So my 95 F150 I6 developed this annoying condition where the Air condition will sort of cut out and divert from the top vents to the under dash heat ducts. My early research tells me this is a vacuum leak some where and if there is a leak the airflow is directed to the under dash heat ducts by default. Anyone else experience or solved this one?

Thanks, Chet

 

Chet, if you have a vacuum leak you can id it by spraying starting fluid around the throttle body and vacuum lines with the engine running. The starting fluid will change the idle speed of the engine. Hope this trick helps.

 

Since many of the vacuum lines are covered by a wiring loom the spray method to locate vacuum leaks is not very reliable IMHO.

Your issue is quite common and is caused by a vacuum leak. The first thing to check is the white vacuum line leading the recirculate vacuum motor on the HVAC/Blower motor case. Very common for that line to disintegrate.

Now is a good time to replace of the vacuum lines under the hood....

 

Yeah, I've never had much luck using that trick even with my classic Oldsmobile (which has 2 or 3 vacuum lines). There are plenty of old timers that swear by it though.


I've also heard you can use a cigar or e-vapor to blow smoke into one of the vacuum lines and look for it coming out of the leak (with the engine off of course). I don't know if this would work though. It's an attempt to simulate a tool professional technicians have that actually creates smoke with a little heating element. FWIW, I've used one of those tools before and they work great.

 

Common issue, and I had it happen myself last year.

You have a vacuum leak, and as engine load increases the vacuum signal decreases, and the door acuatator controlling the routing of the HVAC loses vacuum and it moves to its default position.

Most likely you have a leak in the under hood vacuum lines on the passenger side which feed the HVAC system, specifically the white line controlling the door actuator. There is also a vacuum reservoir (not the can for the emission solenoids) and a pressure switch, looks like a tee with a small diaphragm, which could also be leaking.

The vacuum lines in the cab, the HVAC selector switch, and the different HVAC door actuators could all be potential culprits as well.

Inspect and replace any leaking vacuum lines. Wisdom here on the forum is to replace all factory under hood vacuum lines as they brittle and crack with age.

 

Either that or his vac reservoir has been bypassed. If reservoir is hooked up, I would look for the leak there first as it is tee'd into the vac hose and, if that is where the leak is, it would render the reservoir inop.

 

A hand vacuum pump is very handy in finding leaking vacuum circuits and parts. They are available at most part houses.

By the by, the vacuum leak is certainly causing a lean condition, even if minor, and costing you fuel as the computer is enriching the A/F ration to compensate. This of course assumes everything else is in good working order.

 

Thanks for all the replys, Is the white vacuum line leading the recirculate vacuum motor on the HVAC/Blower motor case readily available at parts stores, is it plastic or rubber and are there pictures of it so I can trace it for leaks?

Thanks again, Chet

 

The original under hood lines are I believe a vinyl product. Most folks just use appropriate silicone rubber or nitrile lines. I like Gates and Motorcraft for hoses. Just make sure whatever you use is meant to be used as an automotive vacuum line and won't collapse under vacuum.

When you cut the replacement sections to length remember that the engine moves as it rev's and the frame flexes over bumps, so give yourself a little extra length so your lines don't pop off, especially those going to the manifold.

 

I used to link some great photos from Subford for the white vacuum line running to the recirculate motor. Sadly Photobucket has changed their policy to not allow forum posting of photos on free accounts. Other forums are suffering from the same thing.

Here is a closeup of the offending white vacuum line:


For reference it's near the passenger side hood hinge.


Here are the rest of the photos in Ken's SuperMotors folder: 1994 Ford Bronco A/C Vacuum Hose pictures, videos, and sounds | SuperMotors.net

The vacuum/HVAC circuits are exactly the same as our F-series trucks.

 

Thanks guys. This is great information, exactly what I needed. I feel more confident on fixing this now!

Chet

 

So a little update..............looking straight at me was that little white nylon hose that goes to the he door actuator. The exposed part was so dry and brittle that it crumbled when I touched it. Now I also noticed that it was "bundled" into a plastic wire manager tube with lots of other wires that disappears into the firewall! My plan though is to cut it back (the white nylon vacuum hose to where its's still intact and "sleeve" it with some generic vacuum hose.

Just curious as to what others do when trying to replace the entire length of this hose?

 

I just exposed my line at the bundle about a 1/2" and put the line one it with a small tie wrap. Worked good for me, Everything in the bundle is probably protected and ok. I don't think you need to do anything more than that. I do check the connection when I am under the hood. Sandy

 

That is exactly what happened to mine a dew years ago.

 

Great picture Sandy. that's exactly what I did with mine and it's now working great. BTW your engine bay is spotless and is making me feel like I'm inadequate!!