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Old 05-26-2009, 12:56 AM
scottmcphee's Avatar
1987 w124 300D
Join Date: Mar 2008
Location: Edmonton, Canada
Posts: 1,518
Reason why IP o-ring leaks after timing change.

Common belief is that the front o-ring is old and cracks or whatever when rotating the IP to adjust injection timing.

Not so. At least on my OM 603.96 with probably original o-ring.

The real reason, I found out, is that for all the internet helps and threads about adjusting timing of the pump, they only mention 3 bolts on the front of the pump. Slack 'em off and use the adjuster nut to move the pump.

FAILED to mention or notice there is also one bolt holding rear of the pump to the block. Sure, you can crank IP over by turning the adjuster or 'helper' nut with just the front 3 bolts slacked... but if you don't slack the rear bolt too, you *will* twist the IP in such a way that it backs out a little on one side where its front gear meets the block - yes at that o-ring.

I bought all the gaskets and rings for the IP thinking I cracked my o-ring. But when I was pulling the parts off the car to do the job.. what's this I see a rear bolt on the IP?!? With the front 3 already slacked a bit, I undo this bolt and the IP breathes a sign of relief and I watch it rotate into a natural position that allowed it meet the block at the front.

Oh, well... gone this far so I pulled it anyway and did the gaskets. But really the front o-ring was in pretty good shape. A little flattened as can be expected but no age cracking or undo hardness... It would have sealed fine if I rotated the pump with rear unbolted too. It has a slider grove that allows rotation and retightening.

SHORT STORY: slack the rear bolt too... when you adjust timing. And chances are you won't create an oil leaker like so many people insist is an o-ring job for even *attempting* to adjust this timing.

Although the adjuster nut has enough power to push the IP around with all the hard lines attached, I recommend you crack all lines at the injectors, slack that nut right off, and take out all the bolts on the line holders connecting lines to the intake manifold, then do your twisting. Then rebend the hard lines a bit back onto their injectors, then put the holder bolts back into the intake.
Scott McPhee

1987 300D

Last edited by scottmcphee; 05-26-2009 at 01:05 AM.
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Old 05-26-2009, 01:16 AM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,028
That's a good reminder but I've had two leaks with the rear bolt loose.

87 300D
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Old 05-26-2009, 04:42 PM
Diesel911's Avatar
Registered User
Join Date: Sep 2007
Location: Long Beach,CA
Posts: 22,726
O-rings do not have to crack to leak. Overtime the conform to the area they are sealing and kind of remain in that shape due to the rubber getting stiffer with age. Moving them to another place means they may not be elastic enough to conform to the shape of the new area.
It also simply could be that dirt or other stuff gets between the sealing areas when you loosen the IP.
84 300D, 82 Volvo 244Gl Diesel
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