First off, hi everybody! I have a 1989 560SL with just over 80,000 original miles. I am the third owner and have owned the old girl for about 2 years now. The previous owner's very good Mercedes Mechanic had done a pretty decent job of maintenance.and all of the filters were brand new when she got passed along to me. I have put about 12,000 miles on, this was a very low mileage car.

About a month ago I notices signs of what appears to be vacuum building up in the tank, with an accompanying "wooosh" when the cap is removed. Then yesterday a couple of miles from the house, all of a sudden very rough running ensued. like 4 cyls dropped, just like that. Made it home. this morning, starts fine, idles good, revs up fine when not under load. I forgot to mention that when cold, seems to be running rich. I have done some searches for certain things, but of course, every problem has its own set of circumstances. Does the tank suck relate to the sudden loss of power? I have done nothing yet as far as diagnosing anything. I'll start with a plug check, etc. I should mention that I am a pretty fair mechanic with a good set of tools. I have a complete set of 560sl manuals, as well as a Bosch fuel injection book. This could well be an electrical problem, like a bad coil. All the rest of the ignition parts are quite new. Before I tear into things, I just want to get a feel for possible avenues to pursue. Thanks guys!!

has always sucked as you describe with no effect on running or starting. It runs smooth and normal and starts on first partial crank.

Don't get off on a tangent. I think your rough running has another source. I would start with vacuum problems. They are most common with our cars, easy to find and cheap to fix. Look at the intake manifold at the rear of the engine next to the firewall. You will find a main vacuum source with a right angle rubber fitting. That fitting lives in high heat and an oily atmosphere and is usually the first to deteriorate.

Keep us posted. If this tip doesn't work out I have plenty more probably useless information for you.

The key phrase here is UNDER LOAD. I have a 93 400 SEL than ran fine even under extreme load when COLD, but missed , stalled etc under load. After trying a lot of things we discovered the coil/s were arcing and one had white residue visible to the eye.

Swapped them both out and presto, problem is gone

If I feathered the throttle , it had no problem . It also gave no codes. Worth a look

Your 560SL has conventional distributors.

I would start with spark plugs, rotor arm, distributor cap and check for wear/erosion/corrosion. Then plug leads. Also check that odd shield behind the rotor arm - I replaced mine albeit at staggering cost!!

Basically, make sure your HT is 100%. I imagine you're going to keep a wonderful thing like a 560SL so anything you spend now will stay with you so buy genuine MB bits (2 years ago I installed some cheap make and they were replaced with MB bits last year after failure).

Then make sure the ignition control module is transferring heat to the body correctly. Remove it, clean the underside and the mating metal surface, then apply heatsink compound and re-install.

For the price of a new ignition coil, I'd be tempted to get one.

So that's the ignition system as new.

Check all vacuum hoses and again, bearing in mind the low cost, replace all the rubber/plastic boots to ensure no leaks. Test the vacuum retention and replace any pipes that leak.

Then try it again!

I do understand that's it's very easy for me to spend someone else's money but you are a special case as you have a 560SL that you're going to keep for ever. Aren't you?

Bonne chance!

RayH

Thought I would give a quick update- The cap appears to have been there for a while, I wonder what sort of "tune-up" the previous owner had It shows some signs of arcing. The plugs are not fouled, but the electrodes are rounded. Bottom line, I'm going to replace all of the ignition parts, including the coil, and then see what happens.

A quick look around with a light does not show any obvious signs of vacuum leaks. I'll do the old water bottle test after the ignition parts are replaced.

And yes, of course I'm going to keep the old girl forever!! I had owned a 74 450SL back in the day, and always regretted selling it. I swore if I ever got another one, I would never part with it. These really are magnificent automobiles. As far as I am concerned, to quote the old country western tune, "she's close enough to perfect for me."

Art, recommend buying Bosch distributor cap, rotor, coil and spark plugs online. Shop around for the best price. No need to buy those parts from a MB dealer, unless you like spending top dollar for the same product.

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Fred Hoelzle

Thought I would give an update, I replaced the plugs, coil, plug wires, rotor and cap. Then took a fairly long test drive, with hard acceleration runs. It seems to be fine. Now, I need to address what seems to be rich running during start up. I did not find any completely fouled plugs, per say, but the plugs had a strong gasoline odor. This sounds like the cold start switch, I'm thinking.