My mom's 86 SL560 has been rarely driven over the past 10 years, maybe 500 miles per year (only 60,000 miles on it). I am getting ready to put it up for sale and it idles a little rough. I'm a decent shade tree mechanic, so thought I'd change the plugs and see if that smooths things out. I just drove the car 500 miles and it ran fine, but seemed to only get about 17 mpg.

Is there a certain brand spark plug that works better than others? I'm assuming Bosch is the brand to use.

Should I expect any strange surprises, like plugs that are impossible to reach or plug wires that have a hidden clip?

I'll probably change the plug wires when I change the plugs. Any recommended brand?

Thanks!

Look at this thread. 420SEL Rough Idle I know this is about a 420SEL but it applies to all 116 and 117 engines. You will find your answer. Read my post and the ones after it. BP5ES plugs are avaliable at Advance Auto if you have one in your area. DON'T ask for plugs for your car, ask for NGK BP5ES Part Number 7832.
Try the plugs only at first. If it still misfires at idle then check for vacuum leaks.

PaulM

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84 500 SEL (307,xxx miles)

Thanks PM. Can you tell me what engine and chassis model my car is? Wikipedia says the car is an R107, but nothing re: the engine.

I pulled one plug last night and it looked pretty good - a little brown, but with a light sticky touch of something on one side of the electrode. The plug is a bosch super (wrote down the number, but it's out in the car right now). My mom had a bunch of work done to track down a fuel problem a few years ago (and about 2000 miles ago). I suspect they changed out the plugs and wires, since all looks very good, so I'm waiting for her to send me her file on the car before I bother with the plugs. I'll put some techron in the tank and put a couple hundred miles on the car and see if that helps.

The idle is only a little rough, you only notice if you are really listening for it. Do these cars have a bit of a lope at idle, or are they smooth like a sewing machine?

Thanks!

What a wonderful car. I won't ask why you wish to sell it!

Do check the distributor cap and rotor as well as the leads and plugs, as you mention. I've been using (300CE-24) MB cap and rotor, NGK plugs and Beru wires and all seems well. It's always worth making sure all the HT is in 1st class condition.

I had a 500SL with 10K miles on it and I regularly got 25 mpg (that's our big UK sort of gallon). 17 seems a bit grim - was that stop/start around town?

The 500 was a stunning car capable of 140 mph all day. Give yourself a treat after you've got it running right and keep it for a year.

Oh, just saw your other post: idle on mine was undetectable, smooth as silk. It was only 2 years old but I feel sure you could make yours do the same. Vacuum leaks are worth checking out as others have said.

The best of luck to you.

RayH

Your car is a r107 chassis with a 117 engine. Your 117 engine is a 5.6 liter overhead cam V8. It uses the CIS-E fuel system which uses O2 sensor and Computer to control fuel mixture.
The 107 SL was made from 1971 to 1989 and used a lot of different engines. The 560 or 5.6 liter was used from 86 to 89. In 89 they changed to the R129 chassis with 3.0 and 5.0 l engines.

As you can see, if you discribe your car as a 107 with a 117 engine it could be a lot of very different cars. If you discirbe it as a 89 560SL (which you did) everyone knows what you have. I wish people would quit using the chassis and engine numbers when asking questions as it leaves a lot of un answered questions about what they are working on.

PaulM

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84 500 SEL (307,xxx miles)

Whatever you choose, they should be NON-resistor plugs. Your car has resistance built into the plug caps. If you use a resistor plug, you will be doubling that value. There is still controversy out there on this but if you do it as MB designed, you can't go wrong. In my case, the only plug I could find locally was an NGK. Don't remember the designation but BP5ES rings a bell.

Your idle should be imperceptible. Mine was smooth as silk up to the day my wife totalled it. I was in the car with her and neither of us were hurt at all, a testament to the rugged safety of the car. Even our marriage survived the wreck but it was touch and go.

It was my daily drive and after initial sorting gave me 4 trouble-free years of service and pleasure. Follow others advice. Drive it for a while. You will probably change your mind about selling.

Yep!

Seems like a conspiracy to influence this lucky fellow?

R

Thanks for the info on the rough idle and the body and chassis identifiers. My mom has sold her house and moved in with my sister and at 82 shouldn't be driving anymore. My wife prefers her sporty Subaru turbo and I'm building an old hotrod pickup on my garage - we don't have the room for the Mercedes or the money to keep it up! My goal is to get as many of the little bugs worked out as possible, then sell it for my mom. That said, it sure is a beautiful car!

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The engine has an aluminum head and block. Don't overtorque the spark plugs.

My best advice is after it's fixed up ..... Don't drive it! If you do, you will build another garage rather than sell it.