

yep thats true.

got that on mine

I also have an 88 300CE, and it was doing the same thing. Replaced the OVP and it cured the problem. Two years later, it started doing it again. Replaced the OVP again, problem cured again.

really? will try to source on and see if it helps! weird because all other functions work well like cold start and A/C compensation.
Thanks guys. will see how i go with a new OVP.

I kind of have the same problem..mechanic told me that it could be the electrohydraulic
actuator that is giving me a rough idle while stopped in "drive". This does not happen all the time though,mostly when exiting the freeway and coming to a stop. My indie
checked current on the EHA and found it to be low ..I probably need to get a new one or scrounge
one from the yards, luckily this part is shared with many 300 series and 190 2.6 series I believe .. I hope to have a new or used replacement soon, and see how it works .
Lots to learn about these fuel systems ..I will look forward to your next posts to see
what results you get ... Thanks css190e 2.6 180k/daily driver/ "it's always something"

Checking the current on the EHA only tells you it is compensating the mixture, it has nothing to do with how it is functioning in relation to needing a new one. It is simply telling you the system is running too rich/lean and the EHA is adjusting for it. Sounds to me like it is doing it's job :)

Check the throttle switch and decel microswitch.

On mine it was the...
 

Little rubber elbow that goes between the valve cover and the cylinder head located in the middle of the valve cover by the air filter. Mine was old, brittle and cracked and it idled fine when replaced.

checked that line mate, but seems ok. (wouldnt it idle higher if there was a vacume leak?)
replaced the OVP with a known good one but problem remains the same. i'm stumped! any ideas?