I have a '98 with dual temp control. No matter which mode is selected, the air seams to come from all the outlets at a relatively
low volume with no change in direction (floor, defrost, vents etc). The blower speed works and temp control works. The A/C turns on in
the defrost position. No HVAC DTC codes. Where do I start looking?

 

Very very common issue. The mode is controlled by engine vacuum. The vacuum line starts the BACK of the intake manifold (its a very thin NYLON vacuum line). It runs THRU one of the main wiring harnesses, under the battery and connects to the plumbing fictures and vacuum resivour in the passengers fender well, from there it goes thru another main harness and connect to a manifold that electricly selects what vacuum servos are actuated.

What usually happeens is:

1. Battery acid eats up the line in the harness under the battery. NOTE! If you see RUST on the frame below the battery,,, you had an acid spill some time in the past and you have a LOT of work to do!!

2. The rubber 90 deg fitting pops off the small vacuum fitting on the back of the manifold.

Start at the back of the intake manifold (its VERY HARD to see) and make sure that its still connected. look at the line in the wire harness and test it for leaks, make sure that the pluming in the fender well is in tact.

BC

 

Thanks Bill, you've been a big help in the past.

Couple of follow up questions?
1) Wouldn't a vacuum leak show up as rough idle and set some codes?
2) Are there any pictures or diagrams of the areas you're suggesting to look at?

Thanks

 

The vacuum line inner diameter is the size of a pencil lead so,, the vacuum leak is so very small that the LTFT, would EASILY compensate for the difference. If it was disconnected, you would be hard pressed to even hear it. No, it does not set any codes.

Pictures,,, Not here at work but maybe when I get home..

If you look in the passengers wheel well, there is an access panel that you can remove to access inside the fender well. Remove that access panel and you can get to everything.

 

Bill, any pictures?

Can I access anything by removing the battery?
How do I check for vacuum leaks?

Thanks

 

Pictures will be a little hard for the area on the back of the intake he is talking about. But if you put your hand around the back of your intake you will have a tiny vacuum line the comes out. Make sure that is in there good and the boot is good. If you have messed around with the area around the back of the intake or intake good chance you might have pulled this line out the back. (it doesnt have a lot of slack)

You can then take your passenger wheel off and take the wheelwell/plastic panels off that go behind your wheel and connect to your fender. You should see the vacuum system he is talking about. (might be a easier way to access but that is how I got to my PCM and saw the vacuum lines and canisters there.

The other end would (I believe) be on the back on the A/C controls themselves, but not positive there.

If you have 2 people you could check for a leak in the line by plugging one end and blowing in the other, as redneck engineering of an answer that is.

Bill is the man... Just follow his advice and you can't fail. Just had my engine out so I know the vacuum line he is talking about and it is so small it wouldnt really cause much of an idle problem. And I have had this issues in other cars with the vacuum line and A/C issues.

If I am incorrect anywhere there Bill (or anyone) please correct me.

 

Wow thanks mine is doing this as well.

 

This might help!

 

And this one...

 

How about a write up.
https://www.corvetteforum.com/c5-gen...with-pics.html