Replacing the air cleaner buffer fixed my idle
 

I was kind of shocked that replacing this part would do more than just keep my air cleaner from rattling. Just thought that I should share this piece of information in case some of you guys are having a hard time getting good idle and you've overlooked the vibration of your air cleaner.

What was wrong / bad about your idle ? .

Do you have a pic? I have no idea what this is.

The idle was rough when the car warmed up to operating temp. The engine felt like it was on the verge of stalling even though the RPMs were at the usual level which is about 825.

Weird. I have a hard time believing a loose air filter would affect idle on a diesel engine.

We are giving you a pass on the initial post since you were asleep at 2 am...
but we still do not know what exactly this thing is you are talking about...
lol

The three 10mm nuts under your air filter go to these: Look for air filter housing mount 1981 Mercedes-Benz 300SD Base Sedan - Intake System - Page 1

I am still shocked and , as others have also mentioned, not convinced this did what it looked like... sometimes things change but not because of the last thing that happened near the car.....

Me too, but what are the chances that it was something else if the problem goes away right after I changed that mount and steady the air cleaner? I've driven the car for about 1/2 hour since replacing the mount and the car is idling like a rock. We'll see if the bad idle comes back later. I hope not.

LIVE LONG ENOUGH and Mechanic ....... 100 percent.

OK - you are talking about the rubber bumpers. Now I see. I have been replacing these for 35 years on my 81SD and have never seen any effect on the idle. In fact I do not buy new ones any more; I drill a hole through the bumper after removing the top and bottom metal screw and plate and insert a long bolt through the entire bumper. Now instead of replacing about every six months to a year, I just about never replace them.

My idle goes from rocking at one stoplight to smooth as glass at the next stoplight to rocking again. Have never been able to figure out why. At some point something just kicks in and it idles smooth as can be but no idea what it is.

But I am pretty sure the buffers/bumpers have no effect on the idle.

:D:D:D

Just like once you fix something, then something else breaks?

Another element to the equation is that the rough idle started a couple of weeks ago when I put 1/3 bottle of Lucus Oil stop leak in the engine(flame suite on) to buy some time on an oil filter housing gasket that I need to replace. The product seemed to helped a little. A week after that, I finished off the bottle and shortly after the idle got rough. I immediately changed the oil with the engine hot. The idle was still bad for another week. I changed the spin on filter and filled it with Diesel Kleen before putting it back on, but that didn't help. I'll post a report in a week or so. If the bad idle comes back sooner, I'll check back.

That's a great idea, I'll have to try that.

Are you talking about a 1981 300SD? Mine has a drop in fuel filter with a bale on top. $10.50 at Pep Boys; changed it this week.

Rough Idle
 

Otto :

That sounds odd to be sure .

When was the last time you cold adjusted the valves ? . everyone thinks because they're quiet , they're fine but they're _NOT_ ~ solid valves require periodic adjustment .

825 RPM's is too high and is prolly masking something else causing the rough idle .

I doubt
 

it cured a rough idle but I'm sure it idled quieter.

OK... Nate gets a trophy on this thread... post number four should have caused several of us to ask what Nate just mentioned... and is spot on concerning the high idle hiding something else....

What spin on filter are you referring to in post #12?

OK, now I get it. The spin on fuel filter. Forgot about that one.

Anytime that filter is off the car... or being replaced.... fill it with Diesel Purge just SOP. Dump the rest into the tank...

I had the SAME issue with my car.
It would idle smooth at one stop light, and idle like a shaky old turd at the next. Sometimes it would ilde smoothly and then slowly start rocking. Once the rocking started it would get worse and worse until the whole car was rocking side-to-side.
Embarrassing; folks behind me could definitely see Betsie rock side-to-side like a cradle, and I was getting seasick. I would drop her in neutral and manually raise the RPM's a little with the throttle pedal whenever that happened.
I had already changed the engine mounts. That did absolutely nothing.

Solution:
Rack damper bolt. Tightened it up a little (adjust with engine hot and running) and no more shaking ever. At first I had tightened it up a bit too much, which results in the engine surging at low throttle input and the RPM's dropping too low when it goes from load to idle (it would almost stall out for a split second every time I release the throttle).
So loosened it back up again. There is a quite delicate balance between the car shaking at idle, and between RPM's dropping to much when releasing the throttle so it requires some messing about.
End result though is smooth idle and a responsive engine.

That rack damper bolt was designed into the system by OCD Germans to allow for taking up wear to the system.... so that will probably fix the problem as described... however, still need to check the valves cold.

@leathermang
Right. Your engine needs to be in relatively OK condition for the rack damper bolt to do anything. It's an adjustment. It won't do anything when things are way out of specification.

As far as I know my rack damper bolt has never been adjusted since 1981. So where do I find it and is it so touchy that I should just accept the rocking? Got a pic?

And, yes, I have sat at stoplights with my foot on the brake and the accelerator at the same time to stop the rocking. Deja vu. Probably should do in another thread so we do not steal Otto's thread.

So, you are in the 98 percent with the rest of us ...
and True....

@Tyl604
Yes, start a new topic to talk about the rack damper bolt. It is a really easy adjustment. You do not need any specific tools (just a 14 and a 12mm wrench) and it's easy to find and easy to get to. You can't really break anything either - if you happen to adjust it too tight your car might be hard to start and it'll almost stall when coming down to idle from higher rpm's.

The bolt is on the back of the IP. The IP is that long straight thing at the drivers side with all the hard fuel lines coming out. The bolt is right at the back, right in front of the oil filter housing.

The procedure is quite simple:
You make sure the engine is nice and hot, and then with the engine running you loosen up the lock nut (counter clock wise, 14mm wrench, you only need to loosen this up like half a turn, don't go crazy unscrewing it, not neccesary) and screw the rack damper bolt in a little (righty-tighty, 12mm wrench)) very slowly until your engine suddenly smooths out. Then with the 12mm wrench hold the damper bolt in place as you use your 14mm wrench to tighten up the lock nut again.
On my engine the engine will instantly become smooth when adjusting the rack damper, ie: It rocks like a turd, and as you adjust the bolt, suddenly it will be smooth. This is the sweet spot. When you back it out 1/8th of an inch, it'll behave like a turd again. At least, that's how it is on mine. Any adjustments further then the sweet spot can cause issues. On my car the engine will start to surge (rpm's going up and down constantly) with small throttle input when the rack damper is in too far.

It might be best to have a helper who can sit in the car, with the transmission in Drive and their foot on the brake to get the best adjustment. As you are moving the rack damper bolt in, you will immediately see (and hear) the change in idling, as your rocking engine will smooth itself out.

Since my wife absolutely refuses to have anything to do with Ol'Betsie (she won't even ride passenger in it, she hates it from exhaust to grill:(), I adjust the bolt with the transmission in Neutral. It just requires a bit more trial and error this way.:)

Ceristimo - that is exactly what I need. No need for another thread.

Thx.

My wife feels exactly the same; I tell her I will get rid of the SD when it dies and have been saying that for 20 years.

It only took three hours for this thread to get hijacked. I adjusted my updated rack damper bolt a couple of years ago when the car was stalling after exiting the freeway. I've had a broken air cleaner mount several times since and it never caused rough idle. One way to resolve this would be for me to remove the mount I just installed and let the cleaner flop around and see what happens to my idle. I don't really feel like testing fate right now and after spending the last two weeks wrestling with my AC system I'm taking a little break from doing anything under the hood.

The car stopped shaking after I replaced the mount, that's all I know right now. To be honest, the rattle wasn't even that loud.

Where exactly do you think it was hijacked ?
The original question / link to high idle and stalling was very strange... air cleaner mount... ?
people talked about things that might affect that idle , stalling situation..

Post #23, but it doesn't bother me. These threads usually go in different directions and I'm not even looking for an answer, just started the thread to point out something odd that happened with my car. Maybe some piece of gunk in the tank strainer broke loose right after I changed that mount.

I can see it happening because the shaky idle is a "comes and goes" thing. It often appears that the engine gets in a "positive feedback loop", aka a microphone too close to a speaker. The feedback might be from the shaking causing the IP to modulate fuel, which amplifies the shaking, at certain rpm's that match the "natural frequency" of the engine & mounts system. Being a "nonlinear effect", it sometimes takes a certain external excitement to get it started, and any little thing like a shaky air cleaner might be enough to push it over the threshold. Think of the "butterfly effect" in world weather. Sometimes a computer model even needs a little starting bump to get it to show oscillations.

Diagnosis of problems should always follow a linear pattern to avoid chasing your tail .

Most of the Vehicles I work on have been through the mill so they tend to have really obscure problems , often DPO & DPM bodges .

Fiddling with the rack damper bolt before you personally have cold adjusted the valves , as a waste of time and effort .

As a Journeyman Mechanic I always like to hear others ideas so threads that meander around are usually very instructive although not always to the original question .

Pure Vivid Imagination.

The rubber mount that I replaced over the weekend has already broken, so yeah, it was just a coincidence that my idle smoothed out at the same time.

If my idle gets rough again, maybe I'll do the same repair as kind of a rain dance ritual thing before I start adjusting valves and changing filters.:rolleyes:

Me too. That's a new one on me.