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  #1  
Old 08-09-2007, 10:10 PM
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Join Date: Jul 2007
Location: Ottawa Canada
Posts: 118
86 420 SEL rough idle, serging goose chase ends

Sometimes I find it hard to find something to write about, but I can talk endlessly about my Mercedes 420 SEL. I know that this page will end up on Google some how and if I can help some Benz owner out there with trouble shooting some of the issues that I had with my 1986 420 SEL. First off this is one of the most amazing cars ever made and I know why it was 60k USD in 86, the car drives like a dream, fast, agile and comfortable. My issue with this car was a group of symptoms that lead me on a wild goose chase. First the car would idle rough, then it started to serge in the RPM up and down by 300 rpm and then it would stall for no reason.
First problem with people working on them is they point randomly at odd engine parts and computers and go ďit has to be that!!Ē Well you play a game of changing these rare parts that are hard to find because the were never needed to be made in large quantities because they never failed and were not needed. I started with the basics by reattaching vacuum lines and cleaning electrical connectors... It helped but still ran rough in heat and heated up quick. So I changed all kinds of other things to no avail and then I discovered that the 2 breather tubes coming from the valve covers were jammed with 21 yr old oil. So I cleaned that out... it got better.
This car still wasnít running great and I spent every flippen day under the hood for 3 weeks. Then I noticed the cat getting plugged... ****... I stop by Young Street garage here in Ottawa and they said put in Bosch Copper Core plugs... Well what do you know, the car ran better and the cat freed up, I went one step further and replaced cap, rotor, coil and wires, including grounding cable. The change was amazing, I had a brand new car that ran cooler, smoother, faster and I have to double check the tach just to make sure itís running because itís so smooth.
The lesson is that if this was a GM small-block everybody with these issues would say change the plugs and wires, but because itís a Mercedes people assume itís some crazy thing like the idle air control valve... Well when you are driving a 21yr old car that has original everything try replacing the obvious.
Oh replace the diff oil with synthetic, you will thank me.

Jake
www.jakewright.ca

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86 420 SEL rough idle, serging goose chase ends-img_3517.jpg   86 420 SEL rough idle, serging goose chase ends-jake420sel_1986.jpg  
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  #2  
Old 08-20-2007, 07:24 PM
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Join Date: Oct 2006
Location: Indian Waters, Florida
Posts: 136
Jake:
1991 420 SEL. What kind of diff oil did you use. How many pints (US)

Thanks, Dave
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  #3  
Old 08-20-2007, 10:55 PM
420benz's Avatar
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Join Date: Jul 2001
Posts: 961
Quote:
Originally Posted by 420SEL Jake View Post
Sometimes I find it hard to find something to write about, but I can talk endlessly about my Mercedes 420 SEL. I know that this page will end up on Google some how and if I can help some Benz owner out there with trouble shooting some of the issues that I had with my 1986 420 SEL. First off this is one of the most amazing cars ever made and I know why it was 60k USD in 86, the car drives like a dream, fast, agile and comfortable. My issue with this car was a group of symptoms that lead me on a wild goose chase. First the car would idle rough, then it started to serge in the RPM up and down by 300 rpm and then it would stall for no reason.
First problem with people working on them is they point randomly at odd engine parts and computers and go ďit has to be that!!Ē Well you play a game of changing these rare parts that are hard to find because the were never needed to be made in large quantities because they never failed and were not needed. I started with the basics by reattaching vacuum lines and cleaning electrical connectors... It helped but still ran rough in heat and heated up quick. So I changed all kinds of other things to no avail and then I discovered that the 2 breather tubes coming from the valve covers were jammed with 21 yr old oil. So I cleaned that out... it got better.
This car still wasnít running great and I spent every flippen day under the hood for 3 weeks. Then I noticed the cat getting plugged... ****... I stop by Young Street garage here in Ottawa and they said put in Bosch Copper Core plugs... Well what do you know, the car ran better and the cat freed up, I went one step further and replaced cap, rotor, coil and wires, including grounding cable. The change was amazing, I had a brand new car that ran cooler, smoother, faster and I have to double check the tach just to make sure itís running because itís so smooth.
The lesson is that if this was a GM small-block everybody with these issues would say change the plugs and wires, but because itís a Mercedes people assume itís some crazy thing like the idle air control valve... Well when you are driving a 21yr old car that has original everything try replacing the obvious.
Oh replace the diff oil with synthetic, you will thank me.

Jake
www.jakewright.ca
Jake:I have the same color car and it needs a paint job.Thinking of changing to black,what do you think?
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  #4  
Old 09-08-2007, 04:16 PM
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Join Date: Feb 2003
Location: Ohio
Posts: 66
Differential Oil

I would also like to know what kind of Synthetic oil to use in my 83' 380 SEC differential.

Thanks,
Bob
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  #5  
Old 09-08-2007, 05:16 PM
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Join Date: Mar 2001
Location: Decatur, Illinois, USA
Posts: 616
A good synthetic rear diff gear oil is either Redline gear oil or Mobil One synthetic. They both come in the regular weight and a heavy ( truck/suv) weight) Use the heavy weight if your rear diff has the diff whine noise from a wear spot in the ring and pinion gears. It will be surprising how it will mask the whine and reduce the noise. Wont hurt the diff either. I've used both in the two weights in several MB cars. Dont use the heavy if you have no whine and dont race the car.
Mobil 1 regular weight at Autozone is about $8 a quart, $16 a quart for the heavy gear oil. Red Line is harder to find, typically for racing and is a good all synthetic, more expensive and likely not any better if you dont race the car.
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1990 190E 2.6 - Arctic white SOLD
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  #6  
Old 09-08-2007, 10:10 PM
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Join Date: Aug 2007
Location: Valley Village, CA
Posts: 1,163
Glad you solved that

Diff will be on my list,,,hmmm

Wonder how the wires, rotor etc made the engine run hot.

My sec ran hot
changed thermo
sending unit
then radiator
now its hi 95

Engine is running strong & smooth though
so maybe your engine running rough generated the heat


What I am asking is if I replaced those electrcal comps
would my engine run cooler??
Thanks

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83 500 SEC Euro 198K
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