Aimbit

I noticed today when I turned on my car after work. It feels like its bouncing a little/rocking back and forth just a tiny bit. After I drove to the first light it idled just fine.

edit: just to be clear... i had to sit there and focus on it to really notice it was happening. I will try again today after work and see if it still happens. It DOES NOT feel like its going to stall.

michail71

Mine does that sometimes on a warm start. I've been meaning to catch it with Torque to see if the engine is in close loop or open loop mode.

I think it happens when the car goes into open loop mode too soon but someone with better knowledge may be able to answer better.

Do you have a C250? It seems there are more reports of this on the 4 cylinder engine.

superangrypenguin

Yes it's normal. Car runs rich. I don't recall if this car has SAI but anyways... This is normal.

Moto_Guzzi

My C350 2008 did it once in its life recently. I changed the oil/filter previous day.

After reversing the car out of garage when cold and parked on steep decline in front of garage to give it a wash. When I then started it, it did this uneven rich running with a bit of smoke, never had this.

I checked the oil level and removed 1 litre, had it overfilled. It never did that again so far.

Alfadude

I would disagree that it is "normal". Should idle consistently when starting, cold or warm. Now is it something serious? From your brief description doesn't sound like anything to be concerned with, at least right now. If it was a one time thing I wouldn't worry about it, but if the rpms bounce around consistently that is not how how it is designed to work.

superangrypenguin

No. When the engine is cold the car is designed to get the catalytic converter to operating temp as quickly as possible. That's why the car runs rich and the idle is higher then about 30 seconds after idle. Almost all cars are designed this way.

michail71

I've seen many mentions of this with the 4 cylinder cars. Mine will do it on occasion if it is warm (had been run about 4 hours prior on a hot day). Once I give it some throttle it will go away. It's always done that. The car bounces around and feels like it's about to stall. Kind of like an old lawn mower that is cold.

I tried to have the dealer look at it but it resulted in the typical "unable to reproduce" diagnosis.

superangrypenguin

Ok, no, it's not supposed to be THAT severe. What mileage do you have on the car?

That to me either sounds like excessive buildup of carbon on the intake valve or a coil pack that's on it's way to heaven. (or in this case, hell!)

Funkwagen

What you're describing sounds like a large vacuum leak almost stalling out the engine rather than running rich...

Vacuum leaks are fairly hard to pinpoint without doing a lot of poking around. Do you have any fault codes that you know of? If you know of a good mechanic other than the dealership you should probably take it to them. It took me weeks to figure out where the vacuum leak was in my car..and it turned out to be two, rather than one.

Alfadude

Yes, the engine management adjusts for a cold or warm start and other conditions, but the idle should be smooth and consistent no matter what. Idle RPM could be higher when cold and then go down as it warms up, but they should not bounce around. At least that is how I interpreted the OP's original issue.

michail71

I'm not the OP but mine isn't that sever. It is a vague feeling like it wants to stall. It can on occasion feel rough when the car is warm but not cold. Typical case it happens is when I go out to lunch from work. The temps are in the 90s and the car has been sitting for 3-4 hours. It is always hot outside when it happens.

It has never happened on a cold start. Not that it gets cold where I live but if the temps are cool it tents to not happen as well.

It may feel rough and not even show any variation on the tack. If I let it sit it may smooth after 30-60 seconds. Or if I just pop the throttle it will smooth out.

It has 50K miles now and it has been doing it since I got it with 20K miles.

superangrypenguin

Fair point.

I think it really comes down to what the definition of "bouncing around" means. On my C300, when the ECU has the engine running rich, I'm in the 850rpm to 950pm range - definitely nothing close to stalling or puttering out.

On my B7 A4 (similar to the C300 - 4 cylinder DI engine), it would idle pretty rough and bounce between 750rpm and 950rpm - again, nothing close to stalling, but it was certainly running rich and the idle was far from smooth.

On the V6 C300 that I have, I can tell it's a bit rough, but it's still pretty vibration free given the fact that it's a V6. On the C250, I can draw an analogy to the A4 but again, I'm not saying that an engine about to putter out and come back to life is normal. That seems like excessive vibration to me.

michail71

It's about the point where an aware driver would notice. I'm not sure the average person would say it seems a bit rich, lol. With the AC and radio cranked up one would not notice what I'm getting at all. There is just an ever so slight bounce in the car. However, it does give me memories of driving my 70s era cars as a young teen. Hell, I think I had a car with a manual choke.

superangrypenguin

I see, then I would say it's around normal. I think it'd be worthwhile taking a video of your tachometer when you start the car.

michail71

What's odd is it looks steady while not feeling that way. I even pulled up the RPMs on Torque once while it was doing it and didn't see anything that looked unusual.

Now that it's in the 90s it seems to happen more often. I'll see if I can get that for curiosity sake.

ezshift5

4-cyl C250 observations.....

RPM at start higher than normal

Higher RPM attenuates w/ warm-up.

ez

95Sinned420

Cold engine does not idle the same as when at operating temp. In fact, the RPM would be slightly higher for about a minute. Why? As mentioned above, for emissions related reasons. Your SMOG pump runs during this time and you can even hear it whine before it shuts off and idle goes to normal.

Aimbit

I am going to let the car idle again when I start it today after work and see what happens. I'll note the RPMs and anything else. But as far as I can remember everything appears normal. The dealer actually called me today asking how the car was holding up and I told him about this he said to bring it in and they will have a look.