My 93 420SEL started acting funning the other day . At first I thought it was a bad shift between the 2nd and 3Rd gear but thats just the RPM range that I first notice the problem

The engine hesitated and almost stalled around 1700 RPM under fairly aggressive acceleration ( heavy load). I sorta babied the throttle, and backed off when I first felt it , thinking it was the tranny. It happened a few more time when I test drove it around trying to figure out if it was engine or tranny .
I was able to drive back 30 miles home at highway speeds and nothing seemed out of the ordinary. It seemed to happen only when pulling away aggressively from a stop.

Engine idled smoothly but quit once after starting it up and being very cold here in NYC

I brought it to my 2 partner mechanics who worked 15 years for Benz dealership and the one mechanic reved the engine in drive (1600 RPM to load it ) and replicated the breaking up. He says it breaks up under load prolly because of the coils. His partner says to replace the rotors and caps. Two partners cant agree and want to start changing parts without even putting a diagnostic tool on the engine? I ran away !

I brought it to the Dealer and the 25+ year mechanic said it gave him no codes but he replaced 3 resisitors that go between wires and plugs and the 8 plugs, scraped the rotor caps with a wire brush and told me put a can of techron in a full tank . His suggestion was if it continues to hesitate, to change plug wires and then possiblely injectors.

Drove 350 mixed miles over last 9 days and in the last 4 days I can usually go all day with 3-4 engine turn offs and restarts and maybe one HICCUP/FLAT SPOT / hesitation per day.

Once past 3000 RPM it pulls like a beast on the highway . So I thought the trend was going in the right direction and dropped by my dealer mechanic to say so, give him a tip etc . and ask if I should try another techron in a full tank of gas and he said ok., and to not change any parts and see if it continues to get better.

No sooner did I leave the dealer and it had that same hiccup . Also after buying the techron and not having a chance to put it in, it stumbled once in 10 mins driving . Then I turned it off for 40 minutes to get something to eat and when I started it up it sorta stubled on idle, and once accelerating aggressively, and then again accelerating aggressively, but this time about about 2800 RPM.

So much for the trend. It never happens if I feather the throttle and when I back off the throttle when its doing its hiccup, it recovers and I feather thru that spot and it drives fine. I have had it running at 60-80 MPH smoothly for more than 300 miles .

He said I can put a resistor on a bench and it tests perfect then put it in a car and it does not perform correctly. What scares me is that even he was not really certain what caused it, and told me to not change anything yet, but as soon as I left his shop it got a little worse.

Sorry for my being so lengthy, any ideas ?

Sounds like a secondary ignition problem for sure.

Caps, rotors, & spark plug wires are the most likely issues.

Can be costly, but since the car is 17 years old..most likely the issue!

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Would you start with wires since plugs have been changed , and then if that does not work do caps and rotors?

OR caps and rotors that the Benz mechanic scraped with a wire brush?

I just ordered Caps and Rotors . Will take about a week to get here. Hope that is the problem.

Will post back results, and if successful or not

you should try Ford caps and rotors.

Is that supposed to be a joke?? What number Ford caps and rotors fit a 119 MB engine?

Yes, it's kind of a joke. There's another thread about Ford caps and rotors fitting on MBs.

Ok Now any ideas as to what is going on with my car. I have heard eveything from caps/rotors, coils, MAF ,and plug wires plus suppressors, basically the entire ignition system .

Anyone want to follow the AND SEE if they can help either from similar symptoms , or from repairing a similar car can also check this thread

http://www.benzworld.org/forums/w140-s-class/1493456-s420-sel-breaking-up-under-load-5.html

All help is appreciated

Peter

Today I looked at my cap and rotor. I pulled off the coil wire from the cap and saw a bit of corrosion. You might take off each wire, check it and put it back on. Also check the coil wire at the coil.

Is your car running well ? If I find corrosion on the wire ends what do I need to do , clean them or replace them ?

I just got mine and it seems to be OK but sometimes it runs rough and sometimes when I give it full throttle it hesitates. I would clean the corrosion because I don't like spending extra money that I don't have too.

Well thats sounds a little like mine. It seldom has a rough idle, occasionally it does, but it hesitates when I give it 3/4 throttle more frequently. A small amount of rough idling is usually accompanied by at least 3-4 flat spots and then I feather the throttle till it goes away , or I get to my next destination.

I just realized that although it is infrequent the rough idle only occurs when it is hesitating . So the 2 are definitely related and part of the same problem.

But I can have the hesitation without the rough idle , but not the other way around.

An easy item you may wish to check is the breather hose that runs from the rear of the passenger side valve cover to the electronic throttle actuator. A member on benzworld had symptoms very similar to yours on his 140 and it turned out to be a large hole in the breather hose causing a vacuum leak. I replaced that hose on my S420 last year because I noticed that it was broken when I was doing a routine inspection of the belts and hoses. My S420 still ran fine even though that hose was broken, though. It was so brittle that it crumbled apart in my hands as I was taking it off. It's an easy part to visually inspect, so you might want to just rule that out as a possible cause.

Is there any way I can find exactly where it is and how it looks?

I just looked and saw a few green and red hoses in that area. Are those the ones you are referring to? They actually looked like hard plastic and almost look brand new, and I was amazed that anything 17 years old could look that good. They plugged into connectors and had 2 hoses plugging into one connector in a few spots.