diagnosing the idle - PeachParts Mercedes-Benz Forum

JHZR2 · #1 11-23-2009, 04:25 PM

Hello,

my 82 300CD-T has very recently developed a slight idle fluctuation. It dod not have this before - it is new.

I have always noted that my tach reads at idle just at 500RPM. That to me is too low, but that said, it also idles perfectly smooth, even when cold.

However, lately I have noticed that when idling in park or neutral, Ill get a surging sound from the drivetrain, and sometimes a slight fluctuation in the tach. In every w123 that Ive had, something in the transmission changes pitch when taken out of gear. I hear a very slight higher-pitched sound - and I can hear its sound changing slightly up and down, with the tach.

Not a very good explanation, but overall two things - 500RPM idle, and then idle fluctuation when warm, only in P/N though.

Any thoughts?

Thanks!

toomany MBZ · #2 11-23-2009, 05:04 PM

If this happens after the engine has warmed up, I'll suggest the rack damper bolt needs adjusting.

JHZR2 · #3 11-23-2009, 05:08 PM

Just started - never before. Not that cold either the last few starts...

Ill look into that. Should I boost my idle a tad too? It IS smooth, just low...

Thanks again!

oldsinner111 · #4 11-23-2009, 05:08 PM

Mine was set at 650 rpm,I noticed the 85 617s they raised it to 750.So I set mine on 700.
Air leaks in the fuel ines always for me caused surging.

Biodiesel300TD · #5 11-23-2009, 05:12 PM

I'd bump the idle up just a bit. It should be closer to 700rpm. If that doesn't make it go away I'd try adjusting the rack damper. My wagon has done this for a few years now. I'm not really ready to slap down $80 for a glorified bolt.

#6 11-23-2009, 05:13 PM

I have turned mine up to about 600, it's smooth at 500 but it's a little happier at 600 when it's cold. I think the book says 750, but that seems much too high.

rrgrassi · #7 11-23-2009, 06:07 PM

Have you checked the fuel filters, air filter, tank strainer

toomany MBZ · #8 11-23-2009, 07:13 PM

Quote:

Originally Posted by JHZR2

Just started - never before. Not that cold either the last few starts...

Ill look into that. Should I boost my idle a tad too? It IS smooth, just low...

Thanks again!

Because this just started, your idle should be fine.

JHZR2 · #9 11-23-2009, 07:35 PM

Quote:

Originally Posted by rrgrassi

Have you checked the fuel filters, air filter, tank strainer

Just did fuel filters and line today, as the line to the prefilter was weeping - no good. Air filter practically new and very clean. Did not look at the strainer, but there is no indication of junk in the prefilter... And power/boost is perfect. Id imagine that throttle response will suffer if the strainer or a filter was going bad and getting clogged, right?

JHZR2 · #10 11-24-2009, 10:30 AM

Quote:

Originally Posted by toomany MBZ

Because this just started, your idle should be fine.

But my idle has always been seemingly too low.

Diagnosed it a bit more last night and today. What I found was that when in drive, there is usually minimal variation in RPM, likely due to the small load produced by the TC and ATF pump. I will see it though when rolling in traffic, when the AT kicks back to first gear before a stop, it will fluctuate a few times.

As soon as I put the car in neutral, Ill get the fluctuation. It fluctuates 5-8 times, then seems to settle. If while in neutral (and stable), the AC kicks on, it does it again, but settles out again too.

Yesterday I played with it at home, after my commute - it was hot and it id it.

Today I played with it before pulling out fo my driveway, and it did it (cold), but when I got to work I couldnt make it happen... So it sometimes happens hot, sometimes happens cold.

But today I also noticed that when I kicked on my AC in my driveway cold, the idle got REALLY rough, kind of like my 240D whern real cold and the idle turned slow, and the tach reported about 400RPM. it smoothed out nice again when I took it to 500rpm by my foot, but that is something that was odd too. That didnt happen, i.e. the rpm compensated for the AC load, after the engine was hot. Strange enough though, once the engine smoothed out a tiny bit, the car still rolls at a good clip (5mph maybe), just idling at 500rpm or so in drive.

So though I did not do anything to the car (and just drive it XC without these issues), Im getting these funny characteristics now. Any thoughts???

TchTchr · #11 11-24-2009, 10:47 AM

Have you adjusted the valves lately? Mine were way out of spec and it would lope at idle but was fine off idle. By fine, I mean it felt smooth. It belched smoke like a chimney though.

JHZR2 · #12 11-24-2009, 10:51 AM

good point and on my to do list...

JHZR2 · #13 11-24-2009, 10:26 PM

So after driving to work I reported that it did the rpm fluctuation when I started it, but then didn't do it when warm this time.

I started the car to drive home and it went right to ~500RPM. No fluctuation. Played with it cold, and then after I got home and it was hot. Nothing. Smooth idle, without fail, just still low at about 500RPM.

So tonight I just upped the idle speed a bit and adjusted the linkage so that everything was still correct. Now Im at about 550-575 and it is ever so slightly smoother and very nice.

Will report back, but it looks like the issue went away. Any thoughts as to why I would have seen this fluctuating idle? 50 RPM and after any drop of load, including going out of drive into neutral, and the AC kicking off?

Thanks!

#14 11-24-2009, 10:40 PM

I wouldn't worry unless it comes back. Maybe it was just that batch of fuel, or a little crud in the fuel system, of the rack sticking a little?

I keep my idle at about 550-600, that seems just about perfect.

toomany MBZ · #15 11-25-2009, 07:38 AM

Yeah, seems to be okay now.