Vent cuts off when engine pulls hard
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Vent cuts off when engine pulls hard
I have a 1999 Explorer with a 4.0L SOHC engine and 5-speed auto tranny. It has a very annoying habit where when the engine is pulling hard at high RPM (usually going up a hill with a pretty good incline), the air blowing out the dash vents suddenly starts blowing out the defroster vents, straight up at the windshield, and in fact there doesn't seem to be much air coming through at all. Sometimes shifting out of overdrive helps, but if it's a REALLY steep hill (engine pulling pretty hard), it does it no matter what. This happens if I have it set hot or cold, A/C on or off, and any other combination you can think of. And it's not really a sudden shift either, just kinda lazily fades from the dash vents to the defroster vents and then fades back to the dash when I let off the gas.
I live in a town that's partially down in a river valley, so going up the hills all the time drives me nuts with this problem. Not knowing much about airflow through the vehicle, my poorly educated guess is some sort of vacuum problem somewhere, but I honestly don't have a clue. Anybody have any thoughts on what could be causing this?
I live in a town that's partially down in a river valley, so going up the hills all the time drives me nuts with this problem. Not knowing much about airflow through the vehicle, my poorly educated guess is some sort of vacuum problem somewhere, but I honestly don't have a clue. Anybody have any thoughts on what could be causing this?
Last edited by ivorycruncher; May 14, 2007 at 11:48 PM.
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From: UT
I don't know exactly how your '99 will be plumbed, but the idea is the same. Somewhere, there should be a "box" or "cylinder" that serves as a vacuum reservoir (on my '92, it's just above the air conditioning evaporator housing). Between the engine and that reservoir will be a one way valve of some kind. This reservoir is there to "store" vacuum for things like your HVAC controls, so they will have a vacuum source even when engine vacuum is low like when climbing hills. When that one way valve fails, the reservoir can't be isolated from the engine at low vacuum, and thus your HVAC controls lose vacuum under heavy loads. Defrost is the "default" position, so when you lose vacuum, the controls revert to defrost. Find that one way valve and make sure it's functioning.
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vacuum holds the AC/ heat in the panel and floor positions. if you loose vacuum, it defaults to the defrost, so yes, you have lost vacuum... at open throttle, your vacuum does decrease normally and in your case can not keep up. you are on the right track looking around the top of motor for a cracked plastic hose, or one that has fallen off, etc. the accumulator "collects" the vacuum as the guys said, but the hoses have to be good to make it work.
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Okay, sounds like I'm on the right track. However, I looked under the hood today, and while I can find a vacuum line here and there, I can't seem to locate the point where they converge, which I assume would be this "reservoir" I'm looking for. The engine compartment is very tight, and on the passenger side in the back, I can see the fan housing for the vents and the A/C system. I'm guessing what I'm looking for is down underneath there somewhere, which is going to be ugly to get at it.
I consulted a Chilton's service manual, which was completely worthless (so far that book is 0 and 4 for having available or accurate information) so I don't realy have anything to go on for tracking down the parts. If anybody has a clue where exactly I can find this valve and reservoir, I would be most grateful.
I consulted a Chilton's service manual, which was completely worthless (so far that book is 0 and 4 for having available or accurate information) so I don't realy have anything to go on for tracking down the parts. If anybody has a clue where exactly I can find this valve and reservoir, I would be most grateful.
