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Old 01-12-2009, 07:23 PM
Richard Howard
Join Date: Nov 2008
Location: Western NC Mountains
Posts: 69
Ip rack return spring r&r

Many of us are familiar with the spring down behind the injection pump on our 4 and 5 cylinder engines that moves the rack back to the idle position. The spring's location is narrow and confined so it's difficult to check the spring ends for wear and the possibility of later breakage.

Not wanting to have a failure that might result in a wide open throttle condition and probable engine destruction, I decided to replace the spring on my 1982 300SD's OM617. I'm glad I did because one of the spring's hook ends had a spot worn almost halfway through the spring wire.

To reach and remove the spring for replacement, here's what I did:

1) I made two spring hooks from a 36" piece of steel 1/8" diameter welding rod, heating the rod's midpoint with a propane torch until it softened enough to stretch the rod to a slightly thinner diameter. You could use a thinner rod but it will be more likely to bend during use.

2) Clamp one end of the rod in a vise and clamp a locking pliers on the other end to help stretch the rod when it softens. Cut the rod into two equal lengths with wire cutters at the center of the stretched area. The stretched ends are then reheated and bent into hooks each having about a 120 degree turn. The other ends are also reheated and formed into a flat spoon shape that will later have a hole drilled through the flattened area. Bring the ends you have produced to a final desired shape with a flat file.

3) Drill about a 3/32" hole through the flattened ends of both rods. One of these holes will be used to support a new spring as it's lowered into the space between the IP and the engine block. The drilled hole on the other rod should be located near the end of the flattened area and then opened into a slot by cutting with a thin hacksaw blade, a three-sided file, or wire cutters. This slot is used when guiding the bottom end of a new spring into place.

4) With a little time, one end of a new spring can be put into the hole of the fixed spring bracket down near the joined surfaces of the oil pan and engine block. Use the slotted end on one of the tools made to guide the new spring's bottom hook into place, while holding the spring's top hook with the other tool having the drilled hole.

5) Next hook the upper end of the new spring into its hole on the movable rack link, pulling the spring upwards with one tool's hook placed through the spring's upper coils. Push on the new spring's coiled area as necessary with the other tool to tilt the top end of the spring so its hook will enter the hole on the movable link.

6) I decided to use two springs, one inside the other, like those often used on carbureted engines, for some additional insurance against damage caused by later breakage of only one spring.

The original spring on my IP measured 2 1/4" long overall, with coil length of 1 1/4", and coil diameter of 1/2". Take the old spring along to the auto parts store to help estimate what amount of tension you want in the new spring. Be sure to select a spring or springs that will allow you to approximate the original spring's hook-to-hook length, so ample closing force can be applied to the rack linkage.

To be able to see down into the narrow space where the springs does its work, a small work light is a necessity. I used the one I made some time ago using a taillight bulb soldered to some hookup wire and powered by a battery charger.

Respect, protect, and maintain complex systems - whether natural or of German design, to benefit from their full potential.

1982 300SD W126.120 with an OM617.950 from a W116.120
1984 300SD W126.120 with an OM617.951
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Old 01-12-2009, 08:37 PM
Registered User
Join Date: Jul 2004
Posts: 7,334
No pictures?

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