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Discussion Starter #1 (Edited)
After stripping the interior of my 89 Veloce to replace carpet padding, I noticed the vacuum sensor, located behind the passenger rear side panel, was disconnected from its manifold hose, which had been capped off and was left to dangle on its own. I reconnected it and sure enough, that telltale whistle greeted me when I started the car. Looking on this forum I found a couple postings from folks who dove in and tried to fix these no-longer-available items. As buying a used one is risky, I gave repair as shot as well.

Not as easy as I was lead to believe, but doable for sure. On a bench grinder I removed the folded-over bead, and the sensor body cracked open. The bolt holding the bellows stem in place would not budge, and the only way to get a 10mm socket onto the bolt was to tear off the cup-shaped metal air vent, and the plastic three-holed 'filter'. Ugh - what do I do now? Turns out a 1/2" copper water pipe end cap is a perfect replacement. I didn't isolate the copper from the steel sensor body since the mass of the steel is far greater than the copper cap, but if you try the same and are concerned about corrosive electrolysis, then a layer of electrical tape between the two should take care of that. Glued a plastic cap on with a few drilled holes to mimic the original.

I found the crack in the bellows, but learned the hard way that soldering the bellows using a propane torch is a big mistake. The bellows fell apart into three pieces - two discs and a copper ring separating the discs. Apparently these bellow segments are glued together with some sort of rubber sealant, which disintegrated within seconds of the torch coming in contact with the bellows. An easy fix was to clean the bellows and copper connector of charred rubber and reconnect using epoxy. Also epoxied the bellows crack, and the sensor body, on both the inside groove and the outside seam.

The sensor once again holds a vacuum, and doesn't whistle anymore, but I can't verify yet if the unit functions properly until I'm back on the road in the spring. I always wondered why my fuel consumption was so bad with this car. I'm hoping this fix, if it works, will help with that.
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Good fr you on your repair, What kind of fuel economy do you currently experience?

If memory serves, my efforts a few years ago with a MAP sensor and electronics (designed by a controls engineer to my spec) to emulate the VSD’s variable inductance resulted in 28/32 mpg over two, same day, mostly flat, constant 60mph, 100 mile round trips, with few stops.

The slight amount of spark advance was obviously beneficial at road speeds but the experiment didn’t pencil out. System remains in my QV, making me the proud owner of the only Alfa so equipped on this side of the Galaxy.
 

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Discussion Starter #4
Good fr you on your repair, What kind of fuel economy do you currently experience?
My sense of fuel economy is strictly anecdotal - the car is new to me and as I spent last summer tweaking and fixing its various issues didn't figure exact mileage. However, my sense is that I was filling the tank a lot more often than I thought was normal for a fuel injected 2L engine. Intuition says I'm getting about 20 mpg, similar to a 4wd 4 cylinder Toyota truck I once had. The only system amiss, as far as I know, is the vacuum sensor, and having read about its effect on fuel economy, thought it could be the reason for what I think is poor fuel economy.

I too wonder about the state of the variable inductance part of this sensor, having torn the sensor all apart and reassembled. It is a fragile little unit, and that part of it did get abused during the reconstruction. I've read that they can need 'readjusting' after repair, but I'm not sure what that actually means. They are fixed in a certain position, so I don't know what can be adjusted.
Actually, now that I think of it, that 10mm nut on the bellows stem would allow the induction element to move up or down, so I'll have to consider that if fuel economy doesn't improve.
 

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Discussion Starter #6
When I have measured fuel economy in our '84 Spider (with functioning vacuum sensor), I get high 20's MPG in around town driving and low 30's MPG on extended highway trips.
That sounds like proper mpgs. This spring I'm going to do the calculations.
 
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