rough idle at 1600rpm - PeachParts Mercedes-Benz Forum

airbus · #1 12-10-2005, 11:18 PM

Weird thing: Our '84 300DT is running fine now (after I put approx. $1500

in parts and weeks of work in it...) except for one thing: I have strange vibrations, occuring exactly between 1600rpm and 1700rpm, no matter if I'm driving or standing (-> engine, not drive train). I asked several people, most tend to the conclusion the IP must be bad (insufficient fuel delivery to one cyl. at exactly this rpm., leading to rough idling

). I'm just trying to save another $1000 for a new IP, any thoughts?

t walgamuth · #2 12-10-2005, 11:47 PM

but bad ip seems not likely to me. did the opinioner offer explanation of why it just happens at that rpm?

tom w

#3 12-10-2005, 11:53 PM

I agree with Tom, a bad IP seems unlikely to give this symptom. Was this vibration there prior to working on the car? What have you done so far?

airbus · #4 12-11-2005, 12:01 AM

That vibration was there ever since we bought it. It had a leaking fuel line from the IP to the 2nd cyl. and bad engine mounts and shocks. So I hoped this would be fixed with replacing the engine mounts and shocks and fixing that leak. Well, still vibrating, and that's annoying, especially since this is exactly the rpm-range where SHE drives it most of the times... (I usually have it at 3000+ rpm

, always )

airbus · #5 12-11-2005, 12:06 AM

Quote:

Originally Posted by t walgamuth

but bad ip seems not likely to me. did the opinioner offer explanation of why it just happens at that rpm?

tom w

Yes, he did:
Any other mechanical defect inside the engine like a bad cyl. would cause it to idle roughly at any rpm, not only at exactly 1600 to 1700.

#6 12-11-2005, 12:09 AM

This really does not sound like an IP to me. The IP doesn't care what RPM the engine is running at, it only knows how much fuel is required (throttle position and boost). The engine is using a lot less fuel if you rev it to 1600 in neutral, than if you are driving at 1600 in high gear. My best guess would be something mechanical that's resonating at the RPM, but I don't know what.

airbus · #7 12-11-2005, 12:16 AM

I'm open to any suggestion because I have absolutely no clou what it could be. Otherwise this car is a beast (well, compared to that 240D that I'm used to so far...), lots of power, no smoke, no blow-by (according to my mechanic...).

ctsoth · #8 12-11-2005, 12:18 AM

If there is no smoke I doubt its a bad cylinder. I would say something is resonating at that fequency.

t walgamuth · #9 12-11-2005, 11:13 AM

are you sure there is actually a problem? if you are used to a 240 the five cylinder sounds very lopey (loopey?). again, i agree witth the poser who said if it is missing it should be smoking whitish bluish smoke from unburned fuel, course if the cause is lack of fuel it might not.

some thoughts

tom w

d.delano · #10 12-11-2005, 01:01 PM

The harmonic damper on the counterbalancer? Seems like this might vibrate if the rubber is delaminating from the hub.

boneheaddoctor · #11 12-11-2005, 01:51 PM

did you order the correct motor mounts when you replaced them.....there are different mounts for turbo and non-turbo models ....and maybe they were cheap knockoffs made with the wrong durometer rubber if they are new.

airbus · #12 12-11-2005, 09:49 PM

boneheaddoctor:
There was no difference before and after replacing the motor mounts, so I'm assuming they are the right ones.

d.delano:

Could you explain that a little further, please?

t walgamuth:
Well, it's not a problem, it's just annoying, like I said before, especially with HER driving.

ctsoth + craig:
What could be resonating? Is the 'thing' that's resonating the problem or is the 'thing' that's causing this specific frequency the problem?

#13 12-11-2005, 10:00 PM

Quote:

Originally Posted by airbus

ctsoth + craig:
What could be resonating? Is the 'thing' that's resonating the problem or is the 'thing' that's causing this specific frequency the problem?

harmonic balancer?

http://www.peachparts.com/shopforum/showthread.php?t=52125&highlight=harmonic

Just a guess.

#14 12-12-2005, 02:44 AM

BTW do NOT rule out the IP as a source of the harmonic vibrations.

Just because it might "idle" smooth doesn't mean it is set perfectly for every RPM across the band. The 5 cylinder pumps are rather picky about perfect adjustment and the shop that I talk to about these say that they can end up having to do timing adjustments (delivery valve adjustements) over and over again just to get it to work well on the car without any qualms.

How bad is the shake? Barely noticeable orrr???

My 300SD used to shake like it had palsy but after some tinkering with the IP I straightened it all out. Wasn't rack damper, wasn't anything else, the IP was just worn and out of adjustment enough that it had harmonic vibrations.

I would describe my adjustments but its not something I recommend anyone do...best leave it to a shop

Describe the shake more to us...

airbus · #15 12-12-2005, 10:54 AM

OK, here we are again. How do you describe a sound or vibrations? It is very noticable. It's a droning sound, sitting inside you can feel the whole car vibrating. It starts EXACTLY at 1600rpm and stops EXACTLY at 1700rpm, driving uphill, downhill, accelerating, decelarating or standing. At he shop I had the timing chain replaced, all belts (and here they probably would have seen anything wrong with the balancer), engine shock absorber and mounts, valves adjusted, stopped some oil and fuel leaks, etc. I'd call it a 'major service', pretty much everything that wears was replaced.
Weird thing is you can't see, hear, feel anything from the outside. I tryed to create those vibrations leaning over the engine playing with the throttle linkage trying to locate the source, didn't work. At the shop they adjusted the IP back and forth, apparently with no success. And actually I trust them, they are specialized in european cars and work a lot on MB diesels. And they are honest: They told me that they had no idea where the vibrations came from. They said the only thing they could imagine was the IP, but taking it out and having it tested was not worth the effort just to get rid of some vibrations. I agree with that one, but at least I'd like to know WTF it could be and if it can cause any further damage to the engine driving around like that. If it was the balancer, it could, apparently

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