low rev troubleshooting

[philmas]

Hi!
I thought I could ask all the tuning specialists on this forum about something I (and obviously none of the mechanics around here) never understood.

When engine is warm, my car's performance at low rpms (from idle to 1500rpm) is poor, bucking in 1st and 2nd gears, particularly when I have to let the throttle off, and then on again.Above 2000rpm, everything's fine.
That makes progression in slow traffic really difficult as I have to play both with throttle and clutch as to keep the revs over 1500rpm!
Idle is just correct at around 800rpm, somewhat wobbling between 750 and 900, but I assume this is subnormal.No stalling or starting issue.

When engine is cold, running is  smooth (idle around 1000rpm) until the cold start system shuts off (i.e. a few minutes after start).

Performance at high revs is perfect.The issue only occurs under load.
Nothing else to be mentioned: spark plugs are new, points almost new; fuel consumption within
average.Linkage looks fine.

I only notice some slight "backfire" (sounds like muffled explosions) in the exhaust  when decelerating in traffic.

I read   a similar issue has  been described by a forumer in the technical manual (old yahoo content), and I must admit his description is very close to mine...
but there was unfortunately no definite conclusion to this, as he said this did not affect automatic cars, and was probably related to (normal !?) hesitation of the IP at low range.


What are your thoughts?

- lean mixture at low range?
- ignition malfunction ( distributor wear...)
- other...?

Thanks for help!
 
 

[jacovdw]

Philippe,

Before delving into the fuel injection pump and mixture, I would verify that the following at a bare minimum is checked.

...Linkage looks fine....

Even if the linkage looks fine, you still need to verify it according to the adjustment procedure in the Tech Manual.

- lean mixture at low range?
- ignition malfunction ( distributor wear...)

It is possible to have a lean condition in the lower ranges, but again that refers back to checking the linkage as above.

With regards to the distributor, grime buildup can cause the centrifugal weights to be sluggish and also impair movement of the base plate.
Dwell angle within the specified range?
Free play (up/down and sideways movement) of the rotor shaft indicating wear?
If you twist the rotor, does it return to it's original position spontaneously?

These are probably the most important things to check first, before you try to adjust the fuel mixture.

Hope this helps.


EDIT: My apologies to you Philippe, I misspelled your name initially.

[philmas]

Thank you Jacovdw.
I 'll first check the linkage as you suggest to see if everything's within specs.I'm not prepared anyhow to put my hands into the FIP,  that was just a first attempt to sort things out and know what to begin with, ignition or fuel mixture.

[Naj ✝︎]

Hi, Phil,

Also check that the wrd if fully shutting off the air supply from the small air filter on the FIP when the engine is fully warmed up.

Naj

[philmas]

Yes Naj, I was thinking today this might be the cause of a  lean mixture.I'll check this too.
Thanks a lot!

Hope everything's fine with you and your car!

[philmas]

Naj,
just not sure I understood the bottomline of this: if the WRD is stuck open, shouldn't this mean I could not get a correct idle?
Idle is near perfect when engine is warm, cold and warm start too (but I must hold full throttle when warm).It's just that performance is poor between idle and 1500 rpm when warm, as if mixture was too lean only in this range.
There is consistent backfiring at the same time.

[ja17]

Hello Phillippe,

Linkage adjustment is very important as stated.

It does sound like a lean mixture situation at low rpms. Make sure that the main fuel filter is clean. If it is clean, assuming all tune-up specs are good, you may try richening the idle thumb screw on the IP four or five clicks. This will richen the low speed range and idle a bit.

[Benz Dr.]

If the WRD was stuck it would either be too lean when cold or too rich when hot, depending on which position it was stuck in. So I don't think that's your problem.

 If it doesn't misfire and seems otherwise to be running OK, I'd lean a bit more towards ignition timing. Good chance your vacuum cell has a leak and you have no vaccum advance. It works first, and the mechanical works with it, but it takes up to 3,000 RPM before it's fully advanced. The vacuum will be fully advanced on a 051 by 1,200 RPM - it's very quick and if you don't have this working, the engine will fall flat in the lower RPM ranges.

[philmas]

My car has been eventually serviced by MB last week.
The shifter bushings needed  replacing, and there was a nasty short somewhere in the # 1 fuse circuit.
I asked them if they could do something to solve my low rev issue.
Following ignition and mixture fine tuning, I must say things have improved...but there is still a slight bucking between 1000 and 1500 rpm, only when engine is warm.Idle is more steady and down to 600rpm, acceleration's smoother.
Linkage has been checked , as well as ignition timing.

Wait and see...

[stickandrudderman]

OMG! You took it to a main dealer? The chances of any spannerists with sufficient knowledge to know how to set up a 113 are almost nothing, certainly here in the UK anyway.

[philmas]

We are so lucky as to have now an MB technical center here near Paris, which can take care of our  beloved cars

[ctaylor738]

We will be in Draguignan beginning Bastille Day.  Bring it down and let me drive it for a couple of weeks and I will give you my diagnosis>   :-)

[philmas]

It would be a pleasure, if you ever happen to come to Paris...I still have to wait until august for holidays