Idle problem - PeachParts Mercedes-Benz Forum

martureo · #1 09-17-2013, 01:42 PM

617.952 stock, sealed ALDA, unknown miles ~290,000 estimated.

I'm not sure when this started as I had a rather high idle (~900) for awhile. I adjusted it back down to 780 (where it was steady) and just barely adjusted the rack damper.

Now it has an odd idle when in gear, but not all the time. It feels like one cylinder is either not firing or isn't firing hard enough. I can't quite describe it, but it almost feels like a car which only had four valves adjusted properly. (No... I've done three valve adjustments in the last month just to make sure)

It doesn't always do it, coolant and ambient temperatures have no effect. I've done a fuel filter change and will run it off of diesel purge in the coming week.

no idea of injector condition (never changed or removed)
smokes a bit, nothing too much (grey smoke)
consistently runs between 26-28mpg
starts right up every time without cranking (even when freezing outside)
less than a quart of oil added every 5,000 miles

This is my daily driver, so I can't have any diagnostics that would have it apart for more than a day.

I can shift it into neutral and bump the revs up just a smidgen to even it out. But other times the idle is smooth as silk (while in gear).

Anybody got any clue?

I'll try to get it to do it again and get video up soon.

cooljjay · #2 09-17-2013, 02:37 PM

Have the injectors rebuilt....no idea about them tells me they are original....

Do you still have the old style primer pump?

Ever remove the fuel tank and clean the screen plus tank?

Ever check the injection pump timing? Timing chain stretch?

Stretch · #3 09-17-2013, 02:43 PM

Quote:

Originally Posted by martureo

...

starts right up every time without cranking (even when freezing outside)

...

So it sometimes shakes like the motor mounts are a bit knackered then?

(I must say you have a special engine there if it starts with out cranking - hee hee - I just couldn't let that one go! I know exactly what you mean though)

martureo · #4 09-17-2013, 03:15 PM

Quote:

Originally Posted by cooljjay

Have the injectors rebuilt....no idea about them tells me they are original....

Do you still have the old style primer pump?

Ever remove the fuel tank and clean the screen plus tank?

Ever check the injection pump timing? Timing chain stretch?

Updated primer pump.

Tank has been cleaned and had a new filter a few months ago.

Timing is dead on. Chain has 4° stretch.

Quote:

Originally Posted by Stretch

So it sometimes shakes like the motor mounts are a bit knackered then?

Yeah, it shakes enough to rock the car at idle. But it's not a continuous shake like the mounts are gone. It's like someone is standing next to my car and pushing in time with a single cylinder.

Quote:

(I must say you have a special engine there if it starts with out cranking - hee hee - I just couldn't let that one go! I know exactly what you mean though)

Stretch · #5 09-17-2013, 04:15 PM

So we're talking automatic transmission - in drive shakes - sometimes. Does the transmission show any quirkiness if you drive it from a shaky start?

martureo · #6 09-17-2013, 05:06 PM

Quote:

Originally Posted by Stretch

So we're talking automatic transmission - in drive shakes - sometimes. Does the transmission show any quirkiness if you drive it from a shaky start?

Yep, automatic trans.

Nope. I had the transmission completely redone, new clutch packs, everything, a few years back. Shifts like a dream.

The thing is that this idle problem doesn't effect anything else. I'm not down on power, I get off the line just like before, etc. It's just really really irritating to sit at a light bouncing around.

cooljjay · #7 09-17-2013, 07:52 PM

Replace the injectors would be my next move......including a valve adjustment....

I would also replace the motor mounts and trans mount....

Zacharias · #8 09-17-2013, 10:33 PM

Quote:

Originally Posted by martureo

Yeah, it shakes enough to rock the car at idle. But it's not a continuous shake like the mounts are gone. It's like someone is standing next to my car and pushing in time with a single cylinder.

Online diagnosis of idle problems can be a challenge.

This SOUNDS like what is sometimes referred to as "lope". It's the rythmic rocking that results from end float in the cam inside the injection pumps on the turbodiesels -- which is what the rack damper bolt is adjusted, to remove. It is quite distinct from a miss (though it can get bad enough to almost mimic one). Usually accompanied by the car rocking from side to side.

Do you have the original, silver colour bolt or the updated gold rack damper bolt?

If you have the silver one from the factory, you should start by upgrading to the gold one. These are longer, the original silver ones were slightly short and can't adjust out lope on the cars as they age.

Not all of the turbodiesels will idle at the same speed, not happily anyway. Idle adjustment is a dance between setting up the idle speed and adjusting the bolt. Even with the new rack damper bolt, it can be a long process. I have owned five turbodiesels and they had about three 'ideal' idle speeds, between them. When the car gets to the mileage of yours, component wear in the pump and the pump governor come into play. Trying to get the idle to a factory spec isn't necessarily going to happen.

Stretch · #9 09-17-2013, 11:34 PM

Well how about engine mounts and engine shocks then?

Diesel911 · #10 09-18-2013, 12:39 AM

Things that effect the idle.
Valve adjustment
Injector Nozzles worn of plugged with Carbon
Motor Mounts
Transmission Mount
Air Leaks in inlet Hoses or crack in the Plastic Filter
Air leaking from Old style Hand Primer that has the shiny Aluminum Body
Fuel Pressure Relief/Overflow Valve not performing properly
Rack Dampener Bolt needs adjusting or Updated Gold Colored one
Idle speed set within the limits that are on your emission Sticker.
The Engine Shock Mounts being rotted out
The Fuel Supply/Lift Pump needs rebuilding

Zacharias · #11 09-18-2013, 12:42 AM

My understanding is that the engine shocks are mainly there to dampen movement on startup and shutdown. They weren't even there on three of the 300sd cars I owned when I got them. None of those cars had lopey idle issues, once they were set up, though two needed the upgraded damper bolt to get there.

If motor mounts were causing this problem I would have expected to hear the OP say something about vibration as well. The particular way he describes it makes it sound like the 'lope' issue, particularly the fact that it is there in gear but not in neutral. That particular aspect can also be linked to the governor being worn, as the car shouldn't drop in idle speed when put into gear, if tghe governor is working properly.

For sure injectors can cause idle issues but I have always seen that occur when the car is either stone cold or more likely when hot, especially after highway runs. The OP says coolant and ambient temps have no bearing on his car's issue. And a bad injector causes a rough idle or a distinct miss, not a lope.

Final note: the idle should be set when the motor and injection pump are nice and hot, preferably within 30 minutes of a 10-20 minute highway run.