1993 mercedes 300e high idle - PeachParts Mercedes-Benz Forum

textech56 · #1 06-12-2014, 09:21 AM

I"m working on a 1993 300e with a high idle after it warms up,wont stabilize to meet idle specs for state inspection ,any suggestions would be greatly appreciated,Im basicly looking for a place to start,seems like when I first start up it doesn't have fast idle,starts around 800 rpm after warms up goes to 1200 rpm

tecqboy · #2 06-12-2014, 05:57 PM

I don't know about 93, but in 91 ( my car ) there is a micro switch on the throttle linkage that closes when the throttle is closed. If the switch is bad or out of alignment, then the ECU won't lower the idle rpms.

pawoSD · #3 06-12-2014, 06:07 PM

High idle if the linkage and microswitch are working correctly is usually caused by vacuum leaks. I have my 420SEl motor torn apart to fix the same issue right now. Replacing ALL rubber components on the engine intake.

liquiddog · #4 06-12-2014, 06:19 PM

I had the exact same problem on my last inspection. Take off the air cleaner and make sure the linkage is snapping back into the micro switch. It should audibly click. Also lube the linkage. This thread contains an excellent photo of the spring setup on the linkage of a 2.6/3.0.

300E 2.6 throttle cable repair - Mercedes-Benz Forum

As another mentioned, could also be a vac leak. Luckily these cars have a built in vac gauge. It's the economy gauge. All it does is measure vacuum.

oldtrucker · #5 06-16-2014, 01:54 PM

Quote:

Originally Posted by textech56

I"m working on a 1993 300e with a high idle after it warms up,wont stabilize to meet idle specs for state inspection ,any suggestions would be greatly appreciated,Im basicly looking for a place to start,seems like when I first start up it doesn't have fast idle,starts around 800 rpm after warms up goes to 1200 rpm

I don't know much about the 300E, however I would also consider the O2 sensor.

Since the engine doesn't idle at the 1000 RPM at cold start (RPM depends on ambient temperature) and rather decides to do 800 RPM, the ECU says you are already running rich or right where you should be, so I won't give you more fuel or less, because I am happy with all sensor inputs.
It is hard to tell by the smell that comes out of the tail pipe.

When the engine reaches Operating temperature, the IAT may signal warmer Air to the ECU and in conjunction with the O2 Sensor might decide to richen the fuel ratio causing an elevated idle.
However to do so, it needs more air not to drown the engine. This could indicate the throttle flap, to be more open than it should be.
So, your problem could be a combination of several things.

I would:
1. Check all vac lines, as other already suggested, in particular the ones close to the Intake Manifold. You can temporarily plug (cap off) lines at the manifold to make sure the line and possibly other devices attached to it don't leak. Observe the idle, if it got better, start unplugging or reconnecting the vacuum lines and see where the condition returns.
2. If there is a purge valve, check if it function's (should be pulsating) In later models they are known to either get stuck open or closed. While closed would be OK (for idle), but open causes a huge vacuum leak.
3. Check Vehicle Test report, if the fail was only caused by high idle and if all other numbers are within spec. e.g. CO being the most important one. If CO is high e.g. more than 0.1 for example, check O2 Sensor. Depending on test and state, 0.1% CO may let it pass, but the CO values could be as low as 0.02%.
4. There are ways to help to find vacuum leaks quickly, but I won't suggest them.
5. If you have maintenance records, when was the last time the O2 Sensor was replaced? If easy to do, take em out and have a visual, look for carbon deposits. You may be able to clean it off.
6. And of course, the idle switch, micro switch is closed or open, dirty linkage or incorrectly set. The latter would have caused high idle at the time the switch contact was lost. How long (weeks, months) does it idle at elevated RPM's? Was it sudden or just creeping up over time?

That should get you going at least.
Hope you'll find the culprit!