I've been searching the threads on this... TOO MUCH INFO!!! the car has a rough idle when the AC is off. instead of a steady prrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr... it sorta sounds like prr prr prr prr prr prrr.... the car has a slight shake kinda feeling like a sparkplug misfire..especially when the car is hot. but the sparks have been changed... twice in one year.. no help... I think wires have been changed... I know this is a fuel injected car and has no carbeurator to clean... are there any ignition coils that should be looked at that would cause such a problem?
What besides the OVP may cause this? Air Mass sensor /Oxygen sensor?, How could they diagnose that? What controls the Idle... I was reading somewhere that there was an Idle control valve that was controlled via vacuum lines? How could I check this? Does the 94C280 have this feature? What exactly should I look for? the people at the MB dealership have not been able to diagnose this problem for the past 3 years already. The only codes I have been getting have been for the EGR valve and that's been replaced. Can anyone shed some light? Please?

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'94 MB C280 Lorinser Sport Suspension

I am experiencing the same problem with my 83 380SL. I am eager to discover what the problem could be.

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Marvin
1983 380 SL

I have a 1982 380 SEC I had the same problem

1. Idle control valve is a silver thing about 5 inches long with 2 prongs on it. Mine is under the air cleaner when I changed that my idle went so high I also had to change the idle control relay a black box on my firewall. when I changed both of the the idle went back to normal.

2. mine also has an air/ fuel mixture the center of my air filter you turn it one way you get more air in the system if you turn it the other way you get more fuel in the system you need to find the balance of air and fuel so the car doesnt rock funny or want to take off when your at a light.

I dont know if your car has the same thing or not if you go to the benz dealer parts they can show you where the parts are for your car. when you find the parts check the hoses around it make sure they are connected and no holes in them.

hope this helps!

benzat
(benz with an attitude for parts but now getting much better just finished a new front end, brake system, fuel system, and new back sway bar bushings, new engine mounts, new engine shocks and much much more but finally getting so much better)

Try fuel injection cleaner, Chevron WITH TECHRON. Put it in with about half a tank, and don't give up if it doesn't work right away. It may take 4 or 5 containers over several fill ups before it works itself out. I'd guess you have a dirty injector or fuel distributor. It's relatively cheap if it works (let me know if it does - I have the same problem and was recommended this remedy)

Fuel air mixture is adjustable, but I believe it's computer controlled also. The idle control relay is more expensive to change so I'd leave that for a last resort, I'm not sure that it would solve the problem. If the techron doesn't work.

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2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey

SOLD:
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino

There are many things that could cause your problem. I will list the easily items to look at first before you spend more money on this. They are bad gas, dirty injectors, clogged fuel filter, and too lean of air fuel mixture. I have particularly good luck with Redline injector cleaner - put a whole bottle in the tank the first time you use them will fix both the bad gas in the tank and clean your injectors at the same time. Have you mechanic run a CO reading to determine if the air fuel ratio is too lean. If the CO reading is way below your state emission limits, have your mechanic richen it to see if it smooth your idle (it did on mine). Good luck.

Sorry guys i have a diesel but my aunt had a lexus showing the same symptoms, her problem was burned sparkplugs. also check your fuel injectors and timing. those problems are amplified under the added load of the AC.
good luck.

Thanks All for the responses.

For me, the problem started after I ran TECHRON. Could possible problem be that all dirty has yet to blow completely through and one of the injectors are clogged?

If I put Redline in now could it possible help?

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Marvin
1983 380 SL

with techron in your tank you have to drive hard.
take your car to the freeway, shift gear to third and push
it close (not over) redline, then release pedal for a few seconds
then repeat the process. after doing it four or five times you
can see the difference. good luck watch out police cars!

Infiniti,
Are you saying that your idle is smooth with the A/C on? But rought when the A/C is off?
If so... then that's weird! Good luck with this one...

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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!)

So, techron is more effective when used under high-rpm driving? I didn't know that.

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2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey

SOLD:
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino

David... that's EXACTLY what I'm saying... that's why it's weird. The car doesnt shake when the AC is on but when off, the car shakes and has a teeny sputter like as if there was a misfire... the engine has that prr prr prr prr prr prr prr noise.. but when the AC is on, it is less shaky. I dont even know if the car has an idle control valve/relay... I think it IS computer controlled... I dont know what to do... I'll check if the car is running lean or whatever...

~manny

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'94 MB C280 Lorinser Sport Suspension

No... I'm not comparing the engine of my LEXUS and my benz.. they are completely different animals. heheeh The lexus is completely smoothe.. I forget it is even on at some times...

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'94 MB C280 Lorinser Sport Suspension

If the car was running too lean, then your Check Engine light would illuminate. I would rule this one out! (Unless your MIL light is in fact on.)
The air/fuel ratio is computer controlled, but you ALSO have an idle control relay (or whatever it is called). I would not think that the A/C is a direct factor though... just results in a symptom, or lack thereof. Damn, I'm stumped... let's see what a tech has to say!

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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!)

yup stumped me... you know what... I was wondering if the exhaust system could have something to do with it... it has been sounding weird for a while...
OK... I added Prolong Octane Booster to the gas and drove real hard today. kept the rpms at around 5000-6000 for a while going up a hill at 3rd gear and 70mph or so... I hope this wont do too much damage to the tranny. Actually... the car is running a bit smoother. The shaking at idle is somewhat diminished but still noticeable.
question: another thing...there is a clunk sometimes when I crank the engine... when I shut the engine off... the same clunk that happens when slowing down and I can feel the transmission downshifting. This might all be interrelated... could this mean that the engine mounts need replacement?
another question: could there be something wrong in the exhaust section? Could an exhaust problem cause this shaking?

no my light is not on... when the light goes on, there is always an EGR problem of some sort... the last one was EGR switchover something...

OK... whatever then!!! this is completely stumping the people here on guam... If I take it to the shop, they will charge me for replacing everything that doesnt need changing. If the techs here have anything to say? I'd be greatful if they might be able to diagnose that!!! I'll be sitting here... waiting .. patiently...

~manny

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'94 MB C280 Lorinser Sport Suspension

Infiniti,
What is Prolong? IT'S JUNK!
Use Techron!! And you don't have to be necessarily that hard on it, but harder than normal.

__________________
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!)