idle problem mercedes c220 cdi w203
 

My W203 has this problem of unstable idling RPM ( the RPM goes up and down between 700 and 900 RPM). This happens only when I start the car and the engine is cold, once it gets warm up the engine runs smoother. I changed the fuel filter, MAF sensor and cleaned the EGR valve. Someone told that the prolem could be caused by the fuel RAIL sensor and I may have to clean it or replace it. how do I clean it or check it?

Oh HS, I would check the emissions system as cold starts require a higher short term increase in rpm------PS you have wasted loads of time and money!

what do you mean check emissions system? What part? All these cars seem to have this idle problem with cold weather

I doubt that the original poster has an American 1.8L supercharged (~192 HP) engine.

Your engine and mine both suffer from carbon buildup on the intake valves. There are many factors that go into how much build up for a particular car/engine.

Carbon buildup on intake valves, on supercharged or turbocharged engines without a very robust catch can system, is very common for many many OEM engines.
Imho, I say it's one of the top engine complaints for OEM boosted engines.
Sometimes, an "earlier sign" of carbon buildup on intake valves is the engine running "rougher" when it's cold. Dealers know to BS the car owner to avoid a very costly repair. Some manufactures approve an "intake valve flushing/cleaning" procedure. "Roughly" similar to seafoam, but done in a more controlled manner, with something not as caustic as seafoam.

Imho, the American version has far too much boost for the minimal self-draining OEM catch-can built into the valve cover.



For many cars, carbon buildup on EGR valves is common. So is buildup on the MAF on the throttle-body, etc.

Then, there's the clogged up cats that, imho, most cars have unless the OEM cat has been changed in ~80K miles.
Many aftermarket cats (imho 90%+) aren't good for even 50K miles.


Many cars may also develop a small vacuum leak in a hose or mating surface. As the engine warms up, the leak may seal. Or, be so small, that it's not noticeable by the driver.

Also, there's engine wear in the rings, the cams, the lifters, etc.


My disclaimer: The above is for engines that ran smooth when cold when the engines were new from the dealer, for that model and year.
Many high performance engines ran like crud when they cold, even when they were brand new.


Imho, rough running when cold and "not obvious" rough running can be very hard - to impossible for the do it yourselfer. Even with the STAR diagnostics, there are many $$$$ tools that the ASE certified dealer mechanic has available.
Even then, it can take tens of hours++ to find some problems. At $100 to $200+ per hour, the diagnostic fee cost alone can make the cost to even determine the cause of a hard to find problem be more than the "book value of the car".

I know a few people that had their vehicle at the dealer for more than a month, under warranty repair, while the mechanic(s) and main office technicians hunted down the issue and tried a number of different things.
Imho, having a car take 1-3 weeks under warranty repair to find a problem happens a lot more than people would think. And, good luck getting any dealership admitting that!

And, yes, for Domestic, Japan, European, etc vehicles.


Note that ~90+% of the problems are easy for the professional mechanic to find, fix, and verify.
Then, there's that small percentage that take more time.
Then, there's the much smaller percentage that can take a lot of time to properly diagnose.

That's one reason, why local mechanics, for anything other than the ~~90% easy to fix problems, they will "throw parts" at the problem. It's often cheaper for the customer. And, being realistic, extremely few people would be happy if a local mechanic charged them $500++ just to diagnose a problem.
So, a local mechanic will throw parts at it. If that doesn't fix the problem, then "there were other problems also that needed to be fixed".

Since you and I have the same CDI engine, we also have the same 1400W Heater Booster fitted to our engines as a standard from the factory.

This booster activates below 0c (if I remember right) and it will suck so much power from the alternator, that your RPMs are fluctuating as you desribe, only when cold. My car has done this ever since I owned it. Are you starting the car outside below freezing temperatures?

Also it is very easy to test, you can turn this heater booster off from the computer menu. See if it makes any difference if you turn it off. Otherwise it could be other problems. Just a longshot, I've heard similar complaints from CDI owners and it was always the heater booster causing the fluctuations :)

https://cimg3.ibsrv.net/gimg/www.mbw...0ecdccef7d.jpg

Do you have any idea about the gasoline C230?
My car also has the same problem when it start in the morning because i'm living in Canadian so the weather now is usually under 0.
Thanks a lot!

My car developed a wandering idle on cold starts recently. I was sorta getting used to it until I noticed fuel mileage getting worse as well. The issue turned out to be swirl flap link bar between m55 pierburg servo motor & swirl flap common rail had galled & disconnected. With this linkage disconnected, engine preferentially closes the swirl flaps, starving the cylinders for air on cold start. Best way to check is to shine a flashlight through opening between #3 &#4 cylinder's intake manifold & visualize that servo motor to insure linkage is connected. If you find it disconnected, try sliding that flap swirl rail back towards the fire wall. This will force those swirl flaps into the 'Open' position. Try starting car from cold & see if idle issue remains. If not, that is likely your issue. If that link bar is so galled out that it won't stay connected to servo motor, try fixing that rail into open position permanently. Post results & I'll show you how I fixed my linkage. :zoom: