About 3 weeks ago I bought a new to me 2000 F350 crew cab with a long bed. So far I love the truck and have made a few changes to it. I bought a billet thermostat housing because the stock one was corroded and began leaking. Replaced the t-stat with a 203 degree t-stat. New MBPR 4" turbo back exhaust and did the Gooch's coolant flush with fleetrite ELC. Now that I got some of the important stuff out of the way I wanted to tackle some less important things. I noticed the vents would occasionally only blow out of the defrost no matter what the switch was set to. After much research on this site and similar sites I decided to trouble shoot it. I turned the key to the on position and the vacuum pump came on which I figured is a good start. After maybe 20 sec the pump shut off. I listened for any leaks but couldn't hear anything. I followed the lines but didn't see anything that looked too bad. Then I popped the glove box out of the way and looked for loose vacuum tubes. everything looked fine so I turned on the fan and sure enough the air was coming out of the correct vents. I went through the different vents a couple of times and it worked perfectly. I know I didnt fix the problem yet and I only happened to catch it on a good day. after a couple of cycles of changing the vents I did notice the vacuum pump come back on. My question (I know its been a long time coming) is, does the vacuum cycle on and off to maintain the pressure because changing the vents will use the vacuum and release it or is that a closed system and once it is under vacuum it should stay like that? I am guessing everytime you take it up to the defrost it has no vacuum and would lose the pressure. I havent tried running the truck and doing the same procedure yet to test the A/C That will be next. If it is a leak in the hoses (normally by the hubs) would it be best to start furthest away and test the hubs with a handheld vacuum pump to see if they engage and then work my way back to the reservoir?

 

Sounds like everything is working the way it should. If you engage the 4x4 ESOF switch if you have a leak, then default is defrost only.

I have never heard my pump except win I first start and wait on my glow plug led to go out. The pump does prime for a few seconds then shuts off.

It would make sense to cycle and repump.

Maybe someone else knows more. But if 4x4 works and blows out vent the it should be working as far as I no.

Later

 

Cap off the line to the PVH solenoid and see if the vacuum-related issues subside. That solenoid is a common vacuum leaker.

 

when diagnosing my system I found that the pump does cycle during regular operation. changing the vents causes the vacumm to be "used" and the pump has to work for it. 4wd also uses it and I found it to be a "silent leaker". best way I found to diagnose is to use a manual hand vacumm pump and check the lines that way. it was very helpful to find which line was leaking. it lead me right down to the wheel without having to deal with all the other lines. just start at the pump and check each line thereafter until you find the culprit.

good luck