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Idle is way too high on my 380SL

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Former Member
pfbraun's picture
Greenville, FL
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1985 380SL
1985 380SE
1984 380SL

Folks, I'm really stumped and hope someone can help me solve the high idle problem on my 380SL. It's idling between 1500 and 1800 rpms. One solution already tried involved replacing the overload relay--behind the kick plate, on the passenger side, next to and above the fuses. When removed, the idle drops to 1500. I found, with either old or new relay plugged in, the rpms are 1800. And, by the way, this problem has not come overnight; rather, it has grown from 1400 on the high end and 1100 on the low (regarding putting in and pulling out the overload relay, respectively). The goal, of course, is to idle around 650. I've lubricated linkages and so forth and can eliminate binding. I've also verified all the fuses are in place and operational by testing with a light. Finally, the cruise control isn't working--in case there is some connection or relationship. FYI, the cruise engages momentarily, but then cancells, which really isn't an issue for me as I've little interest in using it and will solve it in due time. Nope, I'm focused strictly on getting the idle down where it ought to be and would greatly appreciate input toward that end. I'm truly puzzled and seek help.
Paul

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marlinspike's picture
Hampton, VA
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I'm thinking idle control unit (black box up at the firewall near the windsheild wiper pivots) or idle control valve (this thing http://img.eautopartscatalog.com/live/C403028342VDO.JPG
at the front of the engine - just covered by the front of the air filter - and center). Since you say it's gotten worse I'm leaning towards idle control valve.
Richard

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1985 380SE 5.6

Former Member
pfbraun's picture
Greenville, FL
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1985 380SE
1984 380SL
Still seeking solution for excessive idle on 380SL

marlinspike wrote:
I'm thinking idle control unit (black box up at the firewall near the windsheild wiper pivots) or idle control valve (this thing http://img.eautopartscatalog.com/live/C403028342VDO.JPG
at the front of the engine - just covered by the front of the air filter - and center). Since you say it's gotten worse I'm leaning towards idle control valve.
Richard

I think you're right, Richard. After some testing, the idle control valve seems the culprit, but before replacing it, at the tune of a couple of hundred bucks, please, let me verify my findings through further feedback. Turning the ignition on and off, there is no "click" from the unit, signalling it's ready for duty. Hence, with ignition on and the two wires leading to the unit unplugged, current at the socket measures accordingly: 12 volts at the black wire and approximately 9 volts at the black/yellow wire. With engine running, current at black is about 14 volts and black/yellow is about 11.5 volts. Are such readings normal? Before buying a new valve to see if the high idle problem is cured with it, what tests, if any, might remain that I could do to further confirm that this unit is the problem? Can the unit be cleaned and possibly repaired? Also, does this unit commonly fail after a while, and, therefore, although expensive, comes as a cost of ownership--like tires and brakes?

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pfbraun wrote:
I think you're right, Richard. After some testing, the idle control valve seems the culprit, but before replacing it, at the tune of a couple of hundred bucks, please, let me verify my findings through further feedback. Turning the ignition on and off, there is no "click" from the unit, signalling it's ready for duty. Hence, with ignition on and the two wires leading to the unit unplugged, current at the socket measures accordingly: 12 volts at the black wire and approximately 9 volts at the black/yellow wire. With engine running, current at black is about 14 volts and black/yellow is about 11.5 volts. Are such readings normal? Before buying a new valve to see if the high idle problem is cured with it, what tests, if any, might remain that I could do to further confirm that this unit is the problem? Can the unit be cleaned and possibly repaired? Also, does this unit commonly fail after a while, and, therefore, although expensive, comes as a cost of ownership--like tires and brakes?

What year is your car? The service manual job 7.3-112 details how to diagnos this problem. If yours is 1983 or newer, then you did the proper test (i.e. check for a click, if you dont get one check voltage. You're getting voltage so the diagnosis is replace).
Short of replacing it you could do what I did: flush it out with brake parts cleaner. Another person (http://www.mbcoupes.com/repairarticles/idlecontrol/highidle.htm) cleaned his by soaking it in carb cleaner for 4 hours. I suspect his way works better.
As far as is it a cost of ownership part...sorta. They appear to fail every 15-20 years. So they do fail with fair regularity, but not quite comprable to tires and brakes, more like fuel distributors and ball joints.
Richard

PS - the link also gives and alternate way to test it.

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1985 380SE 5.6

Former Member
pfbraun's picture
Greenville, FL
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1985 380SL
1985 380SE
1984 380SL

marlinspike wrote:
What year is your car? The service manual job 7.3-112 details how to diagnos this problem. If yours is 1983 or newer, then you did the proper test (i.e. check for a click, if you dont get one check voltage. You're getting voltage so the diagnosis is replace).
Short of replacing it you could do what I did: flush it out with brake parts cleaner. Another person (http://www.mbcoupes.com/repairarticles/idlecontrol/highidle.htm) cleaned his by soaking it in carb cleaner for 4 hours. I suspect his way works better.
As far as is it a cost of ownership part...sorta. They appear to fail every 15-20 years. So they do fail with fair regularity, but not quite comprable to tires and brakes, more like fuel distributors and ball joints.
Richard

PS - the link also gives and alternate way to test it.
Thanks, again, Richard. My SL is a 1984. The above site is certainly helpful. I'm pulling the idle control valve off the engine right away. Will soak it as you and Serge did before buying a new one. Yep, 15-20 years is acceptable for a part like this, for what it does and so forth. Have no complaints. Paul

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pfbraun wrote:
Thanks, again, Richard. My SL is a 1984. The above site is certainly helpful. I'm pulling the idle control valve off the engine right away. Will soak it as you and Serge did before buying a new one. Yep, 15-20 years is acceptable for a part like this, for what it does and so forth. Have no complaints. Paul

I agree, but you know how hard it is to convince my dad to buy parts (I've been buying the little things, but as a college student I can't afford the $300+ parts) for an old car. I think I've finally won him over by reminding him that the more stuff we fix the less stuff there is to go wrong...either that or he finally decided arguing with a kid is fruitless. :p
Richard

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BTW, in case you were getting your prices from the dealer, youcan get an idle control valve for $250 from germanstar.net.
Richard

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Re: Idle is way too high on my 1981 380SL

I have the same problem in my 1981 380sl with 120000 miles. I've replaced the idle control unit and rebuilt the entire vacuum system to include the removal of the intake manifold, replacing the gasket and sewing things up.
Still same problem. What can I do???:confused:

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wilki's picture
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Re: Idle is way too high on my 380SL

A couple of thoughts on the high idle problem: the things mentioned so far are good, but
when testing the idle valve off the car with auxiliary power, I've read that you should
limit the power to 12 volts for only five seconds at a time to avoid burning out the
solenoid. The other thing is that I read on one of these posts that the oil temperature
sensor also has something to do with the high idle , as part of the cold start
warm up system. Might be worth looking into. Good luck!

Bill

Still flogging away at my '83 380SL

nhdoc (not verified)
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Re: Idle is way too high on my 380SL

When I bought my 560SL it was idling too fast too. It was not the ICV at fault but rather the deterioration of the rubber air distribution tubing system from it which was causing vacuum leaks. You might check them first. That stuff dries out and the joints get leaky. You can replace it all yourself for relatively little money but you do have to get some stuff out of the way to do it- I recall it took a couple of hours and the parts were only about $30.

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Re: Idle is way too high on my 380SL

Roger the nhdoc. High idle problems on the 116/117 engines are most often caused by vacuum leaks. How does your "economy" gauge read? Is it fully pegged in the back at idle? If not, look for vacuum issues.

Look between the engine and the firewall. You will find (or should find) a 90 deg rubber fitting carrying a vacuum line from the intake manifold. That fitting is in the worst environment for vac fittings. It is a hot, oily location and is usually the first to go. In any case, inspect all of your soft rubber vacuum fittings. Find one bad one, replace all. Available at the dealer or some big box auto stores. Cheap easy fix. Do this first before replacing expensive stuff.

Let us know how you do since it will help many in the future.

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Re: Idle is way too high on my 380SL

Thanks to all here & Don350SDL on previous re: high idle issues. A friend asked me to help sell an 86 500 SL with very high idle- it will hold on level road @ 40mph with no foot on the pedal... & a mysterious origin gas leak smell in the cab while driving though no visible signs of gas. It was so strong I had to keep the windows down with little benefit. Just getting started, air cleaner off, no apparent vacuum leaks ( we'll see) nothing visible as yet but will be drilling down thru these great tips. Just to let you know all these notes are very valuable to a Benz newbie! Gotta get this fixed before showing it. (I love how it handles but there isn't room for all my tools, heh)

You know you're outside your specialty when you aren't asking how do I fix that... you're asking "what is that?"

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