Vacuum pump for cruise control?

[Huw Powell]

> Still trying to fix the ailing cruise control on my 87 5kcstq.  Tony 
> Hoffman suggested that my vacuum pump might be weak, because it won't 
> pull the throttle body linkage all the way open--  This sounded like a 
> very reasonable suggestion, so I tested it. (thanks Tony).  The linkage 
> will open all the way using a hand pump- it takes between 5 and 6 in Hg 
> to pull it all the way open (Is this right?).  The servo isn't leaking, 
> I let it sit with the linkage retracted for quite awhile and it held the 
> vacuum just fine.
> 
> I'm using a new, leak free vacuum hose for all these tests.
> 
> I went to a junkyard this afternoon and pulled two other vacuum pumps 
> from wrecked 5000's, one turbo and one nonturbo (part number is the same 
> it appears).  NEITHER of these pumps was able to do any better than the 
> one that was previously in the car.  Maybe I just got two other bad 
> ones, but I did the "run soapy water through the pump" trick and got the 
> same results.
> The sides of the servo "suck in" but the linkage doesn't move.  I don't 
> think it's a sticky valve-- if I compress the servo with my fingers, 
> then release it, the spring will pull the linkage closed even with the 
> vacuum pump running.
> 
> Do these little vacuum pumps fail with alarming regularity like that? 
> Were they not up to the job to begin with?  They don't suck, and yet at 
> the same time it appears that they do suck very much.

When I was rebuilding the cruise on  my coupe to eliminate all its 
little quirks, I dug about 7 pumps out of my spares box and tested them 
to pick one to use.  What I did is run each one on a little power 
supply, and time how long it took them to comletely "suck in" the servo.

Times ranged from 10 seconds or so up to two minutes, with at least one 
not even capable of doing it.  They were all different.  I used the 
fastest one, of course.

I was going to suggest you clean and lube the TB mechanism, but then I 
read above where you retract it all manually and the pump still can't 
keep it in place.

> Do I need to just bite the bullet and pay Rod at "thepartsconnection" 
> the $140.00 for a new pump?

Depends on how many junkyard ones you can try out.  If they let you get 
them cheap, bring them home to test them.  If not, go in with a little 
harness and a battery booster for power and time them like I did (or 
test them 'in situ' against their TB linkages).  If you can't find a 
"good" one then, I guess, yeah, a new one is called for.

> 1) Should a good pump be capable of easily retracting the linkage (I 
> take it the answer to that question is yes).

I've never tried that in place, but it would seem pretty obvious that it 
should, yes.  Whether it should be able to get beyond the first 
butterfly or not might not be an issue.

> 2) How much vacuum (inches of Hg, psi, whatever) should be required to 
> pull the linkage back?  I don't know if you even can adjust the spring 
> tension in the throttle valve.

I think you can only clean and lube them.

> 3) Is there any way I can make one of these pumps work by 
> replacing/refreshing some commonly worn out internal part, or should I 
> start looking for yet another pump, new or used?

I've never seen or heard of a kit to repair them, they don't seem to be 
"openable" - but you never know.  Why not bust one open and see what it 
looks like?

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/