LexBrett2

Apparently it's common that the w203 has a rough idle at start up, especially in the cold. According to my research, the following are likely culprits:

1. $12 hose leak
2. Oil on cam sensors
3. Dirty MAF
4. Dirty throttle body
5. Bad O2 sensor
6. Spark plugs that need to be replaced
7. A unknown vacuum leak
8. Oil on Ecu connector
9. Dirty EGR valve

Can anyone think of anything else to check?? Top on my list is to do $12 hose, once I get the darn airbox off. Have already cleaned MAF, cleaned Thottle Body, checked fuel pressure, and replaced plugs. After doing the $12 hose, plan to clean cam sensors and do a smoke tear for leaks (will have to build or buy smoke machine..)

LexBrett2

bump

samaritrey

How many miles are on the car? How does the car run once warm or on warm startups?
Try disconnecting the MAF wires and see if it seems to help at all.

LexBrett2

car has about 125k miles, and runs perfect after a few minutes to warm up. So disscinnecting the maf will put in open loop? If idle improves then the problem is the maf??

Davis_C320

Did you figure this out?

LexBrett2

No unfortunately have not figured out the rough idle at cold start up. Changing plugs did help a lot though. I also really thoroughly cleaned the throttle body. This weekend I'm going to try to change the "$12 hose", since many have suggested this could cause idle problems .

Davis_C320

did you try the maf thing mentioned above?

LexBrett2

not yet but I will before I try to replace the hose will let you know

ammoun

I think cam sensors are relevant (Number 2) but aren't they supposed to be in contact with oil in the engine?

LexBrett2

Removed the MAF this morning and started her up cold, but the strange idle still occured. If the MAF was not functioning, the theory (I guess) is that the car would run better. Don't think it is the MAF..

Next on my list is
-$12 hose replacement (but having trouble getting stupid airbox off)
-Check and clean Cam sensors, which are the sensors in front of the camshafts in front of the engine??
-Smoke test with cigar
-Smoke test with homemade smoke machine
-Smoke test with $800 machine, which hoping to avoid :-))

2006C230V6

What hose is this?

LexBrett2

See the link:

https://mbworld.org/forums/c-class-w...placement.html

LexBrett2

Last night I checked the $12 hose and it looked fine. Actually the hose appeared to have been recently replaced- there was an aftermarket clamp on the deeper side of the hose. I traced my hands along the hose and did not see wear or leaks, and decided not to replace the hose since it looked good.

LexBrett2

I tried the Scotty Kilmer cigar smoke test last night in hopes of finding a potential vacuum leak. Disconnected brake booster line a blew smoke into the engine. While smoke dos come out of intake, as expected, didn't see smoke leaking anywhere else. Perhaps there wasn't enough smoke or psi. My lungs hurt now. On to the next test in finding cause of rough idle in cold weather at start up!

LexBrett2

Cleaned cam sensors and THrottle body thoroughly, to no avail. Maybe a little oil on cam sensors, but hardly detectable.

2006C230V6

Do you have a code-reader? I have the ELM327 and Torques app for my phone and it has worked pretty well I think.
I used it just after the episode where my 230/203 had rough idle and refused to start.
It couldn't give me a code because the check-engine light was not on, but the app did recommend I check the function of the EVAP, whatever that is.
I didn't pursue it further because at next start up everything seemed to be back to normal.
I did also have the Crankshaft position sensor replaced as well as the fuel pump last year. Two other things you might check on.
Best of luck.

LexBrett2

Yep, I have a handheld OBD2 reader as well as a Dashcommand App. I have no codes or pending codes, although my Long Term Fuel Trim is 11.9, suggesting a leak.
My next test is a more rigorous smoke test, but thanks for the tip on the crankshaft position sensor.

2006C230V6

Maybe this is too simple but are you using Premium fuel? Maybe just a bottle of Heat or Marvel Mystery oil would straighten things out.

LexBrett2

Yeah I always use premium fuel, and use Royal Purple 5w-30 oil. What does marvel mystery oil do?
After I changed plugs and cleaned TB, the poor idle was greatly reduced.. suggesting something not sure what lol

2006C230V6

MMO is an additive for both oil and fuel. I've never used it in the MB, but in other vehicles if I couldn't get a good idle I have mixed one third of the bottle with fuel and the rest into the oil. It is a marvel.
As to why it works? That's a mystery.

AoxoMoxoA

I was affected by the same phenomenon and did the whole checklist thing which produced nothing awry, although my gut told me it was a fuel-related issue.

What alleviated it for me is turning the ignition key to the "pre-start" position for a few seconds to let the fuel pump pressurize the system (you can hear it turn on, then off), THEN proceed with starting the engine. When I follow that protocol (on the cold starts), there is NO hesitation and the engine runs smoothly from the get go...

Try it out on your next cold start and let us know.

miike

The mystery is if it works, not why

LexBrett2

thanks, it's worth a shot!

hadhoud

I have the same problem on my W203 C180K.
I changed :
- Sparkplugs
- Coolant temperature sensor
- Thermostat
- Battery

And I cleaned the butterfly body.
The fault persists and is present only if the outside temperature in the morning is below 8 ° C

LexBrett2

My journey continues in figuring out the cause of my rough idle at cold start up. I have done all of the above to no avail. While the car does run great once warm, the rough idle is maddening.

This weekend I built a homemade smoke machine. It was easier than I thought and works well. I hooked up the smoke to the brake booster line and DID find a leak. The leak was coming from somewhere under the airbox, which I did not remove, so I will have to repeat the test once I have more time.