HVAC stopped blowing around 55mph

[macleanflood]

Am I high, or does this sound plausible?

D - All of the Above

I'll take check valve for $1000, and maybe a side bet on the vacuum reservoir.

-Mac

 

[freedom_in_4low]

Went to tackle the A/C problems tonight, only to find that the dang thing blows plenty cold. I tried to explain this to my wife, but if there's a topic, she knows about it. Turns out what is happening is that the face vent blows cold until (presumably?) there's not enough vacuum to hold things steady because then it starts blowing cold on my feet instead. Then if you slow down, it moves back to the upper vents.

Interestingly, I swear when I looked at this a few months ago it would only blow on feet/defrost, but now the sliders control things 100% correctly, outside of this one vacuum issue. I don't claim to know how this system works, but it would appear that I probably don't have a vacuum leak, since most of the time it's perfect. Or at least not a leak in the typical way. I'm guessing it's a check valve or maybe the vacuum box under the battery or the hose leading to it are the issue. It's not able to store (or deliver) the vacuum the HVAC needs when the engine vac is at a minimum.

Am I high, or does this sound plausible?

That's how it goes. There's enough vacuum at idle, but in higher throttle conditions, like accelerating or maintaining speed up a grade on the highway the engine vacuum can't keep up with the leak, you slowly lose vacuum and the air goes to the defrost.

My 99 was doing that when I bought it, in Amarillo, in late June. I kept cool by keeping it under 65.

 

[hear]

ok, so I'm pretty certain there are TWO vacuum check valves, one right at the manifold, and one near the HVAC tee. I did some work on my 02 a while back, and that's how it is. The check valve is after the tee, so on the branch that goes through the firewall. And of course the other one at the intake. Looking at the FSM for the 97, there's only evidence of there being a single check valve.

On MY 97, I have too much crap in the way to see what's going on.

On HER 97 (the one in question), there is only the check valve at the intake. I didn't feel like getting sweaty at 11:30pm (for this reason, anyway...wink wink), so I'm just gonna order some check valves under the assumption that they're probably bad.

But for the record, how many check valves are there supposed to be? And/or is the 97 yet again different?

 

[macleanflood]

I'll check mine in the morning.

-Mac

 

[hear]

I'll check mine in the morning.

-Mac

You're a dirty liar.


On a hunch, I took one of the check valves off the 02 (which is top off/doors off right now anyway) and put it on the HVAC line on her 97. I didn't get to road test it, but she took it ~40 min for a babysitting job and said the A/C did great.

I did measure the vacuum at idle and it was pulling 16 inHg, which looks like is on the lower end of the limits. I have new vacuum line coming today, I'll just replace all that & the other check valve too and see if it can get a little stronger. It definitely takes a minute at startup before the vents change from defrost to face. Then when I hit the gas in neutral it immediately flipped over. So it's not holding vacuum on the HVAC side when the jeep is off. Is it supposed to?

 

[Peppergat]

I’ve been working on this very thing today. I replaced both check valves and vacuum hoses so everything works as it should. Mine does the same thing. It doesn’t hold vacuum when engine is off.

 

[macleanflood]

Mine appears to be right off the intake.

Got up this morning and went straight into 5300 sq ft of metal roof... completely forgot!

-Mac

 

[B1Toad]

Take a look at this thread of mine from last year where I fixed this very problem. Especially the part about the Motorcraft check valves in post #7 and #8. Don't buy the Dorman.

Post in thread 'Vacuum loss at high RPM'

Jul 4, 2024

The manifold is on the driver's side of your engine. I drew some circles on the attached pic showing the locations (roughly) of the vacuum lines where the 2 check valves are located.

• B1Toad

• 

 

[macleanflood]

I swear I only have one...but I do have a lot of wires around the HVAC hole since I use that for most of my accessory wiring.

-Mac

 

[Rickyd]

You're a dirty liar.


On a hunch, I took one of the check valves off the 02 (which is top off/doors off right now anyway) and put it on the HVAC line on her 97. I didn't get to road test it, but she took it ~40 min for a babysitting job and said the A/C did great.

I did measure the vacuum at idle and it was pulling 16 inHg, which looks like is on the lower end of the limits. I have new vacuum line coming today, I'll just replace all that & the other check valve too and see if it can get a little stronger. It definitely takes a minute at startup before the vents change from defrost to face. Then when I hit the gas in neutral it immediately flipped over. So it's not holding vacuum on the HVAC side when the jeep is off. Is it supposed to?

Do you have access to a hand operated mityvac type vacuum pump? If you unplug the vacuum tubes from the back of the controls and try each line you'll find out which vacuum pod/circuit is the problem. As well as get a leakdown baseline from the "good" ones.

I went through this on an s10 a while ago.that tool was priceless.

 

[hear]

I do have a mityvac hand pump, but seems like I have a hard time getting the fittings on it to seal against the actual vacuum fittings in the jeep (the engine bay ones). There is probably a skill issue involved.

 

[Rickyd]

I do have a mityvac hand pump, but seems like I have a hard time getting the fittings on it to seal against the actual vacuum fittings in the jeep (the engine bay ones). There is probably a skill issue involved.

I'm thinking of the little connector to the controls. A tapered plastic fitting should do the job to test each circuit there no?

Then put a cap on the manifold pipe and test its integrity from the same in dash connector?

 

[macleanflood]

Wrap it electrical tape.

-Mac