86 420 SEL rough idle, serging goose chase ends

[420SEL Jake]

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 “it has to be that!!” 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 wasn’t 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 it’s running because it’s 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 it’s a Mercedes people assume it’s 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