Hello All,
My '94 E320 has acquired a gremlin in the idle. It will rev between 1100 and 1400 rpm in neutral, warm or cold. In gear its closer to 1000 rpm.
Most folks point to the throttle actuator being faulty but I still have cruise control, car not ASR equipped, and I have never thrown up a Check Engine Light. I get the code for "idle control inoperable"

The problem when I began to happen a around summer '09 came on gradually/ would go away as engine warmed up/ was only in neutral. Now I have the above problems.

I have scoured the web and tried as many of the solutions as I could:
Replaced:
Wiring Harness (Jan 09)
O2 Sensor (Summer '10)
New Over Voltage Protection Relay (Fall '11)
Mass Air Flow Sensor (Jan '11)
Intake Air Temp Sensor (Summer '11)
Idle Air Control/Resonance Valve (Late '08)
Cleaned EGR Pipe / Tested Valve (Fall'11)
Installed rebuilt throttle actuator (Summer '11)
Checked:
All the vacuum lines I could find. (Crank case seems to run at a lower pressure then I remember, likes to suck the dip stick in its hole)

Does anyone have any ideas what else could be wrong? I'm so stinking sick of the problem, I have until now been throwing money into best guesses and none have been right.

hello


Suggestions:

I believe that a bad ECT sensor can cause a jumpy idle , until the vehicle warms up a bit.


Vacuum Leak:
from reading up on the forum, common vacuum leaks on these engines occur at EGR valve(tends to stick open) and Purge Valve(also tends to get stuck in open position) both are internal leaks, making them more difficult to find.

see post #1 , does that sound familiar:
http://www.peachparts.com/shopforum/tech-help/258527-what-world-wrong-my-94-c280.html


ETA:
have you inspected the throttle linkages and springs, its possible that you may have disturbed one of the links/springs during ETA installation. have you made sure that the throttle plate moves freely?

good luck

Thanks for the ideas:
By the ECT you mean coolant temp sensor? I've hear of those going bad eventually. I suppose I could check mine with an Ohm-meter. My car is consistently high on the idle though, I don't get surging as described in the post you referenced. Useful reading though, appreciated.

As far as the EGR valve goes, I checked its functionality with a vacuum tester, now that's not to say it couldn't have internal leaks.
Purge Valve? That's new to me, part of the fuel tank?

Throttle Linkages: I had initially left the return spring off the actuator, but noticed it when there was almost no resistance on the pedal. Linkages should be good; I greased the ball and socket link that I had to remove to swap EA's. I am confident the throttle plate closes fully when released, I hear the micro switch close in the EA.

Is there a throttle full closed switch somewhere in the cabin? Pedal area maybe?

throttle can be relearned with shop tester or hht

i would check an make sure all throttle springs are present, then ensure that throttle cable is properly adjusted(See FSM)


check out this vid(read the "show more" part):

Mercedes M119 motor throttle body rebuild. Wiring replaced. - YouTube


good luck

As far as linkages and springs are concerned everything is there as far as i can tell. I only know of springs that connect near the throttle body that keeps the keeps closed when not in use (and it's there), and the other is in the cabin above the pedal. Wiring wise, the rebuilt electronic throttle actuator (ETA) took care of that.

The throttle actuator was supposed to have come set up and ready to go but should I have the car's computer "re-learn" the ETA at a shop with their programming equipment?

I got the impression the computer assessed the ETA every time the key was switched to the run position (the little half second "boot" for the car's systems).

Is there any way the transmission might play a role in the way the car idles?
When I roll up to a stoplight/sign I occasionally put the car in neutral and coast to a stop. When I do this the car's idle jumps to where its happy at 1600 RPM, but the instant the wheels stop turning the idle will drop to about 1000 for quarter second then pick back up to 1600 RPM.

Again, it's a weird problem and I'm pulling my hair out trying to fix it.
Thanks for any suggestions

As an update:
I installed a new ECT sensor to no avail of my idle woes.

As pointed out by the very helpful Timmyr, I checked my vacuum purge valve and found it seems to leak a little then removed from the car. I achieved a drop in the idle when I idled the engine and plugged the hoses that connect to the valve. When unplugged, the idle climbs even higher, to the 2k range. I plan to work on it more this summer, maybe I can coax the valve to working properly again, or pench my pennies and replace it if i can't. (I'm currently away from my automobile; it makes birth in AR while am in school at West Point.)

I'm interested folks thoughts about crankcase ventilation and common/potential problems. The breather hoses (on intake manifold) got replaced with my head gasket 3 years ago, didn't affect the problem.

Also, does anyone have any thoughts about this mysterious computer that is under the front passenger floorboard (I have seen it though only once when looking for my O2 sensor when it needed replacement)? All a couple other threads have come up with is that it says VDO (the mfr. I am sure), I can't find out what its purpose is. Any ideas? One guy said it solved his idle woes, perhaps I'm in the same boat? Though I fear dropping $$$$$$$$ on a computer.

Thanks in advance!