AC blows through defrost vents when accelerating

[saenzm]

Anyone ever have this problem? The AC blows through the dash vents just fine when crusing, but if I accelerate to pass someone or going up a hill, it starts blowing air through the defrost vents only.

Anyone else have this issue? If so, is the fix DIY?

Thanks,
Mark

[DCHitt]

Originally Posted by saenzm

Anyone ever have this problem? The AC blows through the dash vents just fine when crusing, but if I accelerate to pass someone or going up a hill, it starts blowing air through the defrost vents only.

Anyone else have this issue? If so, is the fix DIY?

Thanks,
Mark

You have a leak in the air vacuum system that controls the air flappers. When you step on the gas you loose engine vacuum and with the leak the flapper vacuum goes down also.

[saenzm]

Originally Posted by DCHitt

You have a leak in the air vacuum system that controls the air flappers. When you step on the gas you loose engine vacuum and with the leak the flapper vacuum goes down also.

Is this a DIY fix or better left to the pros?

Thanks,
Mark

[BBQ]

Originally Posted by saenzm

Is this a DIY fix or better left to the pros?

Thanks,
Mark

It is not too difficult... no special tools needed. But it does take time.
Disassembly required.


Check the youtube for video.


ps. I have posted the links here before, but I have forgotten which thread.

[Marley]

I'm guessing that there is a default in the program that controls the HVAC.
Defrosting the windows being more important than cooling the occupants the program may go to that setting under loading of the engine.
A/C compressor 'usually' kicks in to dry the cab air when in defrost mode.
Just guessing on this one though.

[DCHitt]

Originally Posted by Marley

I'm guessing that there is a default in the program that controls the HVAC.

The flappers are usually just spring loaded so when the vacuum is lost the spring wins. I guess that does make it go to a default but I doubt it is in a program.

[Bruceper]

Vacuum drops under wide open throttle (WOT). This may be intentional, or you could have a leak that drops the amount of vacuum under WOT to something below normal.

First thing to do is start the vehicle with the hood open and listen for any noise like a sucking/hissing noise. The vacuum hoses are small and you may not be able to see them within the mess of stuff under the hood.

You didn't state what year of vehicle. But you may also be able to see a vacuum leak code if you connect a scan tool.

[booster]

A lot of vehicles will have a vacuum accumulator tank somewhere. It is a very good place to hook up a vacuum pump and see if it holds vacuum. It will also allow you to check for leaks without the engine running.

If you think you find a leak, or a suspicious hose or connection, you can spray it with a tiny bit of ether and the engine will speed up a bit for an instant. If you are under the dash, you have to be very careful not to gas yourself or start a fire, though.

[peppster66]

I have the exact same problem on my 97 dodge roadtrek, I have replaced alot of the small vacum lines under the hood, but still hasnt fixed it, next I will start under the dash, I dont see one of those vacum storage balls, im actually thinking of adding one if I cant find the problem,,,,,

[mkguitar]

having a yard full of old cars ( and everything on Cadillacs was vacuum...)

you can use a piece of hose as a stethoscope and listen for whistling near hoses

be ready to replace a bunch of hose

golf tees can be used to block off hoses while diagnosing

a vacuum gauge is handy

an emisions manual ( such as mitchell) will have complete vacuum hose routing for the underhood systems- once those are known good you can check the auxillary systems- other wise you can have a pushme-pullyou effect from a bad EGR hose or something.

be methodical

mike

[saenzm]

I looked under the hood for the vacuum leak and didn't see anything wit the naked eye. I keep hearing about a "ball" a vacuum ball that controls the flapper? Does anyone know anything about this? Where I can find it specifically?

Thanks,
Mark

[booster]

Originally Posted by saenzm

I looked under the hood for the vacuum leak and didn't see anything wit the naked eye. I keep hearing about a "ball" a vacuum ball that controls the flapper? Does anyone know anything about this? Where I can find it specifically?

Thanks,
Mark

Not all vehicles have a ball vacuum accumulator.They may have some other shape, or no accumulator at all. I have seen many of them mounted on the engine compartment fenderwells, some under the battery tray, some under the dash. It is best to find a vacuum hose map for both the engine and for the heater in a service manual, so you aren't flying blind. Googling the make and model for one might show one up also.

[booster]

A quick search gave this for a 1998 Dodge, and does show a cannister.

http://cdn.johnywheels.com/2015/12/0...facaf967a.jpeg

[peppster66]

The accumulator ball on my 97 is located down behind the passenger side headlight, and I finally fixed my vaccum issues, turned out to be the 2 check valves, replaced them and it fixed all my problems, learned that fix from the dodge/mopar help site, very cheap and easy fix, hope this helps,,,,,

[leva56]

I have a 2002 Ford 350 PW Excel, I just replaced the vacuum reservoir and it is still doing the same thing, cutting out at higher speeds and blowing thru the defrost. Any suggestions on where to go next?

[GallenH]

Did you check all hoses for leaks as well? I'd start there. Had to TS my 97 dodge based PW because I was getting no defrost when in defrost mode. No hose or cannister leaks though. My pushbutton climate control was very sloppy. Vacuum lines ran through that to the various doors. Hail Mary replacement revealed that to be the cause.

[leva56]

Just replaced the larger hoses that run to the service port and to the intake plus another short hose, they were all crumbling on the outside of them. I don't know if that was the problem or maybe where it connects that the connections were dry and getting air in don't know. Haven't test-drove it yet.

[Kencne]

Originally Posted by saenzm

Anyone ever have this problem? The AC blows through the dash vents just fine when cruising, but if I accelerate to pass someone or going up a hill, it starts blowing air through the defrost vents only.

Anyone else have this issue? If so, is the fix DIY?
Thanks,
Mark

This is almost always caused by a vacuumed leak. Check all vacuum lines, check PCV valve, break booster and throttle body base gasket. If you have a dodge 5.2 or 5.7 engine a very common problem is an intake manifold leak. For some strange reason dodge decided to make the manifold in 2 separate pieces, the top aluminum and the bottom steel. over time the gasket develops a leak.
I found an easy way to check I use an inspection cameras hold the throttle body open and look for oil in the intake manifold. If there is oil there you most likely have found your problem. If this is the case the intake has to come off. Not that hard but take some time. Before taking off the manifold I would check compression and do a cylinder leak down check. Good luck.
I just had this problem and decided to do the engine. found a factory reman engine with 5 year warrantee for $2500.00 so far so good.