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Discussion Starter #1
About 6 months ago my idle rised a bit up to 700rpm while in drive. It might still seem low but does make a difference when shifting and coasting. When you put it in park its the way it should be I think also at 700rpm but when you drop it in drive it goes to 500 then right away to 700.

It did this for awhile then went to the way it should be. Now it came back the other day. If it was a vacumn leak I beleive it would have done it all the time. Where should I look?
 

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Discussion Starter #2
anyone??
 

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1987 420 SEL
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Hey Benzo. I had the same symptoms. I would advise you to start looking right behind the distributor in front of the air cleaner and all around that area.

When my mechanic was inspecting my engine bay area and about to diagnose my timing chain issue, we found a very, very small vacuum hose that had a slight tear in it. I can't remember what this hose was connected to, but the hose was on the bottom under some other wires. My mechanics explanation about the fluctuation in idle was this..."when I would be driving and hit a bump, this small vacuum line would shake causing the tear to open and close slightly but just enough to cause the vacuum leak from time to time."

My mechanic also recommended that I reclean my engine bay area again once the weather gets warmer just in case there are other hoses that need to be either replaced or re-sealed.

Hope it's something simple. Take care.
 

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91 300SEL,2001 SL500
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what is the proper idle speed ? my 300sel idles just over 500 rpm. i do have an intermittant vibration at idle. also noticed my economy gauge reads about 3/4 not to full left. i suspect i have a vacuum leak. any thoughts?
 

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1987 420 SEL
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Hey 300sel4j. You will have to thoroughly look around in your engine bay area for split or broken hoses. When we found the broken hose that was causing my idle problem, the majority of the crack was on the bottom of the hose. The only reason we were able to locate it was due to the fact that we had the car running and when we moved the hose the idle changed. Take your time searching. Also, while you're doing that you should also check all your electrical connectors for any sign of corrosion or lose connections. Take care.
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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Hi 300,

Here are your idle specs below. I've found it easier to test for vacuum leaks by using something like carb. cleaner or WD40 with that little plastic snorkel for aiming accuracy. With the engine running, you just spray a bit at the suspected joint, connection or hose area. wiggle the hose too.

If a vacuum leak is present, the spray will make the engine pick up speed & smooth out.

P.S. Refrain from using Silicone spray for this. Silicone spray will render an Oxygen Sensor DOA pretty quickly.

Cheers, MBL
 

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Discussion Starter #7
I will have a look at this aswell but man so many hoses!![8)]
 

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1987 420 SEL
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I know. That's why it's tricky. Just have patience and take your time.
 

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Discussion Starter #9
The funny thing about this though is the idle is perfect when in Park but when in D or R it rises to 700 which is about what it is in park!
 

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91 300SEL,2001 SL500
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thanks guys. thats what i thought. i fixed/replaced all of the leaking climate control hoses and valves. now i guess its time to fix the engine lines. whos got a free weekend? [:D]
 

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1989 420sel
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300SEL4J - 2/10/2005 7:11 AM

thanks guys. thats what i thought. i fixed/replaced all of the leaking climate control hoses and valves. now i guess its time to fix the engine lines. whos got a free weekend? [:D]
Take a road-trip to the Heartland - Oklahoma! I'll help you out!

As for-out as that may seem, I really do mean it. If anyone is in the OKC area, look me up. Plus, I can get really good deals on Bridgestone/Firestone tires - like almost half off. That offer is good to everyone here on the forum.
Seriously, If anyone wants to make a road trip to somewhere different, let me know... I'll show you a little slice of heaven... [:D]
 

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Discussion Starter #12
I doubt though its a vacumn leak I have on a hose as it would create unproper idle when the car is in Park!

This creates a slightly higher idle when in a gear so maybe its a computer? Would cleaning the contacts on the idle computer (black box) help?
 

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Discussion Starter #13
would me adding a bottle of fuel injector cleaner to a full tank of 94octane gas cause this type of problem?
 

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1990 560 SEL & 1995 E300D
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Hello BenzOnLine,

I fill up virtually every other day and although I am not "religious" about adding injector cleaner, I do, in fact put in a container suitable to treat a 20 gallon tank at least each month. Yes, it will cause a slight rise in idle, but it dissipates as the O2 sensor adjusts for this condition and the idle concern disappears long before you run another tank of fuel. I only use Chevron Techron additive.

Another consideration you may want to entertain is the quality of gas you use. With as much driving as I do I consider myself an authority on virtually all gas brands. I account for every ounce I pump into my sled and I can tell you without reservation that the best gas in the US for our 560's is either BP/Amoco or Shell V-Power PERIOD. A .10 or .20 cent savings at the pump will cost you more when you factor in performance and fuel economy, I assure you. I get 18.4 to 19.3 MPG highway and right at 17 MPG combined highway/city and am very pleased. Also understand that if my car coughs.....it goes straight to the emergency room. It is a fine specimen and has nearly 150,000 miles on the clock.

Gary
 

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Discussion Starter #15
so in your opinion do you think this is my problem? Its been a week now...shouldnt the idle have dropped by now?

Im also guessing it might be the idle control unit but usually those when they go...raise the rpm to 1500?
 

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1991 300 SE
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BOL:

You’ve been a valued member of this forum for almost a year and a half – in that time there have been at least 400,000 threads in the w126 forum about the high idle in the M116/117 engines. A search with my username (with and w/o the ‘a’[:(]) should turn up a lot of info.

Best place to start is the ICV. Soak it in some brake cleaner overnight.

Then move on to the throttle position switch, vacuum lines, ICU, OVP…etc and so on…

Fuel injector cleaner and fuel type is usually not the fix for the problem.
 

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Discussion Starter #17
thx Jdc...me and my father looked at the vacumn to start with and no leaks. Gonna try swapping the idle control unit with another spare on I have to see what happens. Idle valve was cleaned not too long ago....this happened 6months ago and went away on its own so it could be the idle unit.....we'll see.
 

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Discussion Starter #18
Well I figured out the problem!!![:D][:D][:D]

Today the damn gauge cluster went dead!..I went right away to look at the fuses and it basically disintagrated from age. Replaced it and whoalla! I was then reading the fuse descriptions and #11 had "idle stabilizer"....well that fuse was kind of old too so I said ill replace that too. I got into the car and not only did the gauges work but the idle was to factory spec again!!![:D]
 

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1990 560 SEL & 1995 E300D
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Benzonline,

That's fantastic. I have owned my Benz since 10/04 and I have had both the cigarette lighter receptacle and radio fuse blow twice (they are on the same circuit). When I no longer had a spare in the fuse compartment I was forced to purchase an assortment set. Since replacing the old aluminum fuse with the newer "copper-colored" fuses I have yet to have a problem with that circuit (I use the cigarette receptacle to power my XM Satellite receiver and charge my cell phone at the same time).

Your experience has caused me to order a large set of assorted fuses, enough to replace all the old ones and enough to replace the spares, as well. I am glad to hear your problem was an easy fix!

As they say, "Sometimes the elevator.....sometimes the shaft." I'm glad to here it was your turn to take the elevator!

Gary
 

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Discussion Starter #20
Haha. I thinking of replacing all of the fuses in the fuse box after this incident. Does the dealer only sell these aluminum fuses or do they sell the copper ones aswell?
 
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