Kevotron

anybody have a humming sound right after the ignition button has been pressed ??
the noise sounds like is mechanical, but Mercedes could not replicate it when i took it in...

I am Jeff

Are you talking about the humming sound that occurs right after you start the car and lasts about 10 seconds before the idle lowers below 1000rpm? It only happens on cold starts.

Mort

Could be the fuel pump. It should kick in for a few seconds before the car starts to bring the line pressure up to spec. You might be able to hear it for a while when it is cold as the pump might be a little louder sounding when it is cold.

SonnyakaPig

@ Mort,

When you first start the car up in the morning, can the fuel pump make loud thumping sounds that you can actually feel -- the car moves from these thumps.

This is the scenario: The car is driven hard at night, then parked and shut off for the night. The fuel level is near empty at this point. The car is then started the next morning and because I have an 09' the car rewards my spirited driving from the night before by starting off in "Sport" mode.

The car will make loud thumping sounds and the car will rock around a little.

If I press "Comfort" the thumping will settle down a little bit. I then wait for a few minutes and after the car reaches operating temp, I switch it back to "Sport?"

Sorry for the long explanation. Just wondering if the engine is knocking or if the fuel pump can be the culprit because the fuel level is low and my car started off in "Sport?"

Btw, it's 91 octane from Shell, Chevron or rarely 76.

Mort

Hi Sonny

The fuel pump should not cause the symptoms that you are describing. If it was I think it would continue until you refilled or ran out of gas.

When you say you are low on fuel is the reserve light on?

Is this happening when the car is in motion?

The transmission will definitely have firmer and faster shifts in Sport mode and that might be the issue. The shifts will tend to be much more pronounced when the engine/trans are cold especially at part throttle.

What you are describing almost sounds like a miss. That could be caused by the cold dense fuel not being properly atomized by the injectors and would be load sensitive. This situation will cause incomplete ignition of the fuel mixture and result in a hard knock or stumble that can be heard and felt in the car. Not sure why Comfort would be smoother than Sport unless Comfort causes less load on the engine. Sounds more like a problem in the ECU or the cold idle circuitry of the engine. Maybe the adaptive learning from the night before cannot be compensated for by the ECU and the engine needs to relearn its cold start running characteristics.

SonnyakaPig

Hey Mort,

Thanks for the helpful response.

We can rule out the hard shifting in "Sport." My car shifts properly. This is not happening when the car is in motion. I always let the car warm up for a few minutes before I put in gear for the first start up of the day. Also, if it's cold and I let it sit during the day.

The parts that I bolded are also my theories. I really believe this can be the case but no one else is discussing these issues, so why would it be unique to my car?

Plus, my engine sings and makes solid power when I'm driving around, so I don't think my engine has any damage.

But, I do believe that "Sport" has more aggressive timing than "Comfort" and that's why I access "Comfort" when I'm low on gas.

This really only happens when I drive hard the night before and then the car starts up in Sport mode automatically. And the fuel level is low -- Reserve light is on.

I really think it's the adaptation coupled with 91 octane fuel and low fuel level in the tank.

I've been told by other members that push their cars that the C63 is sensitive to low gas and I already know to treat 1/4 tank like it's low and nearly ready for more fuel.

The tune is stock btw.

Mort

Sonny,

So this thumping sound and motion happens as the car is idling cold? I never warm my car up. I start it up, put it in gear and go. I do not drive hard until the engine and trans are warmed up. I have always been told to drive off as soon as the engine does not stumble and I find that to be right away. This will warm up the whole vehicle faster, use less fuel, cause less cylinder wall wash down from a rich mixture, less start up engine wear and be generally easier on all mechanical systems. At least that is the way I see it and the procedure that I follow. You may find you have less thumbing and it will go away faster if you just fire the engine up and go. C mode may be best right off of a cold start.

My car has been performing fine on Shell 91 and all we have here. I think I would use 93 if I could get it and would feel better about it. I am not sure if the 91 is adding to your rough running or not. Low fuel should not be an issue unless you are in motion and the engine is using more fuel. I think you would have a definite stumble all the time until you filed up if it was.

I fill up as soon as the reserve light comes on or just before depending on what is convenient. I have a few issues with running the tank too low and also with trying to cram every ounce of fuel you can get in the tank. I think most of the purge valve and evaporate system issues are a result of overfilling the tank. When the tank is close to full and the nozzle shuts off that is it. Don't try to put any more fuel in the tank as it floods the evaporation system with fuel and it cannot perform as it should. The fuel system is closed to prevent evaporation of fuel into the atmosphere and it needs to function properly or the CEL comes on. The fuel is also used to cool the fuel pump and it cannot do that properly if the level is too low so fill it early. The other problem with low fuel level is potential fuel starvation especially at WOT. The pump is likely in a little bucket to keep it cool and to provide enough fuel for a short burst of WOT fuel consumption. If the tank is low the fuel can be pushed away from the intake to the pump by acceleration forces and the supply in the pump bucket can run out quickly when the engine is run at WOT. This may lead to a misfire and potential detonation and engine damage over time. This definitely is the way it is in my truck and I need close to a 1/2 tank of fuel if I want to avoid this issue when running hard. Found that out the hard way.

My guess is that sport mode increases line pressure, torque converter lock up % and shift points but not timing although I don't really know one way or the other. There are likely a lot a timing modifiers in the tune and it is possible the shift mode is one of them.

A little more fuel for the fire from me.

SonnyakaPig

Mort,

Wow, so much great information. I really appreciate the thorough response.

I think I will drive off a little earlier than I have been after I first start the car up.

This morning, with a full tank of gas, the car started up in Sport and the thumping returned. So, I drove off and I noticed it subsided.

Btw, I don't go WOT when the fuel level is low. And, I never overfill the tank.

I really did learn a bunch from you post.

_AMG_

Mine makes that noise to from time to time, Ive heard its normal.

hhughes1

Very strange that the S or C status could have an effect on a stationary car?? Set it to C before shutting it down and it will still be in C when you next start it up, then see if the symptoms are the same.

link146

Smog pump, i.e. secondary air injector - runs on cold starts and pumps oxygen into the catalytic converter to reduce emissions upon cold starts in order to pass emission regulations. It sounds like a vacuum cleaner and runs for about 60 secs. Next morning, pop the hood then start the car. locate where the sound is coming from. the pump is black and about the size of a softball with one rubber hose running from it. That should be where the sound is coming from. Not a problem. Leave it alone.

khmergod

Late to the party, but in my experience, cold starts are more pronounced in S because of 1st gear. Hope this helps.

enochroot

Could it be that you are subconciously making a groaning noise because you are excited to go for a drive?

AMGC60-3

Are you talking about the noise during cold start that sounds like a mini vacuum cleaner? If it is I believe this is for the proper fuel air mixter for cold start. Or it is the electric fuel pump.

Gamc2010

About 3 month ago, i had my service and mechanic suggested to change tires. Sometimes i get the squeaking and humming noise when making turns in downhill. Recently I hear humming noise when starting the car, especially in cold. If I move steering wheel left to right, the humming noise varies, but this noise disappears after 5-10 minutes of driving I am planing to change tires, but was wondering if there is anything else I should be looking at

Mort

If you are getting a humming - groaning noise that varies as you turn the steering wheel left or right you might have a problem with your power steering pump. Check the level and if it is good get your dealer to check it on a cold start in the morning ie. leave it over night.

roadtalontsi

theres a normal humming noise of the secondary air injection pump - sounds like vacuum cleaner. It is used on cold starts and will last up to 2 minutes usually not that long though. It is also used at occasional stops - like in the drive thru or at a stop light for the self test of the system.

Ookami

I normally hear this and let it go on until it stops before I drive off. One day I was running late and decided not to wait. When I drove off and got to a stop light, I could hear it turning on again and off and at the same time my car felt it was gonna shut off, idle was not steady. Eventually it stopped and the idle was normal and was able to drive off. However during that the check engine light came on and stayed on since.
Any idea on this and what should I do?

Mort

Take it to the dealer, indy shop, a friend or perhaps an auto supply store and get the code read. Go from there.