This one sounds like you might have a belt driven accessory going bad, freezing up, when engaged it causes extreme load on the engine, and when not the engine runs fine, the loud screech you heard gives you a hint that it is likely a belt driven unit grabbing, I would check the compressor first, totally shutting down the A/C, see if it does it again if it does, then check the other units. That is where I would start, Good luck!
Regards,
Brian

Check the alternator next! could have a bad bearing or something dragging it down, the lights dimming too is a clue

Hi,


I have a 1991 500 SL with 115,000 miles. Car runs fine. I recently replaced plugs, distributor caps, rotors, oxygen sensor and had injectors cleaned.

Car ran great until about three weeks ago. The car was driving along great -- but then suddenly engine stumbled badly. From then on, it hesitated slightly at about 1000 rpm just as it pulled away from stops. The problem did not appear the next time I drove the car.

Last week it did it again. Then it ran perfect for a few days. Now it is happening every other day. The problem is getting progressively worse-- the hesitation is now more pronounced.

Yesterday, I started car and it would barely hold idle. Lights dim and brighten as idle drops. The car hesitated badly as I pulled away from a stop. About 10 minutes into my drive, I heard a screech (belt???) and problem disappeared.

Today, started car and, again. it barely held an idle. After a couple of minutes, it idled better but still a little rough. It hesitated after every stop. About 15 minutes into my drive, problem completely disappeared. When I arrived at my destination, it started doing it again.

I have searched the hundreds of idle and stumble postson this site, but none seem like mine where it comes and goes.

Any ideas? Any help would really be apprecated.


Mike

I am confused. The car starts easily but idles roughly for a few minutes. It then has a hesitation just above 1000 rpm as i pull away from a stop.

Sometimes it does it, and sometimes it doesn't.

I will keep you guys posted.

MG

__________________
1991 500 SL
2002 BMW 745i
2003 Harley Electra Glide

Mike,
Your problem seems very frustrating I am sure, keep in mind that the belt driven accessories go on and off. For instance when you first start your car the alternator really kicks in to replenish the battery, and the AC compressor cycles in and out for various reasons as well. That is the first thing I would check. Is your Engine light on? The fact that it is getting progressively worse, you hear the belt slip, and the lights go dim and bright lead me to believe that the alternator is failing. Pull the belt off (The fan shroud comes apart and out in a neat way to allow access) and see if there is any play in the pulley or if it is rough when you turn it by hand. It should turn smoothly. You may have a bearing going out. Check the other accessories as well while you have the belt off. Has the alternator ever been replaced? How often do you change your battery? I have found that a new battery every 2-3 years keeps me from replacing my alternator.
If you have no play or roughness in any accessory, I would then recommend taking it to a shop and having them hook the voltage system up to check the condition of the battery and the output of the alternator. If everything checks out ok, then take it to the dealer and have them diagnose it. I have lots of friends that have Mercedes that take them to shops, and have plenty of stories, but I find that the dealership has the best tools and knowledge for diagnosing these odd problems. Take care and good Luck!
Regards,
brian