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Tom_

HVAC Plastic Vacuum Line

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Hey guys, I have a vacuum leak that causes the HVAC controller to lose vacuum during acceleration and default to defrost.  It comes out the correct vents when you're just cruising, idling, or coasting downhill with your foot off the gas.  I know it must be a small leak and the plastic line going from the check valve through the firewall is my main suspect.  I'm going to test this tomorrow by connecting a rubber vacuum hose from the check valve through the passenger window to the HVAC controller and driving around to see if I maintain the correct vent selection.  The only problem I foresee is if and when I identify this hose as the problem, how in the world am I supposed to pull new line though the existing location in the firewall?  It looks like it's part of the group of firewall connectors wedged between the passenger side strut tower and the firewall. Is there another location to pull through or am I stuck with this spot?

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Benefits of AACA Membership.

I replaced the check valve last summer but to be fair I could have gotten a defective one since the issue didn't change.

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I, like Harry would look at the check valve ( which is a combination valve and "T")  one line goes to the HVAC controller the other line goes to the cruise control.

  I have seen the check valve come apart at the seam where the two halves go together. But check the lines also since we are dealing

with vehicles that are 27 + years old.

Another possibility is the valve at the brake pedal........some places call it the "dump" valve.   It is there so when you hit the brake pedal, it dumps the vacumn and shut off the cruise control.

 

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I will check that cruise cutoff valve but the cruise control works perfectly. Has no trouble accelerating even though while it accelerates up a hill the HVAC loses vacuum.  I replaced the cruise servo and vacuum line a couple of years ago when the old one went.  I'm going to buy another check valve and a T at the parts store and replace that little plastic Y near the reserve tank as well. I trust nothing plastic on these things anymore. Especially near engine heat. 

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Tom if you can't find that little plastic Y connector I can send you one.  I try to pick up at least one every time I go to a PnP.

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Thanks for the offer Stanley.  I have a box of T's, F's, and Y's for different kinds of lines and if I don't find something in there that will work I'll let you know.

 

EDIT:  I just replaced all the small vacuum lines and the check valve around the cruise servo and reserve tank.  I cut back the hard plastic line under the hood to between the fan motor and firewall and put new rubber line over it with a small hose clamp and a dab of silicone around the joint.  I have much better vacuum to the hvac controller and it really closes off the defrosters allowing much more air to come out of the bi-level vents.  The issue with it slowly going back to the defrosters during moderate acceleration still persists.  I am highly suspect of the new check valve.  The old one didn't seem to be working and the new one wouldn't work out of the box until I tapped it against the air box and used my freebie vacuum tester (my lungs) to pull on the output that would go to the hvac controller while I covered the output that ties into the cruise servo/reserve tank side.  Even then it only worked if I sucked pretty hard and wouldn't make a great seal.  I don't know if there's a way to make these little valves work better out of the box but I think I'm going to try switching to a different style check valve like this universal one and put a T after it to replicate the function of the existing valve.  

Edited by Tom_ (see edit history)

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I have replaced the check valve with a new 4 Seasons Part No. 74808 and It is performing better than it has in a long time.  But, under heavy acceleration like up a hill from a stop light it still shifts air to the defrosters.  Is there a way to check if the controller or servos are leaking?  Would that be a reasonable next step in the troubleshooting process?

 

FYI the Dorman Help! check valves are functionally worthless.  They allow for two way flow in 4 out of 4 units I bought from different stores in town right out of the box.  If I have to replace one in the future it will be with the 4 Seasons unit above.

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I did extensive work on an HVAC controller several years ago and replaced the valves with externally mounted Ebay units. That worked flawlessly, but is a lot of work. That said, just drawing a vacuum on the black main feed to the controller yielded inconclusive results when testing to see if the whole assembly would hold vacuum. There is another check valve inside the controller with multiple connections to the four valves, and yes, it is possible to have a valve fail, although rare. The valves appear to be primarily gravity operated to close, although there may be a very light spring, and I have not dissected one. The reason I mention this is they will likely leak vacuum if not stood on end like the installed position. You can also check the individual actuators to see if they hold vacuum. The best way is to remove the entire vacuum connector on the bottom of the controller and apply vacuum to the individual ports. The connector is held in place with a threaded stud and something like a 7mm or 8mm nut in the middle.

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On 5/18/2018 at 3:54 PM, Tom_ said:

used my freebie vacuum tester (my lungs) to pull on the output that would go to the hvac controller while I covered the output that ties into the cruise servo/reserve tank side. 

 

You should consider getting a hand vacuum pump. Then you can isolate each vacuum circuit and pull a vacuum on it while watching the gauge to see how quickly the vacuum drops. I would start by pulling a vacuum on the reservoir tank to make sure it will hold a vacuum as it should. You could also run a vacuum line from under the hood inside the car so you can watch the vacuum gauge inside the car while you're driving to see what is happening with the vacuum.

 

1 hour ago, Tom_ said:

under heavy acceleration like up a hill from a stop light it still shifts air to the defrosters

 

That sounds like the way a vacuum system would do if it didn't have a vacuum reserve tank and was relying on just the engine vacuum. The default is air coming out the defrosters when there is no vacuum in the system.

 

Vacuum_lines-4.JPG

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Does the Reatta share a vacuum tank with any other vehicles of the era or is it a Reatta specific part?  That would make sense as the check valve requires vacuum to be held on both ports and the ports are free flowing between each other.  So if one branch leaks the other will lose vacuum.

Edited by Tom_ (see edit history)

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X2 on checking the reservoir tank. I just checked mine by pulling the line from it and it held some amount of vacuum after sitting for six hours, so it would seem the controller should hold reasonable vacuum also if tested separately. 

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