"rumbling" when pulling away from a stop

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hova
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I've got a bit of a weird one... my 463 300GEL seems to have a faint rumble whenever I pull away real slow from a stop. As in, if I put my foot down on the accelerator pedal, there's a bmbmbmbmbmbmb that sort of grumbles then disappears as soon as I have picked up some speed (but still in first gear, I would think). I can feel it under my left foot as well, so it feels a bit like there's a vibration in the chassis or bulkhead somewhere. I don't know if its an engine issue, or something loose that's vibrating at the exact hertz/frequency my engine makes at those rpms.

Is this common? I don't feel like it's really affecting my performance, and it's quite faint - but it's there, and it's disconcerting. Is it simple? Is it complicated? Shall I drop everything and replace the lump with a low mileage brabus 36 engine? :twisted: I have been toying with the idea as my brother has found one back home and offered to send it over to me.

HELP!

Many thanks,
Hova.

fixwin38
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Re: "rumbling" when pulling away from a stop

you might look at greasing all the drive shafts and check the exhaust mounts.plugs and leads sometimes break down under load..always very difficult to diagnose "from a distance" may even be a failing catalytic converter.....

Pistonhead
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Re: "rumbling" when pulling away from a stop

If this rumbling is heard whilst starting as well, I reckon, it would probably be the exhaust fouling with the body work, the exhaust bracket hanger weakening the exhuast fouls against the chassis by the down pipe.

Alternatively, it just might mean the baffles of your KAT, if fitted are breaking up, resonating as it does.

Good Luck,

hova
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Re: "rumbling" when pulling away from a stop

No, no rumbling when I start the car. Just faintly when I step on the gas LIGHTLY. If I gun it right away off the stop, then there's no rumbling either. Are my fuel lines plugged up? Thought that might be an issue.

As a side note, when starting the car from cold, it sometimes takes a while for the engine to turn over and catch. As in, the starter motor will crank for a while before the engine roars to life (ha, roars... it's only a 300! I mean roars, in relative terms, of course). Is this common for the straight six? (someone told me it was, but could be complete and utter BS). HELP! :D

fixwin38
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Re: "rumbling" when pulling away from a stop

Hi the engine should start at cold on second or third revolution rev to 1000/1100rpm and gently reduce the revs over 120 seconds to 650/700 Automatically!! when hot engine should rev on startup to 1000/1100 for 5 seconds and drop to 650/700revs steady idle...if you do not have these parameters there is something wrong with the cold start management system/vacuum system....

hova
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Re: "rumbling" when pulling away from a stop

The revs going up to 1000/1100 then idling down to 650/700 seem right on point - no wiorries there.

It's more an issue of the number of revolutions/time elapsed before the engine itself starts (when starting from cold) - it sometimes takes a bit longer [than any other car I've owned]. How many seconds, in your honest opinion, should it take for the engine to start from cold?

With a hot engine I've got no problems, but from a cold engine, sometimes it takes... oh... maybe 3-4 seconds? Is that normal for the M103 engine?

Pistonhead
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Re: "rumbling" when pulling away from a stop

Questions on the general conditon of engine need to asked, when was it last serviced? spark plugs, air filter, fuel filter and oil with oil filter renewed. Check and Inspect the distributor cap and rotar arm.

All being well, then you have a fuel related problem, cold start sensor or injector need checking. Cold start battery draw? Fuel pressure issues, does the engine start any quicker if you were to prime the fuel by turning on the ignition key about three times on and off then start, will she catch straight away?

Good Luck,

fixwin38
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Re: "rumbling" when pulling away from a stop

Any smoke on cold start up? 3/4 seconds is reasonable provided the revs are right...the fuel system is pressurised by the electric pumps and has an inline filter and accumulator.. might be losing the pressure a bit at the first cold start.on a cold day...is it the same delay on a warm day ?

Russ280
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Re: "rumbling" when pulling away from a stop

Could be the water pump. Mine sounded pretty much as you describe.

Process of elimination involved:

Replace water pump with genuine MB one, no change.
Changed just about everything that revolves on the front of the engine, no change.
Got to thinking it was the gearbox input bearing :cry:
Last ditch attempt to cure it (I was convinced the noise was still from the front of the engine) rip nearly new MB pump off and replace with cheap pattern pump.

Noise disappeared :) :)

Russ280
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Re: "rumbling" when pulling away from a stop

hova wrote:
The revs going up to 1000/1100 then idling down to 650/700 seem right on point - no wiorries there.

It's more an issue of the number of revolutions/time elapsed before the engine itself starts (when starting from cold) - it sometimes takes a bit longer [than any other car I've owned]. How many seconds, in your honest opinion, should it take for the engine to start from cold?

With a hot engine I've got no problems, but from a cold engine, sometimes it takes... oh... maybe 3-4 seconds? Is that normal for the M103 engine?

Fuel Pressure accumulator?

phileas
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Re: "rumbling" when pulling away from a stop

Hova and all,

Regarding your starting issue, I have something very similar....it takes more than a few revs to get the thing going in the morning. the problem is slightly better now than it was in the winter. i replaced, the cold start injector and idle control valve (all MB original parts...not cheap!.....but I did have idling problems which are now cured) and still have the same issue. I've checked the plugs, leads, points, fuel pump and these are all fine. somebody mentioned something about a vacuum/accumulator? Have I missed something obvious?....any ideas appreciated.

thanks,
Phileas

M2dxb
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Re: "rumbling" when pulling away from a stop

The accumulator sits next to the second fuel pump. If it has failed (i.e. has a hole in the rubber inside which you cannot see) it will return fuel to the fuel tank (from the return hose, it'll be obvious which is which when you have a look at it) whenever the fuel pumps are running without the engine (like when you prime them by turning the ignition to ON but not cranking). You can take the hose off the back of the accu and have someone turn the ignition to see if anything comes out. Danger of spilt petrol of course.
The cleaner way is, prime the fuel pump a few times by turning the ignition to ON and not starting, repeatedly, every time until you hear the pump go off, THEN try to start it. If it starts straight up it might be the accu.

I think a more likely culprit (which I discovered on mine after a similar process of elimination) might be the idle setting of the fuel/air metering body. Check your last MoT certificate, what is the CO reading from the "krypton" test? If 0% or very near 0 then it might be running too lean, hence takes a while to start. Adjusting the idle screw on the fuel metering unit should sort that out, while connected to the CO-meter, but if you go down that route ask advice first or get it done at an MB specialist whose worked on the M103 engine before, otherwise it'll confuse them no end.
My engine used to shut down very very rarely while idling, regardless of temperature, turns out it was the idle mixture running too lean. "Enriched" it and it has never happened since.