29th Dec 2010, 10:15

High idle - On the front of the engine, to the right of the warm-up regulator, there is a round cylinder with two wires coming out of it. Push on the wires when the engage is idling high. If it slows down, the wires are just loose. Pull them off and squeeze lightly.

If this doesn’t work, then the cylinder needs to be replaced.

17th Mar 2011, 22:44

I just bought a used 1985 500 SEL for 200 dollars.

It also leaks water into the engine. The person I purchased it from had done a complete top end rebuild. Does anyone know what is involved in repairing a bad rebuild?

I know the heads are cheap enough, but how about if there is a crack in the intake manifold?

Anyone have some experience, I would really appreciate it.

19th Mar 2011, 16:52

I recently purchased 1985 Mercedes Benz, clean in and out with 113,000 miles, runs like a dream. It only has a clonking noise when I engaged in gear forward or reverse. Also it needs a fuel injector. When idling or at a stop light, it shakes a little.

I purchased it at a dealer, out the door tax and LC, registered for $1,400,00..

I truly enjoy driving it. Once going it runs perfect, especially on the highways. I'm looking forward to fixing the small issues it has... it's by far the best car I've ever driven. Only wished I had purchased one long ago.

1st Apr 2011, 08:22

I have the same problem. Have you found out what the noise was all about! I love the car, and that reason will keep me from ever selling it! So please help me with the problem.

12th May 2011, 15:48

If you have purchased a 1984 or 1985, the Nada classic book puts them at 14,400 in excellent condition, average retain is 10,600 with 117,500 miles on it.

In reference to the owner who had vibration problems, it's definitely your guide and bushing. Have the tires checked first, and balanced and rotated to see if this stops your problem.

I'm a collector, and I have had almost every car you can imagine. You guys are right, it's hard to beat that old Mercedes ride! You know you have a quality machine with you when you drive it. These cars are heavy, especially the gray Mercedes that were built overseas like mine when I was in the military, but all I can say this car has been the best used car I have ever owned. I have had my 500 SEL for over 20 years.

10th Jul 2011, 17:22

I have the same problem. It feels like I'm out of control on the highway; the steering is way soft and really wanders. I don't know if it's the tie rods, didn't know about the "steering box".

29th Jul 2011, 22:12

We own a 85 500 SEL. Found out that the idle control has a fuse that went bad, it's in a larger fuse panel under the hood of your car. My car was idled so high that the car would move on its own at 20mph. I hope this helps, I'm a lady restoring my baby. Good luck.

9th Sep 2011, 16:07

You need a rebuild. I have just done my one for $2700.

17th Oct 2011, 05:15

Replace the vacuum line from the modulator is my first guess.

Ben from Ohio.

18th Feb 2012, 11:07

Hi, I own an '85 500 SEL, and I love it! The car works in tip top shape, and I love how the rear seats recline back and forth.

The only issue I have with the car is the blower, either AC or Heat, only blows air from the top, and not directly at me. Thank you for help.

Chris from NJ.

20th Mar 2012, 03:47

Cannot raise the bonnet on this 500SEL 1984 Benz. The bonnet pull raises only one side, and I cannot get the other side to release. The car has had an electrical failure, and is currently on the side of the road. Can anyone help here??? I have managed to accidentally break the passenger side headlamp glass in an attempt to lever it up to access the catch. I am trying to help a mate out here, and have ended up in the s**t.

27th Oct 2013, 22:46

Run as fast as you can.

27th Oct 2013, 22:56

I still own the car, 81000 miles. The rear defogger worked after I replaced the fuse located on the fender on the driver's side. Just replaced the belts and auxiliary fan switch, and repaired one of the vacuum lines. My paint is starting to fade though.


15th Dec 2013, 22:58

I have a 84 500 SEL, miles unknown (meter broken), but it overheats when traveling for about 40 miles; the engine steams when I park it. It reads at 100 Celsius. What do I do to stop this? It's a good car; that's like the only major problem.

17th Dec 2013, 06:27

Sometimes it pays to tow a car to the dealership. It's amateur prying and breaking out a headlamp, with no guarantee it will work. They have the knowledge to do so.

I had a 450SL and it cost a fortune to keep it on the road, especially emission issues. Oil changes, shocks and brakes I did. As well as a Crutchfield self installed sound system. Just don't drill into the gas tank to mount the rear speakers! I also replaced the ignition switch as well.

I added a lot to make it look like a 560. But I also knew to go to the dealer when it was necessary. If you can't fix something right, pay an expert.

24th Aug 2014, 20:50

On my 1985 500 SEL I had to replace all vacuum bellows in dash for about $377. A small price to pay for cool comfort in the Texas summer heat.

1st Oct 2014, 20:30

Fast! The specs say it'll do 0-100 in 8.1 seconds. Of course I had to test that one for myself. After almost 30 years, it can still do it!

5th Oct 2014, 18:43

Try changing the ignition control module. It is located on the driver's side front fender.

8th Apr 2015, 01:56

I recently developed a high idle on and off for 2 weeks. I read some of the comments here and tried to diagnose the problem. It turned out it is the idle control unit that is defective. I ordered the unit from a friend working for the supplier, and today I replaced the ICU and my problem is gone. No more high idle. If you have this problem, try replacing the ICU first before you go to a mechanic. The ICU is located in the middle below the windshield wiper area.

Thank you to those who posted their experience. It allowed me arrive at a good diagnosis. Have a nice day.


26th Aug 2015, 21:26

Next time pry the plastic box off the idle unit; two plastic tabs, and it then slides up and off. Then take a soldering iron, a small cheap one is better, and touch all solder points till they turn bright silver... just before they melt. You have to do this this to all solder points. Over time they loosen inside where the connections are. You can't see them, but they do become loose thru vibration and temp changes. Try this - it works most times. I had a Mercedes mechanic tell me this years ago. I have owned and still drive the 1984 and 1985 U.S. spec 500SELs; it has always worked for me.

These cars have been called the gold standard in the "STAR" Mercedes Benz magazine. If you own one, you know why... fast and comfortable. The engines last longer on U.S. spec cars... lower compression ratio and milder cam timing, not as fast as euro 500s, but the engines just loaf along without much strain. I have one and I'm still using it daily with 376000 miles.

9th Mar 2016, 23:56

When I pull the hood release latch from under the dash on my 85 500SEC, only one side releases. You need to get someone to press down directly above the latch that does not release when you pull the lever from under the dash.

11th Mar 2016, 11:50

My SL had the worst release catch ever for the top. So bad I used the hardtop only a few times.

19th Feb 2017, 21:12

So how would I find the vacuum emissions diagram for my 1984 500SEL? I can only find the USA diagram, and my euro with 142.000 miles doesn't have a lot of smog stuff. Thanks a bunch.

19th Oct 2018, 14:56

Transmission leak - make sure bolts are tighten properly on your pan.

High idle may be a vacuum line leak; fuel distributor needs replacing usually around 100,000 miles.