Missing when coming back to idle - PeachParts Mercedes-Benz Forum

bbboomer · #1 11-08-2009, 09:36 PM

My 300SD has developed a miss when coming back to idle.
It seems to run fine except for the idle miss. The miss is very brief then it returns to a smooth idle but while it is missing it almost dies.
It kind of acts like air in the fuel but I can't find any leak.
I have checked everything I can think of but found nothing yet.
Anyone have a similar problem or any ideas?

Diesel911 · #2 11-08-2009, 09:55 PM

How did you check for the Air Leak?
At Sea Level Atmoshperic Pressure is 14.7 psi. This is enough pressure to keep Fuel inside of slighly cracke Fuel Hoses; cracks that can allow Air in; and even more so when the Engine is running and there is some Vacuum inside of the Fuel Hoses and 14.7 psi out side of the hoses.

Check the hoses under the Fuel Tank and see if they are seeping or cracked.

Also is your idle speed within spec?

You can replace the 2 pieces of Fuel Hose that hook to the Fuel Supply Pump and Plastic Filter for a few dollars with 5/16 SAE 30R7 Fuel Hose that is sold in most any Auto Parts store. Most ofte it is GoodYear but sometimes they sell Gates Hose; it is rated for Diesel Fuel.

If the other stuff does not seem to be the problem and you have a Rack Damper screw on your Fuel Injection Pump maybe fooling with the adjustment would help.

bbboomer · #3 11-08-2009, 11:51 PM

The two hoses at the tank are the only ones I haven't changed and they look fine.
The idle speed is fine, the valves are adjusted, the chain stretch is fine and the lift pump has a new primer pump.
I am beginning to think it is either the new primer or maybe the lift pump itself.

bbboomer · #4 11-17-2009, 06:18 PM

Okay,
The rack damper screw is the old style and screwed nearly all the way in.
I turned it in another 1/4 turn and magic! the miss is gone.
I can't find the rack damper screw on Fastlane, does anyone know if they are available?

MBeige · #5 11-17-2009, 09:17 PM

The rack dampener is also called governor idler pin. I also adjusted mine because mine did a miss and actually died on early morning starts. I must have adjusted it too deep, so it damped the idle too much. Now it's fine and dandy with a good strong start (before, it sputtered to start) just like how it was prior to the miss incident.

Phil should be able to get the part for you. But since you've already found the "sweet spot" I would just keep the new one as a spare.