My wife had to have this car and I have to admit it is a cherry, I don't think it ever came out of the garage it was in when we got it. Exterior mint, interior mint, even has that new car smell. Under the hood was dusty and had a smell of oil couldn't see any leaks but it was dirty. I got some engine degreaser and washed it off .... now there is a miss or the idle fell to low (around 5-6 on the tack) at 10 it is smooth but at a stop it started to stumble. I took off the air crossover and cover where I found 3 coils and 3 sparkplugs top end full of water. So I used my shop vac and a straw to empty the water. I got a set of OEM plugs and wires. When I changed them I noticed that two of the plug wires were cut by a metal plate between #1 + #2 spark plugs. Thought new plugs and wires would do the trick but just my luck still stumbles and the check engine light is shining brighter then ever. I disconnected the battery for about 15 minutes and when I restarted the car wow at idle beautiful .... no light no stumble .... so I punched it and it had great response but ... POW on comes the light and stumbling is back to vibrating.... Other then new coils I made sure everything was dry. Now to the question what under the hood would cause this problem by just getting wet ? The MB dealer wants $250.00 to hook it up to their computer so I sent away for a obd2 reader that has code reset options and should fit the plug under the dash. Just thought I would ask if anyone had this problem or if you know how to read the codes without the machine (like with the old gm's jump two pins and it starts to blink the codes)

 

Is the air horn connected to the air filter box, is connected to the opening behind the grille? if not water could get in there, other then that, I am puzzled?

 

In the process of fixing the a/c the engine was steam washed and the wiring harness was damaged and I experienced a pblm(stumbling and then vibrating with the check engine lights) similar to yours. I have no clue as to the mechanics of a car, but you can look at my thread "Am I being ripped off" for advice and suggestions given by experts in this forum.

 

I checked the wiring and all connections to the air intake, and every box under the hood, everything is dry. I found fuses that I never knew existed. The I tried moving around the coils still seems to feel like a miss I hate to change parts without knowing that their bad. These coils are odd to me because there are 3 and each one controls 2 plugs ..... I can pull off a wire to see if the plug isn't firing but haven't tried pulling a coil to see if the plug under it is firing ? Does anyone know where I can get the shop manual on CD or a copy of one, or even the test procedures for this problem.

 

Well I will make it short and sweet. I got a Equus 3030 code scanner and ran thru the tests found #2 cyl not firing . Found a place online got a 3 pack of oem coils installed in about 8 minutes. What an easy car to work on I can't see how they charge so much at a dealer. after putting the car back together I reset the code and fired it up WOW smooth you could put a glass of eater on the top of the motor and not see a ripple in the water
Idle dropped to around 6 on the tack . So I crossed my fingers and punched it WOW again. I don't like to drive my wife's car (because I get blamed for every bird $%$# on it) but I had to take it for a ride. So I took it out on RT 3 here in NJ and before I knew it I was doing 120 and had plenty of peddle left GREAT RESPONCE AND HANDELING now I know why she wanted it. so the total for my Fu%^ up $60.00 plugs and wires $70 for the code reader $150 for 3 coils my free tine total $280 The closest dealer wanted $250 just to hook it to their computer $190 for 1 coil $145 for wires and $8.00 for each plug all in all today was a good day
Hope you all have the same luck and good fortune