C 230 Rpm Idle Fluctuation - Page 3 - PeachParts Mercedes-Benz Forum

mpolli · #31 12-17-2007, 11:45 PM

I think you have what I have. Here are my exact symptoms:

Once (or maybe twice) a day, at a stop, after fully warmed up, the rpm stumbles for about 1/2 sec. That is it. No other symptoms. It is a curiosity.

Is that exactly as what yours does?

Will_w202 · #32 12-17-2007, 11:46 PM

[QUOTE=N41EF;1707142]Lore, it seems to me that Will already told you, that it's a "normal" self test to have an idle surge during long idles.

I never said it wass normal. Doing it for excessive lengths of time would tell me that something is wrong with the pump or emissions system, but in 100k miles there have never been any diagnostic codes related to these symptoms.

I have learned to LIVE with it, but it's not normal

Will_w202 · #33 12-17-2007, 11:50 PM

Quote:

Originally Posted by mpolli

I think you have what I have. Here are my exact symptoms:

Once (or maybe twice) a day, at a stop, after fully warmed up, the rpm stumbles for about 1/2 sec. That is it. No other symptoms. It is a curiosity.

Is that exactly as what yours does?

I think both Lore and I have the same issue Mike - mine will do it for 2 minutes, the quick "blip 'n dip," multiple times, ryhthmically. It's not a 1 or 2-time deal at first startup. It's a lengthy annoyance that is definitely abnormal, but again, in the OP's defense, the "great advice" of checking the system with SDS is probably futile. Neither of my local dealers could determine a "problem" in teh past

So, now it comes down to ingenuity - and I would think a failing throttle actuator would cause the problem ALL the time, not just for the first 20 mins

mpolli · #34 12-17-2007, 11:58 PM

Please let the poster answer my question Your Honor!

Will_w202 · #35 12-17-2007, 11:59 PM

Quote:

Originally Posted by ILUVMILS

Man, this thread is all over the place! Spark plugs, Air Injection, Transmission adaptation, etc. The answer probably isn't too hard to find, as long as you look in the right place. I realize not everyone has SDS at their disposal, but here's what I'd do if I was working on the car.

It sounds as though the throttle actuator could be the problem. The first thing I'd do would be to make sure the fuel trim values are within specification. This is a reliable, fast way to eliminate the possibility of a lean/rich condition causing the problem. If that checks out good, I'd activate the throttle actuator using SDS. It's a simple test, done with the engine at operating temperature/idle speed. The throttle is slowly closed, lowering the idle in small steps. Then I'd see if I was able to open the throttle up again. Sometimes the actuator simply isn't strong enough to overcome the throttle return spring. I see lots of this on higher mileage M111's. The result is a fluctuating idle because the actuator doesn't react quickly enough to stabilize it.

Of all the posts, I'd say this one is the best (and hey! He's a tech so he should know

)

Next time I'm in for service, I'll see if I can bribe my service writer with a Panera lunch card (saved me $120 last time

) and get some reduced-cost disgnostic time. I will point him in this direction, ILUVMILS

Lore, I'd say this is out best best - specifically, the lean/rich/fuel trim issue

Mike, sorry for hijacking! - just trying to help because I am 100% certain our problem is the same

LORE BENZ · #36 12-18-2007, 11:37 AM

Mine does not last for 2 minutes, more like 2 seconds at stop light, did it today only one time and that was after 20 minutes of driving at long light.

R.P.M. dips down and back up to normal once only during the whole stop of 1.5 minutes. OTHER TIMES TWICE, but not at same light or stop.

IF I TAKE LONG 2 HOUR DRIVE, NO problems at all!!!!

IF it is rich mixture, what can tech do about it?

SOME dealers are notorious for fixing that and selling throttle actuators, PARTS CHANGERS IF YOU EVER HEARD TERM. Thats why I am leaning to someone who has machine or dealer to ONLY DIAGNOSE if anything wrong.

THAT IS MY CONCERN!

WILL, THROTLE BODY IS NOT CHEAP, so better be sure that is cause of problem before changing it and RPM DIP is still there after that!!!!

mpolli · #37 12-18-2007, 03:08 PM

So I think from what you say that you have the same as I have which is not as bad as what will_w202 has. If I understand correctly this is simply a curiosity and does not effect driveability or performance in any way. Is that correct?

If so what is the big deal?

I think to put this to bed, you could do an EXPERIMENT:

Unplug the air pump motor (I assume it has a connector). Then I believe your symptom will disappear. Also your CEL will probably go on since the pump will "fail the test". Also your emissions will be worse. Then plug in the pump and the symptom will return. Then you will know if it is the air pump or not.

LORE BENZ · #38 12-18-2007, 04:34 PM

Quote:

Originally Posted by mpolli

I think you have what I have. Here are my exact symptoms:

Once (or maybe twice) a day, at a stop, after fully warmed up, the rpm stumbles for about 1/2 sec. That is it. No other symptoms. It is a curiosity.

Is that exactly as what yours does?

yes!!!

But I don't know how to remove 'air pump motor" and wait for one day or two to see if it will do it again???

LORE BENZ · #39 12-18-2007, 04:36 PM

Quote:

Originally Posted by Will_w202

Of all the posts, I'd say this one is the best (and hey! He's a tech so he should know

)

Next time I'm in for service, I'll see if I can bribe my service writer with a Panera lunch card (saved me $120 last time

) and get some reduced-cost disgnostic time. I will point him in this direction, ILUVMILS

Lore, I'd say this is out best best - specifically, the lean/rich/fuel trim issue

Mike, sorry for hijacking! - just trying to help because I am 100% certain our problem is the same

HOW DO YOU FIX RICH/THIN MIXTURE if that is problem?

LORE BENZ · #40 12-18-2007, 04:56 PM

Is that normal to dip like that?

Do you need to replace air pump motor??? expensive!!!

THROTLE ACTUATOR???

MY symptoms are as I stated above, very specific.

SO what parts or problems can do this ONLY??

Will_w202 · #41 12-18-2007, 06:08 PM

We're going to talk this thread to death. The probable culprits have been identified. I'd do the tests prescribed in previous posts, starting with disconnecting the air pump as was just stated. I don't think you NEED the air pump, it's just a $700 part that makes the EPA happy by changing the fuel mixture during cold starts to lower emissions. If mine ever dies, you can rest assured I won't be replacing it.

ILUVMILS · #42 12-18-2007, 08:39 PM

Quote:

Originally Posted by Will_w202

....If mine ever dies, you can rest assured I won't be replacing it.

Eliminating the air pump won't cause any drivability problems. You'll just have to get used to driving around with the Check Engine light on all the time.

LORE BENZ · #43 12-18-2007, 09:31 PM

THANKS FOR POST GUYS, I APPRECIATE IT!!!!

I am going to drive car as is and if decent indie shop with SDS machine pops up I will consider hook up with this rich knowledge in hand!