Rough idling - PeachParts Mercedes-Benz Forum

AleksACK · #1 11-04-2008, 03:15 PM

Hello everybody!
Need some help from professionals…
I have 00’ ML430, with 100k and some troubles.
The whole truck is really vibrating (almost shaking) while idling at 600 RPM. It idles OK at 3000 RPM but “backfires” at/above 4000 RPM – scary picture, looks the like engine is going to break off the mounts and fly away. Feels like it missfires – but I think it doesn’t because CEL is off. I was trying to keep it at 4000 RPM for some time to get a code, but, like I said it’s scary and I don’t want to make any other damage – no code.
I’ve replaced mass airflow meter a year ago (check engine light was on), also crank position sensor was replaced 6 month ago because intermittent no-start. Spark plugs were replaced a week ago, spark plug wires tested and cleaned – no result.
I don't know when this problem occured, as this is my wife's vehicle, I would guess 3 or 4 month ago.
Don’t know where else to look, please help.

mbdoc · #2 11-04-2008, 03:47 PM

The rubber engine mounts do break. Also the trans mount will transfer vibrations.

AleksACK · #3 11-04-2008, 04:09 PM

Thanks for reply.
I'll check the mounts as well, as I have to crawl under the truck to find the source of another broblem - whining noise is coming from the front at speed above 35 MPh. All driveshafts' rubber boots are OK, so I'm kinda hoping it can be one of the wheel bearings. Otherwise it has to be something really bad - front differential or even transmission. Any known common issues? This truck was used on a really soft beach this summer on a regular basis - at least once a week (I have to say it outpeformed brand new F-150 and Titan), so everything is possible.
Regarding the idling problem - I was told that I should try to replace camshaft position sensor as well and it may sollve the problem??? Like I mentiond before, it doesn't vibrate in a range 800-4000 RPM, also feels quite well while driving.

Gene Horr · #4 11-04-2008, 05:33 PM

Dont start replacing sensors unless you have an error code.

Have you changed your spark plugs? It's time if you haven't. Also the mounts as mentioned earlier. Problems like this are usually regular maintenance items.

AleksACK · #5 11-06-2008, 03:58 PM

Yes. Spark plugs were changed last week, wires cleaned and tested. Engine mounts are fine.
The problem is - it will not go above 4000 RPM! It sputters and RPM jump between 4200 and 4000.
I'm really confused now.

Gene Horr · #6 11-06-2008, 05:57 PM

Just because the CEL light is not on does not mean that there aren't any codes. Get it scanned. If you have access to a scanner that will give real time readouts from the sensors that would be even better.

I now suspect a fuel delivery problem. If you have a partial blockage or partial pressure problem this would allow it to run fine at lower throttle positions but give you problems at higher ones. And it may not trigger a CEL. The CEL is more related to problems with the exhaust. Not having sufficient fuel does not hurt the exhaust and so the CEL may not be triggered.

How old is the fuel filter? Note that IIRC there should have been a recall done on the fuel filter area. Also the fuel pump wiring is prone to failure.

AleksACK · #7 11-07-2008, 01:41 PM

Still nothing. I have an OBD interface I connect to my laptop and a simple software - no codes. Unfortunetely, it doesn't show real time fuel pressure - not supported. But it shows real time MAF air flow rate. It reads 5 g/s on idle - I was told it should be around 16?. The MAF sensor is 1 year old - do they really go bad so fast?

mbdoc · #8 11-07-2008, 03:30 PM

Was the replacement MAF a new OE Bosch unit? They have a 1 year warranty.

They don't fail that quickly!

AleksACK · #9 11-07-2008, 03:55 PM

Yes, it is OEM Bosh part, bought from egermanparts.com. But, does it indicate that MAF has to be replaced? I mean - MAF air flow rate 5 g/s is not what it should be?

mbdoc · #10 11-07-2008, 04:06 PM

At idle on a V-8 engine the NORMAL minimun reading is 18-20 KGH.