Hi folks,

I don't plan on doing the timing chain work, but I may attempt to check the stretch myself. Also, I have not looked at the R&R work for the vac pump, but I don't imagine it would be difficult. That might be something I'd do.

My question is should both be done at the same time or can one be done and then the other done later?

Bob

it is 100 times easier and safer to roll in a new chain with the vacuum pump removed

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Why?

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You can see you've got it seated on the IP's cog easily I suppose.

-J

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I have the same question............why?

the force from the vacuum pump on the timing gear pulls the chain down into the recess causing many of the well-known inaccuracies.

if this, and compression are removed, the timing chain is under complete control by the movement at the crank only (and the chain's own weight, of course..).

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the chain cannot come off the IP gear unless the entire set of safety pins is removed. I believe there are two and one bolt on the side of the casting facing the fender.

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Is it the vacuum pump...........or just the compression of the engine?

I agree that the elimination of the "forward jump" is desirable.

it is certainly both*.. but the vac pump is not mentioned in the manual. So anyone following the manual (as a novice) will not be aware of this force.

Manual says to remove the glowplugs and tensioner.

If you happen to set your engine in such a way that one of the two lobes on the timing gear are near the max compression of the VP bellows, you will find a considerable opposing force.

People doing this for the first time will not imagine that pulling up with force is a good idea to remove slack thus created.


*this is not to say that the crank will jump forward at the speeds involved

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Where is this opposing force coming from? Is it from the vacuum in the system? If so, the system could simply be opened and the lack of any vacuum would kill that deal in a hurry. Why bother to remove the pump?

you would have to APPLY vacuum to the pump in order to remove the force of the spring on the IP gear lobes.

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Well, in that case, you'd definitely need to remove the pump.

I'm curious as to whether the pump can push back on the chain, and drive the engine backward during the process and thereby cause an issue with the feed of the new chain.............the act of compressing the spring will simply require more force at the crankshaft, but won't create any catastrophe if the engine doesn't move in reverse.

I've not encountered the desire of the engine to move in reverse when it's slowly advanced to set the valves. The speed of rotation might be the issue here.

My experience (which is "once") is that the pump makes the crank that much more difficult to turn, at least when the lobes of the timing gear are pushing on the spring of the VP. The process of threading the chain is already so precarious that an extra amount of opposing force creates conditions where error is more likely - when the opposing force releases is a moment where the ill-prepared could make mistakes. Forward rotation of the chain/sprockets can cause the chain to jump teeth as well - especially if the MB chain retainer tool is not used as is the case with most of us. Many variables have to be taken into account and prepared for so that the chain can be replaced with maximum accuracy. For me, removal of the vacuum pump is insignificant on this scale of importance.

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The injection pump, compression/expansion and the camshaft has far more to do with that than the vacuum pump.

I am not advocating the removal of the VP for chain renewal procedures. Just to fulfill the OP's Question.

I doubt the VP can pull the crank backwards, but it will definitely put tension on the chain along that length (from crank to timing gear)

But I see what you're getting at with the speed of rotation.. compression must be removed to avoid this.

If all precautions are met, and the installer makes no error in turning the engine and laying the new chain on the cam sprocket, this force is negligible.

..it is when a small mistake is made in laying the new chain and removing the chosen clamps from the sprocket that this force becomes an obstacle. The installer pulls on the chain until it is taught, not until he overcomes said force and removes ALL slack on the IP side of the engine.

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