Ventilation System Troubles

Jeff Strickland · 2011-01-29T01:34:09+00:00

I have a '97 F150, and the life support system is acting up, and I need help.As I drive on the freeway, the motor is turning about 2,000 RPM (a little under, actually), and the vents churn out air wherever the dial tells them to. The trouble is, if I press the gas -- hill, speed up, whatever -- then the airflow stops from the selected set of vents and goes to the Defrost ducts and seems to switch to Recirculate. If I set the Fan Speed to HI, I can hear the ventilation system change settings from whatever they are to blasting air onto the windshield.It does not matter what the setting is, it will change to Defrost and Recirculate. I don't know if the AC compressor cuts off, or not, but the cabin gets very stuffy if I let the system do whatever it wants to do. Whatever is going on goes on at any speed under any load, but if I change the transmission to a lower gear by going out of OD, or physically shifting into 2nd, then the engine load changes and the vacuum apparently comes up, and the ventilation system returns to the settings dictated by the dials.If I switch the O/D off and force the downshift, then the RPMs jump to 3,000 and the ducts return to spitting air out according to the dial.I was thinking of a vac leak, but there are no other problems that normally go along with a vac leak. I don't see or hear any hoses or connections that could be leaking. I don't know how the control module works, but I have an instinct that this is the problem.How do I troubleshoot the ventilation system? I suppose that's akin to asking how a woman works, what makes her laugh or cry, happy or mad...

Jan 29, 2011 24 Replies

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Old Crow 15 years ago

No, I'm not talking about the charcoal cannister, that's emissions. This is some sort of reservoir(they call it a reservoir, but it functions more as an accumulator)in the vacuum lines to provide a steady source of vac. when the engine vac drops. Usually located on a fender apron or in some out of the way corner of the engine compartment. I've worked in a GM shop too long to be able to say exactly what it looks like or where it is, but Ford used to be good about using what looked like a

1 lb coffee can painted black with a hose going to one end of it. GM uses plastic ball-looking things.

-- Old Crow '82 FLTC(P) 92" '87 FLTC '61 F-100 302/C-6 BS#133, SENS, TOMKAT, SLOB#13, MAMBM

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Jeff Strickland 15 years ago

Thanks guys.

I found a small check valve behind the battery with a standard sized vac hose going in and two tiny hoses coming out. One of the tiny hoses feeds through the firewall, and the other goes into the right side fender. The plastic hose going into the fender was worn through/broken. I repaired it with section of 1/8" rubber hose.

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SC Tom 15 years ago

That was the one I mentioned in my post. Glad you found it and it's all fixed!

-- SC Tom

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Old Crow 15 years ago

Betcha if you look under the fender you'll find the vac. cannister I was talking about. Great that it was such an easy and cheap fix.

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Jeff Strickland 15 years ago

Cheap? It broke the bank for all of $0.42. And that bought way more hose than I needed to fix it.

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