let's start from the very beginning. about half year ago, my check engine light came on and stayed on for no apparent reason. i took it to my mechanic and he told me it's a faulty oxygen sensor. the car drove fine with the lights on, so didn't have it replace (i know, i know, but now is not the time for lectures). about 5 weeks ago, i cannot start the car one cool afternoon, i turned the key but no clicking sound at all. the car finally started after i turned the key and hold it down while stepping on the gas padel for about one minute. i drove it for about 170 miles straight and turned the engine off. i tried to restart the engine right away and it hesitated alittle and it made a high pitch grinding noise for about 2 second before it started again. after that no problem starting it for about couple weeks. had my battery and alternater checked- both ok. also had my mechanic take a look of the flywheel as was suggested on this forum- he said it was fine also (i still wonder if he actually looked). lately, the car have some problem starting (worst when engine is hot)- the engine will start and rpm shoot up to about 15k and dip right down to about 500 (it died twice) and bounce right up again.
finally changed my oxygen myself two weeks ago and disconnected the battery for about an hour and reconnected it- voila, check engine light is gone!! however rpm dipping at starting persisted. this past saturday, a very very hot day, i started the car and the rpm dipped as usual (the only difference is that this time the air condition is on auto and full blast at start up) and the car start to vibrate slightly as i pull out the parking lot. as i started driving down the road, the engine persisted to vibrate/quiver only slightly. as i pull to a stop or when the rpm dipped too low, the engine started to rock/vibrate violently and check engine light came on again. i pulled over and turned off the enging after about 3 min of driving. the car restarted fine (with the usual dip in rpm of course). what's going on with my car??!!??!!??!!?? please help.
sorry about this awfully long post. i want to tell the the whole truth and nothing but the truth
i am going to change the infamous ovp relay this weekend and see if that helps.

Good, change the OVP ASAP!!
Secondly, it sounds as though your check engine light came on because of a cylinder misfire. THe rough idling probably triggered the CE light because of a misfire in one or more cylinders.
Unplug the battery to turn off the light and see if it comes back again after chaning the OVP.

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)

Ditto on that...the rough idling and possible misfiring would freak out the sensor, and cause it to spit out an error code.

Disconnecting the battery and waiting a few minutes before reconnecting will reboot the computer and reinitialize the diagnostics.

I think we need to start a forum on the OVP, as the posts in here weigh heavily in that aspect!

The OVP is a relay designed by Mercedes engineers to do one thing only...plague unsuspecting Mercedes owners with a whole host of problems that would stymie any logical troubleshooting process. Fortunately, the cat was let out of the bag early on, and we all know their secret.

Seriously, though, the OVP relay has been the source of a lot of peculiar problems, rough idle being the most common. The unit is small, easy to replace, but unusually expensive compared to a typical relay ($65).

Do a search on "OVP" and you will learn more about the care and feeding of your OVP.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

thanks guys for the replies,
that darn ovp relay is not in stock in my local mb dealer but they will get it before friday.
any ideas on how to check the starter without taking it out?
what about the flywheel? anything else? why do the cylinders misfire? i use only 93 octane gas and i just changed the spark plugs about 20k miles ago. should i change the plugs again alone with the wires?

Apprentice,
How many miles are on your car? The plug wires are usually changed around 100k miles, that's when they are supposed to changed anyway. If they've never been changed, that could be your culprit. My rough idle which caused the CE light came from an old fuel filter. Have that changed also.
GOod luck and keep us posted.

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)

i just changed the fuel filter myself about 200 miles ago could it be something wrong with it?
my car has 95k miles on it. i think i will change the wires with the ovp relay this weekend.

Good idea, change the wires and the OVP and see what happens.
If you just changed your fuel filter I wouldn't worry about it... but your symptoms did start when you changed it... (?) Hmmm, worry about it after you do the wires and the OVP. Let us know how everything goes.

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)

any more thoughts? anyone?

i went to my mechanic this past weekend and without touching my car and just by listening to my description of the problems: rough idle, lack of power, and and car vibrates badly when i steps on the gas and when i turn on the air conditioner, he told me that i need to change the coil packs (ignition coils). can anyone confirm this? my car is a 95 c220 with 95k miles. it still runs except the problems above. i just changed the ovp relay with no noted difference. i was going to change the spark plug and ignition wires this past weekend but now i am thinking about changing the ignition coils too so i can do it all at once. but the question is, do i really need to change the ignition coils? it's $82 at fastlane and i needed two of them.