??RPM drops every 3 seconds?? - PeachParts Mercedes-Benz Forum

mroliveira · #1 04-15-2007, 09:20 AM

Hi All,
I have a 1996 S420 w/41,000 miles. As I am driving, the RPM drops 200 revs every 3 seconds, and stays down for 1 second then back to normal.
...In other words, this is a 10 second snapshot:
2000, 2000, 2000, 1800, 2000, 2000, 2000, 1800, 2000, 2000.

I have tried it with the cruse control on or off, (makes NO difference).
I have tried the grear shift it in automatic O/D position and in the manual position 3,4 (makes NO difference).

Happens on lower RPM as well. (30 miles per hour up to 70 miles per hour).

But as it's happening, if you step on the gas, it does NOT happen, It accelerates as normal, but when you get to the desired speed and cut back on the throttle, then it happens again.

It's not real bad, dropping 200 RPM, but it gets REAL annoying at times.

ANY THOUGHTS ON WHERE I CAN GO FROM HERE?

MANY THANKS FOR YOUR HELP!

stevebfl · #2 04-15-2007, 11:56 AM

I'm guessing that you have a misfire. Does this only happen at 2000 or at any steady throttle position? Can you duplicate it out of gear with the engine held at a steady speed say 2000?

Being a 1996 I presume you have Motronic engine control, which means coil on plug rather than distributors. I would guess the problem could be a coil to plug connector. Unfortunately it could be all manner of things though.

mroliveira · #3 05-25-2007, 08:58 AM

Thanks for the response.
I have tried it with the car in park and reved to 2000 rpm. It runs PERFECT. I've held the throttle at various positions upto about 2400 rpm without any problem.
BUT when you put in drive and get up to about 30 miles per hour, is where it starts to be noticable. The RPM drops a couple hundred RPMs every 3 seconds (stays down for one second, then goes back up for 3 seconds).
One thing I don't think I said was that I just bought the car and the altanator was making noise, so I put in a new altanator. (the bad thing is I DIDN'T NOTICE THIS PROBLEM UNTIL AFTER THE NEW ALTANATOR). It could have been there before, but I didn't notice it (was more worried about the noise from the altntr). Is there a possibility it was the new altanator? (or just coincidance?)

juribe2 · #4 05-25-2007, 12:18 PM

Sounds like a bad coil or a bad coil conector.

mroliveira · #5 05-25-2007, 09:28 PM

How can I check if it's a bad coil or connector?
Why would the RPM's drop only when I'm driving and not do it while in park?

.......if this were my wife, she'd buy a new engine...... then a new outfit and a new pair of shoes to go with it......and perhaps even a new purse too.

Hatterasguy · #6 05-25-2007, 10:28 PM

Won't the miss fire code be stored in the computer?

Now that you have the W140 time for the FSM DVD set.

just-n-time · #7 05-26-2007, 11:30 PM

Missing under load,thats why it did not show in park or neutral. How about the Maf can you spray clean it?

stevebfl · #8 05-27-2007, 10:05 AM

A misfire can go un reported by fault code for considerable time. There is a level of misfire and a rate of occurance that must be exceeded and the earlier the car the less critical the criteria. A 96 would probably be the first year for an "S" class to report at all.

Unfortunately, real tooling will probably be required to observe the problem. Either the smooth running function on SDS or the ignition firing values in EI if it isn't a ME car. it could also be a throttle problem and not be a misfire. The condition needs monitoring with the appropriate tooling and TRAINING.