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Vent vs. Defrost Vaccum Issue

knightgang on Mon July 20, 2009 10:15 PM User is offline

Year: 1997
Make: Olds
Model: Cutlass Supreme
Engine Size: 3.1
Refrigerant Type: 134a
Ambient Temp: n/a
Pressure Low: 40
Pressure High: 200

This car has been sitting for a while and I know I have a small leak in the system on the High Side port. Found it when we charged up the system and we will fix that later. THe issue right now I suspect is a vacuum issue.

With the A/C system on and blowing out of the dash vents, everything is fine until you step on the gas to pull off from a stop or accelerate in speed. When you do that, the air diverts from blowing out the vents to blowing out the floor and the defrost. One the RPMs level out for a cruising speed, the air will return to the vents.

I found the vacuum block and did not find a leak around it or any of the lines. I also found the vacuum actuators under the dash. There are two that I saw. The one that moves when the air direction knob is turned (from vent to floor to defrost, etc.) you can see it move when you push ont he accelerator. It seems that even at idle, there is a flow or air around it (or even out of it).

So, can anyone direct me to proper trouble shooting of the vacuum system. It would be great if I knew how the vacuum system works and the proper name of the components. This issue gets annoying on hot days when the air does not blow directly onto you.

Thanks

GM Tech on Mon July 20, 2009 10:27 PM User is offline

Get out from inside the car- go underhood to the battery and look at the black plastic hose that runs under the battery down into the vacuum reservoir in front of left front wheel- the black plastic line will be unplugged, or broken- sucking air- seen dozens like this-- the outgassing of the battery hardens the plastic - it becomes brittle and breaks...repair by splicing a rubber hose ove both broken ends....W-cars are famous for this failure mode-- exactly as you describe- vents go to defrost while accelerating-or when climbing a hill-- is a sure sign the reservoir is unhooked-- just did one two weeks ago.....takes about 5 minutes to fix...

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Edited: Mon July 20, 2009 at 10:36 PM by GM Tech

knightgang on Mon July 20, 2009 10:36 PM User is offline

So, I have to remove the Washer reservoir, and the battery to get down to the vacuum line, correct?

What about the small amount of airflow I found under the dash around the actuators? Is this even a problem I should be concerned with?

Can you explain how the vacuum system works? Thanks for your help...

GM Tech on Mon July 20, 2009 11:03 PM User is offline

I never remove the battery- or the washer fluid reservoir- I come up from below and see the broken line- or pull it out from under the battery from above and then splice in a new piece and route it back down to the vacuum reservoir. You may need to open up the splash shield in front of front wheel though- to see the vacuum reservoir.

The vacuum is generated by the intake manifold- usually runs at 17-25 in Hg- it takes at least 15 inHg to operate the vacuum actuators inside the cabin- if there is a leak, it runs around 16 inHg, then when pulling a hill, or accelerating, the vacuum in the manifold drops, due to increase air inlet through the wider opened throttle plate, if it drops below 15 inHg, then actuators give up and are spring loaded to divert all air flow to the windshield to default there for safety reasons-- if by chance it was rainy windshieldcould fog up, then warm air can still be used to clear the windshield so a driver can see where he is going-- even if there is NO vacuum to the dash..so it is for safety, that the air blows out the defroster. The vacuum reservoir is there to allow temporary wide opened throttle to keep constant vacuum to the actuators- so they don't react as quickly. There is also a vacuum check valve to keep vacuum in place with engine off-- A good way to test a vacuum tight system- is to shut car of with vent mode selected-- wait several minutes and start car-- does air come out vents immediately- or is it coming out the defrost ducts? If it comes out defrost, then vacuum has dimished due to a system leak-- proof positive you have a cracked or broken hose....all new cars are tested to see if vent actuators stay in vent mode after any engine off time......

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

NickD on Tue July 21, 2009 12:14 AM User is offline

With the climate control off before turning off the engine the vacuum line running should hold a steady pressure with a hand held gauged vacuum pump. There is a vacuum switch inside of the climate control, in most cases, the grease, a silicone dries up and causes leaks, this switch provides vacuum to the various actuators to control airflow either to the defrosters, dash or lower heat vents. Actuators should also hold a vacuum, use a diaphragm, would have to be replaced if they leak. This is the hard part

The easier part is tracing the vacuum source from the intake manifold through a check valve, only permits air to flow toward the intake manifold, can blew in it on either side, source side should blow easily toward the manifold, manifold side blowing back should be be blocked solid. The through the reservoir, use a hand held vacuum pump on that to see if it holds a vacuum with all good hoses and connections.

Vacuum systems should work down to 7"/Hg, if yours drops below that, it will switch to default, the defrosters. Of course, you also want to check your intake vacuum, at idle, should be 20"/Hg minus an inch for each thousand feet you are above sea level. I am suspecting your check valve is not closing when your intake manifold vacuum is less than your reservoir vacuum, that would suck your system down quick since you seem okay at idle.

What really causes intake manifold to drop is that big hole in your TB with the throttle valve wide open, also burns your tires, stresses your drive train, and eats gas like crazy. If you drive your car so your defrosts do not open, you will save the planet from excess CO2 production and have more of your money in your wallet.

knightgang on Tue July 21, 2009 9:20 AM User is offline

Quote
Originally posted by: NickD
What really causes intake manifold to drop is that big hole in your TB with the throttle valve wide open, also burns your tires, stresses your drive train, and eats gas like crazy. If you drive your car so your defrosts do not open, you will save the planet from excess CO2 production and have more of your money in your wallet.

NickD:

I thank you for your explanation of the vacuum system and the check valve operation etc. to help me understand the problem and look for a fix to the issue. However, the comments you posted above about driving habits are uncalled for. This issue occurs under normal driving conditions and normal city/highway acceleration changes. I am not drag racing the car off from a stop sign of traffic light, flooring on and off etc. So, I would appreciate it that you keep your uninformed opinions to yourself. Thank you.

NickD on Tue July 21, 2009 10:51 AM User is offline

Just adding a bit of humor to aid in the explanation, but if you are dropping below 7"/Hg at normal highway speeds and not driving on a mountain top, sounds like you also have engine related problems. Didn't mean to ruin your day.

knightgang on Tue July 21, 2009 11:44 AM User is offline

Not a day ruiner. I am new to this board and am not familiar with the people that are posting. I have been on other boards, boats, diesels, BBQ, etc. and there always seems to be a Butt head that has to over simplify issues and arrogantly point the finger to other issues that either are not there or already ruled out. I did not mean to be to touchy about it.

On the other hand. I did find the break in the vacuum line that GMT pointed me to. I did have to remove the washer res and battery to fix it because it was broken right at the passage form the engine compartment to the res tank on the underside of the car. I cut out the damaged section and repaired it with a section of small engine fuel line that would tightly slip over each end of the vac line.

Test drove and for the most part it has fixed the issue. In normal driving there is a slight transfer to the floor, but the majority 80% or better i guess still comes through the vents and a pretty fair CFM rate. But, when you really get on it in hard acceleration, the transfer is more like 85% to the floor/defrost and 15% to the vent.

I can only assume that the small amount of airflow I noticed under the dash yesterday around one of the actuators could indicate that one or both of the actuators have a small leak and when enough vacuum loss is there from hard acceleration the actuators are not holding. Could that be it? Should I replace the actuators? GMT?

Thanks

knightgang on Tue July 21, 2009 11:47 AM User is offline

BTW. This car did sit for about a year and a half until I was able to get around to repalcing the intake manifold gasket. The vent vs. defrost issue had occured before intake gasket gave up. The car has been back in running order for about 8 months now and has no engine trouble.

GM Tech on Tue July 21, 2009 11:55 AM User is offline

You have fixed it-- Not to worry about very small leaks- not worth it in my book...

It is a "W" car- they fail just as you described- Once you see a couple of dozen- you just fix what is wrong--GM never made just one of anything-- so the trend presents itself- and you go on from there....

Glad we got it right......

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

knightgang on Tue July 21, 2009 11:57 AM User is offline

Thanks for the help!!!

NickD on Wed July 22, 2009 7:40 AM User is offline

Minor leak, major leak, what is the difference? And will a minor leak change into a major leak? Shouldn't hurt as long as the vacuum source is greater than the load. But when the load exceeds the source and you are stuck in default mode, have to make repairs. Local store sells vacuum hose for 20 cents a foot, good quality, so buy five bucks worth, that's 25 feet and replace all of it. Always find leaks in the climate control mode switch that is normally both electrical and vacuum, take those apart and clean them with new grease, dried grease is usually the major problem. But didn't do that on my daughter's 98 ZX2, Ford sold me the switch separate for $9.50, couldn't believe that low price.

Anyway, when it's done, with the vehicle sitting overnight and before you start the engine, can manually work the mode switch, and hear doors opening and closing.

May not even be the climate control vacuum, with other vehicles, the reservoir vacuum source was shared with the cruise control and/or the parking brake release, and if any of those fails, nothing works. That was last month with my 92 DeVille, the line from the parking brake release had to be replaced, about 15", was that a dog, didn't think my old body could still bend that way. But always defrost, no cruise, and no parking brake release.

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