what causes a high idle on 300e

[hihosilver]

well changed idle air valve it idles alot better but sometime the idle will sit at 1200-1500 unless I push the linkage back, lubed everything but still doing it whether its cold or hot, is it just the linkage or?

[sbourg]

If you are saying that pushing back on the linkage will drop the idle, then I would definitely start looking there. The throttle should drop back against its stop when the accelerator is released - it should not be able to be pushed back further.

Steve

[gmercoleza]

Is the high idle intermittent? Do a search on microswitch or cleaning the throttle position sensor. Worked for me...

[Duke2.6]

Did you check the throttle valve switch function?

A misbehaving microwitch will not cause high idle - just effect fuel cutoff on trailing throttle.

Duke

[meangrean]

like you sayed unless i push the linkage back.there is to much tension on the linkage or cable.It would also be nice to see what car you have

[hihosilver]

86 300e, the cable to the pedal has a spring but it barley touches, it did not do this before the valve job

[sbourg]

It seems silly to overlook, but happens to ours - heavy inside snow mat jams accelerator enough to keep throttle slightly open. Despite trimming the mat, we have to re-position it on rare occasions, as it gets twisted getting in and out. Mimics a binding throttle linkage problem.

Steve

[autozen]

Went through this once on a 90 300E. Just lubing the linkage didn't work. After checking many things including changig the speed sensor on the back of the inst cluster. I solved the problem by removing all the linkage and cleaning and lubing the pivot shafts.


Sbourg,
Your right corner says SoCal. What the h*ll are you doing with snow mats?

[sbourg]

Originally Posted by autozen

Sbourg,
Your right corner says SoCal. What the h*ll are you doing with snow mats?

We have a cabin about 60 mi north of LA at 6000'. In the winter, we leave the house in tees and shorts, and 90 min later we are shoveling furiously at the cabin driveway for at least another 90 min just to park the car. Maybe we should have bought a cottage at the beach....

Steve

[joel]

idle control valve, water temp sensor fixed my problem.

[hihosilver]

already changed air valve, I live in desert no snow or snow mats

[Newt]

I have a 1991 300E that had a high idle problem. It would start OK and run fine; however, the idle would not get below 1200-1300RPM's. I disconnected the connector for the Idle Control Valve when the engine was running and at operating temp and the idle speed would drop to about 750 RPM's. It would stay that way until I restarted the car. I checked the Overload Protection Relay and it was the old style, I replaced it and the problem remained. I replaced the Water Temp Sender and still no change. Lastly, I took some time just to give the engine a good visual inspection and I noticed something that didn't look quite right on the Throttle Microswitch. I ended up removing it and sure enough the rubber boot that covered the switch button had cracked and jammed the cam arm and would not allow the switch to operate. I replaced the switch and idle returned to normal....problem solved.
This is an outstanding forum with tons of good info. Thanks to all and I hope this info helps someone else

[pifcat2]

I'm reviving this thread because my dad has similar problems on his 91 300E. The head was rebuilt while he lived in another state and the mechanic wrote on the receipt that it needed an idle control valve (my dad didn't have time to have the work done). It's in my hands now and I noticed the ICV was unplugged which I rectified. Still high rpms so I replaced and the problem persists. [FYI, the ICV can be tested by taking it out of it's bracket while still plugged in and turning the ignition key to power it but not to start the engine. You should be able to see the valve open and close with the key on/off.] I'm told I need to fix this high idle or it will damage the transmission.

When replacing the ICV I found the left hose had a crack at the opening. I temporarily tie-wrapped it to see if it leaked. I don't think it does but read I should shoot some carb cleaner to check. I will replace them both but it doesn't look easy to get to under the manifold.

I noticed a helper spring (see lower right in pic) was added to pull on the long throttle linkage closest to the firewall to reduce the rpms. The rpms start at ~1k and rise to 2k until I push back on the linkage. I removed the spring since it's a band-aid and still doesn't prevent the rpms from rising. I'm concerned what other short cuts this mechanic made. I also adjusted the length but it didn't help. There is a shorter one next to the throttle body and I tried to adjust it but it wasn't easy to get a wrench on it.

My next task is to remove and clean all the throttle linkages and check the micro-switch. I can run a continuity test to check it.

I also noticed my head temp sensor is leaking which I'll remove to add pipe thread compound. Interestingly there is an sharpie drawn arrow to it. The mechanic noticed it but didn't fix it...

Any input is welcome, thanks.

[pifcat2]

I removed most of the linkage, cleaned and greased it. The link bars weren't easy to pry off the ball ends. I didn't get the link bar off that's next to the throttle body because it started to bend. It appears to be spring loaded. I'll clean it as best I can and re-assemble tomorrow.

I disconnected the plug to the ICV while it was running and the idle dropped to under 1k until I restarted the engine. Seems to work fine.

I checked resistance on the micro switch with a voltmeter and it operated correctly. I noticed there is no adjustment on the switch body as it's held on a pin and screwed to the bracket. It's cable sheath on the connector mounted at the firewall side of the head was hard and a little frayed likely from the heat. The switch connector seems ok too and no obstructions to the switch.

I found this link for more info: High Idle 300E -88 (Have preformed search)

[pifcat2]

After the linkage cleaning and lubrication, it idles fine around 750rpm. The only thing I notice is sometimes it takes 2 turns to start the car.