97 F150
4.6L V8
4x4

I have read threads pertaining to the vacuum leaks causing the A/C to switch from vent-defrost under load, or at higher RPMs. Over this past summer, I checked the reservoir by the battery, the lines coming out of the firewall, the lines behind the heater core box, and also lines/vac motor asy's behind the steering column/dash. To no avail on finding any broken vacuum lines or leaks around elbows. I didn't have a vacuum gauge to test professionally, so I just took the vacuum lines and blew threw them myself. After a quick second, I could blow no more through the vacuum, therefore, I assumed the line was good.

I have inspected one other thing. On the passenger side valve cover is the PCV valve. I've checked the hose and hose elbow at back of TB. Nothing. However, the driver's side valve cover has a plastic pipe that comes out of the front of the TB and hooks up to the driver's side valve cover. From the Hayne's manual, it is labeled as "Crankshaft Air Intake." PCV hose and all looks OK, but the opposite side hose has a hole in it. Could this possibly be my vacuum leak? As far as I can tell from the diagram in the manual, the PCV and "Crankshaft Air Intake" are supposed to work together. I was just wondering if the hole being their would cause the vacuum to run low. Although the engine runs smoothly with no problems.

 

Probably not. But, fix the leak anyway before you set "lean" codes.

 

Ok. Well if its any of the thin/hard plastic vacuum lines such as the ones by the battery, they're are not obvious to cracks or leaks. What about the check valve? I could blow through the valve one way, but not the other. How to test? I have also replaced the vacuum motor assembly found underneath the steering column (red vacuum line from controls). Unless the PCV hose could be built up with carbon? I have had to take the TB off and clean because of annoying P0401 - "Insufficient EGR Flow" a few times since I've had the truck.

 

If the vacuum check valve flows one way but not the other it should be OK provided it's installed in the right direction. Flow towards engine vacuum. Is it possible the valve has deteriorated from crankcase fumes?

You mentioned you changed the vacuum motor for the red vacuum line. That controls the defrost to floor door. In a no vacuum situation the door will be closed to the floor thereby giving you defrost air. There should also be a yellow line on that motor which when Control placed in def/flr position, vacuum is delivered to give mid-position door.

The vacuum motor for the panel registers is fed by the blue vacuum line. In a no vacuum situation that door will be closed to the panel thereby directing air to the defroster/floor duct.

The vacuum motor for the outside/recirc door is fed by the white vacuum hose. No vacuum leaves the door closed to the recirc thereby allowing outside air in.

The black line is Source from vacuum canister to control selector.

Have you run the engine, shut it off and pulled the check valve off to the vacuum canister or black line going to Selector control, not the engine, to listen for vacuum hiss?

 

The way mine is set up, the door/defrost motor has a blue/yellow combo and the panel/defrost is a single red line. I know which motors control each because I have inspected these things when I had to do a Evap/Heater Core replacement. Also checked the vacuum lines that are behind the dash as well.

By having Max A/C on, I unplug the red line from the vacuum plunger and all air goes to defrost. Plugging it back up sends it back to panel. The blue and yellow control the floor for mine.

No I have not tried running the truck and shutting down to check for the hiss. If there is a hiss, the hiss should be my leak source right?

So when I run this test, I need to leave the check valve connected to the engine side of the vacuum?

If I'm not mistaken, the check valve should allow vacuum to flow from engine to the canister, but not from the canister to the engine.

 

Do you have an electronic A/C control or a 3 **** manual?
If electronic - do a search for EATC and you will find how to replace the o'rings inside the controller.

 

I've got the manual 3-**** system. I did see a post about repairing EATC, but I'm not lucky enough to have the EATC.

 

Ok. Just went out and did some more hunting. I did the test and did hear a hiss coming from the 2-way check valve. I think it was on backwards. If I'm correct, it should allow to suck from the motor to canister, but not from reservoir to motor. It is now on to where it is sucking from the engine and not the reservoir.

Is there another check valve in that area. I've got the one 2-way valve, but I see another thing that looks like a check valve, except its 3-way. There is a black vacuum line hooked to the top from the reservoir, another black on the side(thats the line that runs through the firewall, and the bottom just has a small black rubber hose that runs to a chunk of vacuum lines that comes out of the throttle body.

If it is a check valve, then I am pretty sure it is failing. On that one, I can blow through each one of them. I'm assuming that the bottom hose should be the check. After finding out that I could blow through each side, including the bottom, I took the valve and tapped it on the top of the battery and tried blowing through the bottom again but I couldn't. So now I can only blow through the top and the side, but not the bottom. This is what makes me think that it is another check valve.

Also, the first check valve mentioned earlier goes to one half of the reservoir, and the other to the other half. I guess the bottom half is for my 4x4 system and the top is for the A/C vents. The top half of the reservoir runs that black line to a "valve?" and cuts through the firewall. But for some reason, I think that 3-way "valve" is failing.

 

Sorry for the line color confusion. The vacuum diagram I was using was for a 99 F series. Because it's for hvac it doesn't show the resevoir as a 2 piece.

The check valve I was talking about is as you described being a 3-way. I think you've found the problem. Perhaps it's just gummed up with crankcase fume residuals and just needs a solvent cleaning. As for the 2-way check valve you reversed, I guess you'll find out if it was on correct if another problem arises.

Good luck

 

Actually, the 3-way I am talking about is a 2-way, and the 2-way is actually a 1-way. One path for vacuum one 1-way and two paths on 2-way.

Just put on a new 2-way check valve and so far it seems to have fixed the switch to defrost at constant highway speeds. But as soon as I gun/mash it to the floor, it switches to defrost. This makes me think maybe I might have broken/cracked a line when putting on this valve. For now, it only switches to defrost when I mash down on the pedal. Better than before because I drive a constant 55mph a hour back and forth to college and it doesn't take but about 10 or so secs before it diverts to defrost. Hopefully thats not a concern anymore, but I would like to fix the mashing down on the pedal issue.

P.S. I must've had the 4x4 lines reversed as well. I found out that I drove to work with my front end engaged. LOL, so after reversing the 1-way valve I had to reverse the lines as well. On that note, the bottom section of reservoir is strictly for the 4x4 stuff and the top part is for A/C controls, etc.

 

Found a little crack/hole in the black vacuum line coming off the reservoir, figured it happened when I was changing out the check valve. Anyway, I patched that up but I still have a problem with it going to defrost under acceleration. Before I put the new valve on, it didn't go to defrost under acceleration, however at a constant pace anywhere from 40mph and higher is when it would change to defrost. It did that once or twice earlier, but hasn't done it again ever since. Just wish this problem would be easier to find.

 

[quote=Chris 97 F150;9261025]Ok. Just went out and did some more hunting. I did the test and did hear a hiss coming from the 2-way check valve. I think it was on backwards. If I'm correct, it should allow to suck from the motor to canister, but not from reservoir to motor. It is now on to where it is sucking from the engine and not the reservoir.


Did you change this check valve back to before you reversed it to see if it affects your defrost under accel problem? Won't hurt to test.

 

The check valve for the 4x4 is in the correct position I assume now. I got the side that you can suck through towards the throttle body.

On the 2-way check valve, which vacuum lines should be where? Earlier, I had it set up where the top nipple was connected to the reservoir and the side connected to the line going through the firewall to the controls. The big fat nipple by itself is hooked up to the line going to the throttle body.

After messing around with the valve in hand, I tested to see which sides pulled more/less/no vacuum. By sucking from the main (leading to the throttle body), I found that more vacuum is pulled from the side, rather than the top. Wondering if the side nipple is supposed to be connected to the reservoir, therefore the reservoir has more amount of vacuum. In other words, I swapped the lines on the L-shaped part of the valve.

Now I think I have stronger vacuum from the motor to the reservoir, instead of before having a stronger vacuum from motor to controls.

I hope I worded that to where you can understand it well. It's kind of difficult to explain, so I'll photoshop a drawing and try to show you what's going on.

It's not the prettiest picture but I think it will work.

 

I've looked everywhere I could possibly think of, but still haven't been lucky to find that darned vacuum leak.

 

Go take out the battery, there's a vacuum canister there and the battery tray usually rubs through it and or the vacuum line.