Good morning everyone. I seem to have an intermittent vacuum leak on my 2003 f250.

Some history: A couple of years ago I developed a vacuum leak and the air would only blow out defrost. I took it to the shop and they "plugged the leak" but the service writer didn't know what was done. The AC worked fine since then. My ESOF had not worked for some time at that point

Two weeks ago I went through the process of replacing ball joints, knuckle seals, and O rings. ESOF still didn't work so I went looking and found the 2 hose plug at the bottom of the 4x4 solenoid unplugged with a bolt in it; I'm assuming that is what the shop plugged. Plugged it back in and ESOF worked again.

The past 2 days when starting the truck the vents automatically go to defrost and then after several seconds will go back to vent. It has only seemed to do it with the AC thermostat in full cold or on max AC. Checked the vacuum pump and it ran continuously with key on. Pulled the line to the 4x4 solenoid and plugged the end with my thumb and the pump still ran. The hoses to the knuckle were replaced a few years ago and seem in ok shape. The 4x4 solenoid was replaced a few years ago when i was trouble shooting ESOF issues.

Any idea where to start looking for this leak?

 

How long did you let it run with the line plugged? if it was all the way down it takes a bit to build the vacuum back up enough for the pump to cut off.

A mighty vac pump can be a handy tool to chase down the leak. Don't know anything about this vendor just grabbed it off the interweb for show and tell. Yes it's a fluid pump but I've also used it to put vacuum on a system and see if it's bleeding down.

https://www.tooldiscounter.com/produ...f743b1bac0edf2

 

I only had my thumb over the line for 30 - 45 seconds. I can plug it completely later this evening and see how long the pump runs. If I plug that hose again and the pump stops running continuously, could that indicate the solenoid is bad?

I have a hand held vacuum pump but so far I have only used it on the back of the knuckles. I couldn't get it to pull enough vacuum to lock the hubs with it but the ESOF was functioning after getting it all back together. At least it was last week.

 

Not sure if it would indicate the solenoid bad or a leak on the ESOF side. I can't remember if the ESOF operates on a pulse or continuous vacuum. I do know if you have a big leak on the ESOF side your AC will go to defrost when you switch into 4x4.

Hopefully someone more knowledgeable than me will jump in shortly.

 

I can say that it has not gone to defrost when switching to 4x4 and only seems to go to defrost (so far) on initial start up then will eventually go to vents.

 

With that said you may have a slow leak in the cab, either a line or motor. I think I saved a schematic of the system. will try to find it.


Edit: found it. Again if you have the ability to put vacuum on parts of the system and watch for bleed down it will help.

 

Thanks for the replies. Just to clarify, this had not happened for the past couple of years until I had the vacuum line hooked back up to the esof solenoid and all of the seals replaced.

 

I had a few minutes before lunch and went out to do some trouble shooting. I unplugged the line to the 4x4 solenoid and plugged it with a bolt. Vacuum pump shut off within about 1 minute and 20 seconds. Pulled the bolt and plugged the line back into the solenoid and the pump came on again and ran continuously for about 3 minutes before I shut it down. the air went from defrost to vents when I turned the key, then back to defrost when I pulled the bolt, and then back to vents after a few seconds. Switched to 4x4 a few times and it stayed on vents.

Edit: The vacuum solenoid is described as pulse vacuum.

 

Thank you for commenting on this thread and providing the diagram. For an update, I went out and got a dorman 4x4 solenoid from oreilly during lunch and swapped them out. The vacuum pump shut off after a short time and seemed to function normally. I let it sit for about 20 minutes and checked again and the ac stayed on vents so it did not appear to leak down. ESOF functions.

 

One more update: I let the truck sit for a little over an hour. Went back out, key on and the ac came on through vents, then went to defrost, and then slowly came back to vents. I'll try to check with the vacuum gauge tonight.