Wonky Vent Controls -- Vac Leak

[Skandocious]

So I think we've had threads debating this before but I'd like to start with a clean slate because I don't think I read a definitive solution...

My vent controls are all wonky. I'll have the air blowing through the dash vents, ie if the setting is on Vent, A/C, Max A/C, and when I start to climb an uphill grade (putting a load on the engine) the air will divert away from the dash vents and start blowing out from the floor and/or defrost vents.

I understand that the door that controls the flow of air is controlled by vacuum, and that engine load is directly proportional to how much vacuum I've got at the manifold, but I just can't figure out WHERE the vac leak is nor really how to properly check. I removed some dash pieces and check the lines in the cab, as well as the vac connections at the back of the dash controls-- it all looks great inside the cab. All of the vac lines are very rubbery and pliable, not brittle or cracked from what I can tell.

Now the stuff under the hood is a different story... The vac lines under the hood are rigid feeling, but I get the feeling that they're supposed to be rigid from the factory, unlike the softer ones under the dash. I've been under my hood about 3583 times and I've never seen any cracked or disconnected vac lines. Is there anywhere in specific that I should be looking? Problem spots on these trucks?

The one vac line that WAS busted was the white one that controls the air door near the blower box in the passenger rear corner of the engine compartment. When I got the truck it was completely broken apart so I replaced the broken piece with a piece of rubber vac hose and some clamps. What does that blend door (is it a blend door?) control? Better question-- which door is in charge of diverting air to the dash vents? I get the feeling that the default position (with no vacuum) is to push the air through the defrost and/or floor vents, and thus when I lose vac to the cab it's default back to that position, then when the vac builds again (after I unload the engine) I hear a pop noise (door closing) and the air shoots back out the dash vents again.

Any vacuum experts around that can provide some advice?

 

[radialarm]

Check the connections on the tomato juice can, then double check the white plastic hose/tube( mine got to doing what you described and I cut open the wire harness and got a good flexable end and ran rubber hose, end of story for me)those at one time flexable tubes under the hood sometimes rub through on the bottom or where touching something else double check them all. You might invest in a Mightyvac hand pump w/guage to help find the problem!!! Good luck!!!

 

[fatherdoug]

Was that white line attached to something that looks a like small blimp? I think that is a vacuum reservoir. I have had one leak around the seam of the "blimp" after a number of years(91 Aerostar). Just a brain fart....

 

[Skandocious]

Some good ideas, I'll check that stuff in just a bit.

Can anyone give me some kind of diagram that will help me trace the exact vac line path all the way from the manifold to the door that controls the dash vent air diverter?

 

[Skandocious]

Just went out and checked... So the white vac line in the engine compartment (mentioned above) appears to connected and healthy. I didn't pull back the wire loom yet to check the internal connections. I then moved to the cab and found the air diverter under the dash, with a light blue vac line running to it. I turned the vents on without the engine running and of course it came out of the defrost vents (no vacuum to power the diverter). I moved the diverter plungers by hand and the air starting pumping out the dash vents. So the diverter under the dash is the one that is clearly getting low vacuum. I traced the blue vac line and it goes over to a male-to-male vac line coupler with couplers for 6 or 8 vac lines. From there it goes up to the dash controls.

The blue vac line looks very healthy and pliable, as did all the others going up to the controls. So the problem must be a vac leak in the engine compartment. Which vac line/color is the main vac line that supplies all of the vacuum to the controls in the cab? I saw a black one that was going from the engine compartment to the cab-- is that the main supply line? Anyone got a diagram? I think I might have one on my Davis computer but I'm not there right now.

 

[fatherdoug]

Chris, I have tried to copy a scan to my gallery, but it is the wrong format(tif) The diagram shows several black vacuum lines coming from a central header. The diagram is a generic one for 90 - 95.

Black lines go to: vacuum check valve, manifold fitting(vacuum source), and the auto transmission.

Black with yellow letters go to vacuum brake booster

Black/yellow goes to air cleaner

Black/brown goes to speed control

Pink goes to solenoids

Red goes to distributor

 

[Skandocious]

I'm headed back to Davis tonight so I'll look in my manual and see what I've got. Thanks for trying FD, I appreciate it.

 

[89frankenford]

check the vacuum reservoir and make sure there is no rust holes in it....belive me those cans rust out VERY easily even in the non rust states....check the edges of it and look for pin sized holes.

 

[tomtoc]

i've heard of using a propane torch (unlit obviously) to shoot at the lines in the engine compartment. if you've got a leak, the propane will get sucked into the line and the engine will idle higher.

apparently lots of people do it even though it sounds a little scary. it's supposed to be a lot easier than doing a smoke test or whatever. i need to do the same thing b/c i have no max A/C if i step on the gas at all (no need to thread hi-jack, just saying)

 

[john112deere]

i've heard of using a propane torch (unlit obviously) to shoot at the lines in the engine compartment. if you've got a leak, the propane will get sucked into the line and the engine will idle higher.

I have a dim recollection of reading about a safer way to do this test. Danged if I can remember what you used instead of propane, though.

 

[1985 Ford F-150]

You could probly use starter fluid or carb cleaner.

 

[john112deere]

Sounds right- something in an aerosol can.

Maybe even WD-40 works, but I can't remember for sure.

 

[Skandocious]

I've tried the carb cleaner trick-- did nothing but make a mess of my engine compartment.

 

[tomtoc]

Sounds right- something in an aerosol can.

i guess you can use anything you can spray and is flammable. bug spray comes to mind. either way you're playing with fire.

 

[Old_Paint]

Propane = SAFEST

I've tried the carb cleaner trick-- did nothing but make a mess of my engine compartment.

That's the beauty of the propane. It IS the safest, and CLEANEST way to check for vacuum leaks. A split plastic line may not let liquid through, but will easily suck the propane in. The best part is, NOTHING to clean up after the testing is done. ANY liquid solvent you use can at least momentarily puddle and will most certainly drip or run, and leave something to continue to burn long enough to catch other things on fire that normally are pretty hard to catch on fire. The propane, being a gas, will simply give a small flash of blue fire for a quick instant, and is GONE. Worst case, you might singe a little hair, but you're not going to melt your face off. If the torch lights, you know two things then. Time to move the torch back, and you have a totally other problem (sparks where sparks shoud not be). I used this method to find my leaking plenum gasket, and it worked VERY well. Just try to think about it this way. You're pouring very flammable liquids on very hot surfaces. Do you REALLY want to be that close to a possible flash fire?

A small propane torch (unlit) with a standard tip on it or a rosebud tip if you want to cover larger areas, with the valve barely opened will tell you all about a vacuum leak in short order. If you're trying to pin-point a leak, you can remove the tip and just use the feed tube to make a very small stream of propane. One leaky plug wire with solvents dripping is asking for a disaster. The WORST one I've heard of yet was alcohol. You cannot see an alcohol fire until it's TOO LATE, because alcohol burns invisibly. When you can see the flame from an alcohol fire, something else is burning with it.

Use the propane, Chris. It's easier to get it into places you cannot squirt fluids, and above all flashes quickly if ignited. It's lighter than air, and will simply blow away with the fan turning. IIRC, you have an E-Fan mod, so you could simply disconnect your fan while you're checking, and use even less propane.

 

[Skandocious]

That's the beauty of the propane. It IS the safest, and CLEANEST way to check for vacuum leaks. A split plastic line may not let liquid through, but will easily suck the propane in. The best part is, NOTHING to clean up after the testing is done. ANY liquid solvent you use can at least momentarily puddle and will most certainly drip or run, and leave something to continue to burn long enough to catch other things on fire that normally are pretty hard to catch on fire. The propane, being a gas, will simply give a small flash of blue fire for a quick instant, and is GONE. Worst case, you might singe a little hair, but you're not going to melt your face off. If the torch lights, you know two things then. Time to move the torch back, and you have a totally other problem (sparks where sparks shoud not be). I used this method to find my leaking plenum gasket, and it worked VERY well. Just try to think about it this way. You're pouring very flammable liquids on very hot surfaces. Do you REALLY want to be that close to a possible flash fire?

A small propane torch (unlit) with a standard tip on it or a rosebud tip if you want to cover larger areas, with the valve barely opened will tell you all about a vacuum leak in short order. If you're trying to pin-point a leak, you can remove the tip and just use the feed tube to make a very small stream of propane. One leaky plug wire with solvents dripping is asking for a disaster. The WORST one I've heard of yet was alcohol. You cannot see an alcohol fire until it's TOO LATE, because alcohol burns invisibly. When you can see the flame from an alcohol fire, something else is burning with it.

Use the propane, Chris. It's easier to get it into places you cannot squirt fluids, and above all flashes quickly if ignited. It's lighter than air, and will simply blow away with the fan turning. IIRC, you have an E-Fan mod, so you could simply disconnect your fan while you're checking, and use even less propane.

Very good points Tim. I think I'll try just a bit of propane when I go up to my moms house this weekend.

 

[Skandocious]

Finally got around to trying to propane trick. Sprayed every vac line and connector that I could find and the engine never skipped a beat.

What now?

EDIT: Just read that Tim found his leak at the plenum gasket Didn't check there.

 

[Old_Paint]

Propane will be the safest, trust me. Any liquid solvents around a hot engine is a recipe for disaster. Dunno why everyone thinks propane is unsafe, but don't hesitate to spray starter fluid all over a hot engine with 65,000 V going through each spark plug wire about 10 times per second.

Back to the original question, though.

It may well be the check valve in the vacuum supply to the HVAC system is shot. The reservoir on the right fender has NOTHING to do with the HVAC. That's for EGR, TAB/TAD only. There should be another mounted on the left frame rail near the rad support for the cruise control, also a "tomato juice" looking can.

The reservoir for the HVAC is inside the cab, under the dash, and is a little larger than a softball. If you have a vacuum pump, pull a vacuum on the white line with the controls in the OFF position. It should hold, or at least drop very slowly. If that's the case, then yes, the white vacuum line is probably in good shape. You can't determine otherwise without testing it. Check the check valve in that line. You should be able to blow through it from the end closest to the reservoir and away from the engine. You should NOT be able to blow through it in the other direction. If that check valve doesn't work right, then when manifold vacuum drops, so will the vacuum in the reservoir. Lose the reservoir, you lose the damper controls.

One more thing to check will be the damper actuator(s). Make sure they're not leaking too. If they've aged and split inside, and won't hold a vacuum applied by a hand pump, you just found TWO problems. 1, your annoying automatic damper control, and 2, an intake leak.

 

[Skandocious]

Interesting... I do have access to a vac pump up at Tom's place. I'll give that a try next time I'm up there. Thanks Tim.