Pttrader

New to this forum, I have a 96 purple C4 automatic like the one in the upper left corner of this website. Hoping someone can help me with this particular problem. This car is stock (Not for long) and has the manual heater/ac control.

While I am driving with the fan on Vent it will sometimes switch to the Defrost position and then back again, especially when I let off the throttle. It acts like a vacuum leak. If the car has a vacuum actuator I'd say it's leaking, or the hose attached to it.

Worse yet I looked in the Chilton's and they show a vent duct assembly, nothing else.

Mr. Peabody

Welcome to the forum. You've got one of the best year C4 vettes there is

Mine is the auto system and when in auto mode it will switch from vent to defrost. That being said, pop off the injector cover cover from each side of the engine and check the thin black vacuum tubes the come out from the passenger side of the engine. There is a sort of 2 into 1 coupler that often breaks.

Check on the for sale section of the forum or on ebay and purchase yourself the two volume red-cover shop manual for your 96, that's what we all do. You should be able to get a set for $75 - $150.

Most likely that someone who has the answer to your exact question will chime in soon

Pttrader

Thanks for the info. I'll check them in the morning. Yes, I need to get the shop manuals.

pianoguy

If the check valve under the passenger side fuel rail cover isn't the culprit, then you may have a problem with the HVAC programmer (fairly common for '96), and at some point, it'll likely get stuck on defrost. Here's a good thread on it:

http://forums.corvetteforum.com/c4-t...l-problem.html

LannyL81

Does the 96 cars with the manual temp controls have the same programmer as the ones with the digital temp controls?

If so, then the problem is the vacuum connector at the programmer. Easy fix is to remove the programmer and insert stiffer tubing into the vacuum connector lines to keep the lines from closing off under vacuum.

Pttrader

I looked under the plastic covers on the motor, nothing glaring at me. I guess a hand held vacuum pump and if I remember a can of carb cleaner one could spray near the hoses and the rpm would change indicating a vac leak. Not sure this is a good idea, or that it would work well on a computer controlled car.
I started looking closer for leaks and will check out the items mentioned in the link. Looks like this has been an issue for the 96'.

pianoguy

Under the passenger side fuel rail cover, toward the back, you'll find one of these:

http://www.napaonline.com/MasterPage...One+Way+Vacuum

You can pick up a replacement for a couple bucks at any auto parts store. It also is plumbed into the cruise control, so if that is flaky as well, this could be the culprit.

Pttrader

Thanks for the part# pulled off the right cover and inspected the check valve. The cruise works great. Gonna go to the parts store and put a new one in anyway before the contortist act of getting under the dash.

Pttrader

I replaced the vacuum check valve. Napa couldn't find one but Autozone had one. Problem still there. Gotta start stretching for the contortionist act under the dash.

pianoguy

Just pull the driver's seat and track out - makes getting in there possible without having to be a member of the cirque-du-soleil. I removed the carpeted hush panel and also the metal panel underneath it, and one small elbow duct. The programmer box is just a bit above the gas pedal, and is held on by one bolt (snaps in on the upper end). The worst part of the job is getting the vacuum harness off of it, as they provided very little slack. It is held onto the box by a ridiculous little push-nut. I used a small-diameter long blade screwdriver to pry the push-nut out until I could grab ahold of it with a needle-nose pliers and twist it until it broke. You'll want to reinstall using a small nylock nut instead. Once you have the box out, you can choose your method of fixing the "soft nipple" problem - some have trimmed them back a little, others have inserted some rigid tubing, or else you can replace the vacuum manifold with one out of an earlier model that has harder nipples (that's the route I took). Meanwhile, this is too much nipple talk for this early in the morning ;-)

Nick DL

Thanks for the detailed location, I need to get up there and check it out. I already checked the vacuum and it is working just fine.

Also, I just wanted to say that it is never to early to talk about nipples.

Pttrader

Being such a beautiful weekend I was out cruising, seems it had rained the last two weekends. Sure thank you for all the detail on getting to the bottom of this. Yes, I'll yank the seat out and crawl under there and remove it sometime this week.

I had just got the car out of the shop Friday with new Corvette rubber door seals. It seems the door panels at the top are not hooked in properly and need to come back off and rotate them down and in correctly so the tangs are in the slot so to say. The weather strip on the back edge of the door wants to push the door panel out away from the door in the top rear area. So much for bringing the car to what I thought was a good body shop. I had talked to the shop and got a song and a dance. I noticed they used masking tape (The cheap yellow stuff) to hold the hush panel in tight. Only problem is that I could see that the tape had already let go and I had told them about the corvette fever article on the internet expaining that these should be reattached using 3m strip caulk. I had them replace the window guide cause it was broken. The told me they had to remove the glass on both sides to install the outer door seals. There again I remember reading how you loosen the window stop and that will allow you to run the window down enough to drill out the rivits. So when I returned Sat morning to show him how the professionals made some obvious boo boos all I got was back peddling on getting things resolved.

Since I put it on my credit card we shall get things resolved, been down this road before. Here I thought I would let the professionals do this so everything is right and I would work a weekend doing what I do to pay for the quality work I was gonna get. Well, the best laid plans...

Pttrader

At long last after 3 trips under the dash (Working upside down) I finally tackled the programmer swap. The other 2 trips were for the failed power brake booster, and the bad ground of the horn circuit.
Once again thank you Pianoguy for your step by step instructions and link which were spot on in getting er done, only I let it go for over2 months.
The programmer was more expensive than I wanted to pay, seems the cheaper ones were gone so I bought NOS for $250, still about half of what the dealership wanted with taxes.