Odd vacuum pump activity

[Jcsws6]

I recently picked up a clean 90 which has been working near flawless. I noticed tonight when I moved it the vacuum pump didn't coming on. Previously, it was cycling for the first 5 seconds or so just like my 91. After verifying voltage and the pump, I found that if I unplug the airline it runs. If I've had it unplugged with the car off it will cycle for a second or so then turn off. It has plenty of built up vacuum when I pull it off (noticeable pop). Is it possible the one way valve has become clogged or could it just be holding vacuum?


I'm about to do the fuel pumps and so maybe this is a sign to just pull the plenum and do the injectors as well.

[Paul Workman]

Sounds good to me! As long as it runs when you open the vacuum line and then when you reconnect it, the pump shuts off after a few seconds...is how it is suppose to operate.

Ordinarily there is a little seepage of air around the various check valves and rubber connections so that the system looses vacuum after a while and the pump will run, periodically, to restore vacuum (with the motor OFF with the ignition switch in RUN). The less leakage of air, the less the pump will have to cycle. But, when the pump runs continuously (or more often cycling in less than every 10 or 15 seconds) then the system needs to be investigated for excessive leak(s).

[ram_g]

I have found my secondary vacuum pump cycling interval to be all over the map. At different times it has ranged from as low as 2.5 seconds to as much as 30 seconds. When I had the plenum off I tried to zip-tie various connections which made absolutely no difference. I did not do any further detailed diagnosis.

According to Haibeck everything will work as long as the cycling interval is at least 1 second and no codes are set when the secondaries are activated for at least a couple of seconds. I monitor this routinely and so far so good (and yes, my secondaries definitely kick in).