Not sure about this one....vacuum problem maybe? Just returned from a few days in the mountains of Colorado with the 97 Aerostar with 4.0L and auto transmission. With no AC running, but pulling hard up when going up steep grades, the flow of air through the vents stops entirely. Let up on the accelerator and the air flow returns. Accelerate hard and the air flow would stop again. Even at 70 mph on interstate highway, when pulling hard while on a long incline up hill, the air flow would stop until the hill crested, then air flow would return to normal. Not sure if the blower fan is shutting down, or if there is a damper in the vent system that is closing. Either way, there is no air flow.

Anyone here ever had this happen with their Aerostar and found out what the cause is?

 

There is a leak in the vacuum lines that activate your AC doors. This causes the doors to default to windshield defrost when the engine vacuum drops (manifold pressure increases due to high load). Normally a vacuum reservoir and a one-way valve keeps doors in their position when the engine vacuum drops. Look under the hood around the blower area for broken vacuum lines. They're plastic, about 1/8" or 3/16" diameter, and tend to crack with age.

 

Thanks. As suspected, a vacuum related problem. So it makes no difference whether the AC is on or off, the vent doors are affected the same? If the vehicle had no AC system, the vent doors would still operate using the vacuum system to keep them open, or closed?

 

That is correct; the vent doors are actuated by vacuum, whether you have AC or not.

 

Thnx for confirming how the system operates. Not sure if it might also be the source of a mystery noise, too, when accelerating hard. Thought it might be exhaust related, but haven't been able to locate any exhaust leaks at all. Will find out if it is vacuum related once that tube is replaced...

 

Springs return the damper doors to their "fail safe" positions with the loss of vacuum. That position is bringing in outside air and blowing it at the windshield.
Often, when pulling hard, engine vacuum drops to 0 - there's a check valve that should prevent the loss of vacuum back to the engine. A common problem among my Fords occurs under a hard pull when the check valve doesn't hold and the vacuum in the duct system drops to 0 until I let up on the engine load (get over the hill). I've also found cracked hoses on the connectors. Using a hand vacuum pump, disconnect the engine's vacuum and connect the vacuum pump in it's place. If you can pump a vacuum and it holds, there are no hose leaks - try replacing the check valve. If it leaks back check all the hoses at their connectors first then along their length. If you find no leaks, connect the vacuum pump directly to the damper motors and pump a vacuum on them - if they hold it, they're good. If they leak, replace them.

Good luck hunting,

 

DFord, thanks for adding these helpful tips. Looks like I could have a few more hours of work to get the leak issue tracked down. Getting any vacuum leaks repaired may also help with gas mileage possibly?

 

easy fix, look at the black & white plastic vacuum "tubes" which start at the Air Box, where the front fan blower is, and follow each line to where it ends. The plastic tubes become brittle and start breaking with engine heat.

you might find a way to splice them with a rubber tube if the break is halfway from point A to B, or you might find they are cracked where they connect to either the heater valve, the recirculation actuator, or the front top of engine.

never mind the gas mileage, the 4.0 liter engine is terrible with gas mileage. Ask me how I know, vacuum has nothing to do with it. But it sure sounds good when you step on it and burn 1/8" of a tank beating a Honda at the red light.

install a K&N Air Filter, that will improve things slightly, but don't expect a Hot Rod.

 

Jose A, thanks for your suggestions. Last night, I did an inspection of alll the tubing. All seems to be in good shape, still plenty flexible with no breaks, cracks, or signs of brittleness anywhere. Looks like the next step is to do an actual vacuum test to see if it holds. I don't have a vacuum pump, so will need to check around to see who has one locally. If vacuum holds, then to check individual components where the problem may lie.

 

you welcome DarylJ.

next you might check the vacuum "tree" on the rear driver's side of the engine top, above the rear spark plug. Another plastic tube goes from that tree, routed through the rear of the engine, then towards the front where the front fuel injector is on the passenger side. That one tipycally dries out.

if no problem there, remove the glove box, open it, then squeeze the rear edges to remove the entire glovebox in one piece;

you will see another "tree" sort of hanging there by the interior air box cover, check that all tubes are well connected, this one has red, clear, and black or white vacuum tubes.

if no problem there, then two more checks:

you'll have to remove the black metal cover over the dash, 9 screws which are a pita to get to, then follow all the tubes checking each one.

the last check will be to remove all the plastic dash facias to be able to get to the Switches Pod, where all those tubes finally connect to the Main Selector Switch.

and the last last check will be to remove the front blower fan to check the tubes inside the air box that might have been damaged by the blower cage.

 

I have the same problem now. Must go through all the suggestions listed here

 

it's always one of those plastic tubings in the engine compartment.

The red tubing shown in the picture above is from 1980-1987 Jaguar XJ-6 that I just happened to have handy. Same size tubing as Ford but nylon instead of cheap plastic.

 

Thanks again, Jose A. I will hope to find the leaky tube sooner rather than having to wait until the last options to find it. Having to lift the dash....well, probably not as bad as replacing the blower motor on a Volvo 240, but maybe not far from it. No fun. :-)

 

This is a common occurrence on my vehicles and you may not have a leak - except for a leaking check valve. You can borrow a hand vacuum pump from Autozone or other parts stores to test your system to see if it's leaking down. The check valve is a disk about an inch in diameter in the vacuum line and it's a convenient place to pull loose and connect the pump. Pump it to pull 10-20 inches of vacuum and let it set to see if it holds. If you can't pull that much vacuum, you've got a leak for sure and need to look further. The diaphragms in the damper motors are prone to leak - if the system is leaking down and you can't find a cracked/split hose, check the motors individually.

 

DFord, thanks for adding this to the conversation. Will check with the local Autozone for a hand vacuum pump. Re the dampers: Are they checked the same way as the check valve?