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Oil_burner 03-10-2000 11:13 AM

I have an '84 300SD, 110K mi.
The idle is a bit off, and consequently the thing runs rough. If I barely touch the gas pedal it is fine. Guessing by the almost lack of movement by the tach were talking 50 to 100rpm. My local repair shop said there is no "easy" idle adjust - it is in the injector pump. Am I being hosed, or is there a fairly easy means of adjusting the idle?

stevebfl 03-11-2000 12:28 AM

The idle is adjustable at the pump stop. The rythmic oscillation type of idle is often caused by TOO much idle. To test if this is so just lightly push the "STOP" link till you have reduced the idle to various speeds. Also the vehicle tach isn't suitable to establish proper speed. The oscillation type of lope also is to be dampened by the proper adjustment of the rack dampener on the back face of the inj pump. Twelve wrench bolt with 14mm jam nut. This is too be screwed til it just dampens the oscillation.

Most are screwed in too far which causes motors to start and die when cold.

Another problem that is fairly easy to fix is fuel line bleed back through the pump element check valve seals. This leak-down causes the quantity of delivery to be different from cylinder to cylinder and a rough idle is a can get worse. The seals are a flat copper washer. They can be changed on pumps that don't have external mixture correction on the elements.


[This message has been edited by Webmaster (edited 03-16-2000).]

stevebfl 03-16-2000 10:02 AM


Look to the current threads about rack dampeners for your cold start/run problem first.

The dampener works nicely when new but many a tech goes overboard on the adjustment. The intent is to just touch the rack with the tip of the spring loaded screw. If the screw applies too much pressure the car will die on cold starting or under the influence of the P/S right after start up.

I don't think you leak at the tank will cause this problem. The low pressure fuel in the pump cavity (which is necessary) is checked by the feed pump from backing down the system. Besides if there is air in the pump cavity you probably would find it hard to start and pick up cylinders slowly until it ran fine.

Thanks for the complement.


Alec 03-16-2000 02:15 PM

I have an 85 300TDT with a similar idle problem during startup in the morning. It ildes rough but when I depress the accelerator the engine smooths out. Could a simple solution be air in the line (I have noticed bubbles in the fuel filter)? Fuel pump? Injectors?

Thanks for your time.

stevebfl 03-17-2000 10:50 AM


From your description it sounds like the dampener screw is too tight.

The screw runs horizontally into the back plate of the injction pump. Its head is 12mm wrench size and the jam screw is 14mm. If they look like they are made together (no threads shown) then you should replace the screw as the spring probably is too loose. Just backing it waaaaaay out will check to see if it is the reason for your sensitive cold idle.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Oil_burner 03-20-2000 10:10 AM


You hit it on the head. My original post spoke of the idle being off - it sure seemed that way. It appears that it was the dampener screw. It was adjusted almost all the way out. When I tightend it to most of the way in, the problem vanished. After two days of fairly cold starts (no problem), and a couple of smooth trips around town; I am ready to declare success!

Thank you,

Dale Mitchell
84 300SD

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