HVAC Control Issue

[Tom_]

On a 600 mile drive over the weekend I noticed a problem with the HVAC damper system that has not happened before.  While cruising or decelerating with the vents set to "BI-LEVEL", everything functions as it should.  However, while accelerating the air slowly stops coming out of anything but the defrosters.  Now I understand that this is because of a vacuum problem but I cannot for the life of me find a hissing or whistling sound under the hood.  Do our cars have a check valve in the line somewhere?  What are some common places to find leaks on these cars?  Does this sound like a check valve issue to any of you?   

[2seater]

Yes, there is a check valve in the tee at the firewall directly behind the throttle area. One leg of the tee goes to the HVAC controller and the other feeds to the cruise servo and vacuum reservoir below the air inlet tube to the throttle body. One of those hoses may be deteriorated or possible the vacuum fitting on the bottom of the HVAC controller is loose, located on the far right below the glove box.

[Tom_]

I thought so.  Does anyone have an image of their check valve location?  There has very clearly been some vacuum lines replaced and I'm not sure it's all connected as it was originally.

[Ronnie]

[Tom_]

Thanks Ronnie.  It was there and the line over to the check valve had been replaced with a slightly longer hose with a hose clamp.  I have inspected the lines you showed and they are pliable and there are no signs of leaks.  So I have ordered a direct replacement and I'll update when it comes in.

[89RedDarkGrey]

9 minutes ago, Tom_ said:

 So I have ordered a direct replacement and I'll update when it comes in.

 

Of what?

[Tom_]

The vacuum check valve.  It was only $13.  The universal ones are available for around $6 but I wanted an identical replacement.

Edited April 18, 2017 by Tom_ (see edit history)

[89RedDarkGrey]

I brought this up a while back- a few Members thought it was sound advice. At the rear of the throttle body, coming off the EGR adapter is a black metal tube. The large diameter tube is where a vacuum brake booster would attach. Hardly anyone notices this, but there is a rubber cap over the end. Rear of engine heat plus 28 years really deteriorate rubber hoses and the like. I renewed mine- when I found it was severely cracked and leaking. You don't want a chunk of it ingesting into the engine.

 

[drtidmore]

 

The vacuum line to HVAC controller needs to be carefully inspected.  It is a hard, rigid plastic line and it has been exposed to 28 years or so of engine heat as it sits directly above the rear exhaust manifold.  I have first hand knowledge of how brittle it can become as while changing spark plugs not long ago, the simply pressure of my arm against it caused it to snap and even worse, the break was just inside where it vanishes into a wiring loom.  I was able to fish it out sufficient length from the loom to get a short piece of rubber hose over it to bridge the broken ends.  

 

Thanks to Ronnie for posting the excellent pics and hope you don't mind me borrowing this one as it clearly showed the line I was discussing.

 

 

Edited April 18, 2017 by drtidmore (see edit history)

[Tom_]

I replaced the check valve and the problem improved but did not completely go away.  I assume there's still a small leak in the hard vacuum line going to the HVAC controller.  I am replacing the evaporator following a long and expensive trip through the A/C system this weekend and I will check the line when I have it undone and out of the way like Ronnie's site says in the instructions.  If I find anything I'll snip out the section of effected line and put a length of soft rubber vacuum line in its place with zip ties as a patch.  

 

Do any of you gentlemen have tips for the evaporator replacement procedure?  It's my first one and I'm getting kind of spooked from all the posts about how its a major PITA.

[89RedDarkGrey]

27 minutes ago, Tom_ said:

Do any of you gentlemen have tips for the evaporator replacement procedure

 

http://reattaowner.com/roj/component/content/article/95-accessories/heat-a-ac-system/325-evaporator-replacement   is an excellent tutorial, very concise.

[Tom_]

Well I got the evaporator replaced this afternoon.  It was as big of a PITA as I was expecting but I got it done in about 4 hours.  The plastic cover had at some point gotten oil or power steering fluid on the bottom of it and a few pieces came of in my hand as I removed it but hey, that's 1 less screw to put back in on reassembly.  Why that thing has 11 bolts holding it on I could never tell you.  At any rate, the evaporator I received had a vacuum on it and a little plate over the fittings with seals holding the vacuum which was a nice little touch to let me know it was at least good when I put it in.  Now I just need to take it to get charged and hope it holds for my trip to Florida in a couple weeks.  That tutorial came in handy in finding the bolts I couldn't see 89DarkGrey so thanks.  I've attached a little picture showing just how nasty these things get when they aren't kept clean and leak slowly.  Keep in mind I cleaned this in 2013.

[89RedDarkGrey]

That must've smelled nice     I can't wait to do mine later this year- not just for cold, but the compressor also runs when MAX HEAT DEFROST is on- to dry out the cabin. Without AC in the Winter- your breath condensates on the windshield, making a mess of it.