I have replaced almost all vac hoses, but a couple are left. I want to replace the vacuum hose that goes from the top of the plenum to the vacuum reservoir under/next to the ignition coils:

A piece of regular vac hose goes from the top of the plenum to a green plastic tube that disappears under the manifold and emerges at the other end of the manifold as a regular vac hose. It attaches to the round vacuum reservoir.

Who can tell me why there is that green plastic tube, and how long is it? Did they use that plastic tube instead of regular vac hose, because of the high temperatures under the manifold?

What's the best way to remove and replace the hose? I assume it's just routed through under the manifold, or is it somehow attached? Should I just disconnect it and pull it out?

and if you replace it with standard silicone or rubber it's possible it can collapse (suck in on itself) under vacuum due to length and surround-heat. Silicone vac hose enjoys incredible longevity when subjected to heat without drying out, cracking but it is also much more pliable at room temp and even softer when hot. I think it would get hot, then lay on the valley pan floor and likely suffer wall collapse under vacuum. Rubber vac hose in a similar wall thickness to silicone is more rigid and stiff but wouldn't survive very long in the surround-oven between the valley pan and intake.

I don't know the silicone hose would collapse as fact but it's what I've conjectured in the past when looking at it and wondering about it like you are now.

Disconnect the vac hose on the plenum, then unbolt the vac. res. from the coil platform.

Pull the vac. res. out with the green plastic hose; there exist no attachments--it just runs the length of the manifold.

Replace the little piece to vac hose from the res to the tube, then slide it back under the manifold. Use a piece stiff wire, e.g., a hanger wire, to guide it into place. Depending on the exact shape of the tube, you may need to get creative. Once I had difficulty getting it to come upward to the plenum so I worked the hanger from above; another time I had difficulty getting it under the manifold so I attached the hanger to the tube with a piece of tape and worked it from below.

Reattach at plenum and bolt the vac. res. down, and you have reached the finish line.

green line completely with silicone tubing.

I thought the scary issue concerned whether it had another attachment under the manifold that one cannot see.

years (since '77) and especially when there is a constant vacuum environment - such as the vacuum reservoir, lines to the HVAC and power locks, all gated by a one way valve to the manifold. Oh yeah, Benz used them for years too. Must be a german thing :)

Don't worry, I wouldn't replace the tube with regular vac hose. I was pretty sure the material they used had to do with the high temps under the manifold.

After doing that, there will be only one original vac hose left (The one that goes in the dash and to the A/C).

the door locks, trunk and fuel vac hoses are a pain to replace too :)