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Old 04-06-2010, 05:21 PM
JamesDean's Avatar
Electrical Engineer
Join Date: Apr 2005
Location: NE Ohio
Posts: 5,009
Custom Idle Adjust Screw OM617

For any of you who have managed to break the idle adjustment screw mount on the side of your injection pump this is for you:

My "repair" for the mount can be found here...quite a while ago actually:
help! anybody

I grew tired of the idle always being so low (just a hair above 500) and not being able to do anything about it. The engine would turn on/off poorly. I'd rather it be around 750 as it should be. So I spoke with a machine shop locally and they pointed me to this: Part Number: 98861A324. Cost $17
M10x1.00 Rod Length 1 meter.

I plan on using this, in place of the factory adjust screw to compensate for the height at which my "fix" is currently mounted.

The downside of this is that I will loose the spring loaded STOP feature. The cons of this have been pointed out here:

617 Idle Adjust Screw Replacement

Originally Posted by OM616
The spring allows you to move the rack to the stop position via the external throttle. This is how the manual red "STOP" lever on the engine works.

If you put a solid bolt in place of the plunger the red STOP lever would not work. Then, should the engine run away, the only way you could stop the engine would be the ignition switch. As an added bonus with a run away condition and no manual stop available, if you vacuum system is not in the best shape, the stop actuator may not have the ability to pull the rack back and you will need to get creative to get it stopped, like squeeze the life out of the fuel lines.
However the vacuum system in this car is in very good shape. The ignition tumbler has been replaced as has the modulator that sits on top the IP. So, I am not too terribly worried about having the STOP lever functional. I've only used it a few times and that was because I didn't feel like going in the car and turning off the key. I'd much rather have the motor idling at proper speed all the time than worry about a condition that might happen (I've been working on the car for 5+ years now and haven't had it happen yet.)

I plan on cutting the rod to length with an angle grinder (cutting wheel) and then taking the cutting wheel perpendicular to the rod and cutting a notch for a flat head screw driver. The important thing here is not to make the rod too long as I still need to be able to get a deepwell socket over the locking nut.

Thus far in in-garage testing its working fine. I got the idle to sit at a nice ~750 rpm. Engine start/stops much better than before. Its not drivable yet, I think you'll see why:

Additionally, someone mentioned using the knob style adjust system from an older 617. I actually forgot about this until I was looking at old posts while writing this one....I wasn't really sure how it works and if this would provide with sufficient length. I would imagine that MB kept the same range of adjustment with both implementations...

I hope this helps someone out there who made the same mistake I did a few years ago.

Any comments/suggestions/critiques are welcome.

Cruise Control not working? Send me PM or email ( I might be able to help out.
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