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While replacing all the seals on my IP today, I also tore into my ALDA. It has been leaking badly. I could not pump up any pressure with my MityVac, it just leaked as fast as I could pump. The car ran great, but sometimes at part throttle there was a sudden power increase as the ALDA decided to seal and increase fuel delivery. So it had been low on my priority list.
I was afraid I would not be able to open it, as I have a spare bad ALDA that must have red LocTite on the screws or something, which I never could break loose. But my "good" leaky one opened up easily. It was nice and oily inside, from intake air pushing oil past the leaky seal. Once I cleaned it all up I finally understand - sort of - how it works! :) I had bought a new shaft seal from a list member and was hoping that would cure my problem. The old seal didn't appear any worse than the new but I swapped then anyway. Put it back together and tested.
And... It still leaked! Now what? Well, before I could hold my thumb on the bottom opening and it held pressure. Now it didn't. I suspected the large O-ring sealing the two halves together. There were remnants of some old sealer, so I figured Bosch may have intended sealer be applied as well. I cleaned it up more, then applied Yamabond (non-hardening sealer) sparingly with a toothpick, re-assembled, and voila! Working ALDA.
On a side note, now that I have seen the shaft seal, I understand the pressure readings better. I noticed that below 10psi or so, it would slowly leak. But above 10psi it would hold pressure. This happened on a couple other ALDAs I tested as well. It is due to the seal design. As pressure increases, the seal gets tighter, that's why it leaks very slightly at lower pressures. So to test, pump up with MityVac, and it should hold 10-15psi with nearly zero leakdown. Some leakdown below 10psi is probably normal. (?)
Also, my ALDA had been installed with TWO copper seals under it. I'm not sure if this was an error, or done on purpose to simulate a 1-turn CCW adjustment by raising the ALDA relative to the internal IP shaft, without cutting off the top seal. Fascinating idea. I removed the extra seal though. Wonder if that's why this car ran SO good even with the ALDA unplugged. Will have to run some more tests when I'm done.
Anyway, if your ALDA leaks, there may be hope to revive it without plunking down $300 for a new one. I took photos of it while apart
Dave Meimann
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