When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
stuck on defrost vacuum issues. all in one place now
Stock XJ Cherokee Tech. All XJ Non-modified/stock questions go hereXJ (84-01)
All OEM related XJ specific tech. Examples, no start, general maintenance or anything that's stock.
So how about this? Start the motor. HVAC controls all OFF. Does it idle OK? Blip the throttle a few times. This helps build up vacuum. Shut the engine down. Then pull the line off the "T" that feeds through the firewall to the HVAC controls. Do you hear the whoosh of air relieving the vacuum? You can repeat this if unsure. Also do this again, but instead of pulling the line right after shutdown, let it sit for 15-20 minutes before you pull it. Do you still hear the whoosh? Is it the same or a bit weaker?
With the HVAC controls off, all vacuum ports at the control should be closed. If you're able to build and maintain vacuum, this would indicate that the lines from the intake port to the reservoir are all good. Waiting 15-20 minutes (you can wait longer), will help determine if there's a very small leak or bad check valve. If no "whoosh", you have a big leak. If weak whoosh after a brief waiting period, you have a small leak.
If all good so far, start the engine and build up vacuum, HVAC controls OFF. Then shut down. Then quickly slide the controls from off to Vent. You should be able to hear the vent doors operate. You can then try sliding over to heat. Again, you should hear the doors moving. You won't get much more than 2 maybe 3 setting changes before you run out of vacuum. If you get no movement, and you're sure you have vacuum at the "T", then you have a leak somewhere in the dash. You can try removing the radio to see the HVAC controls below it. Then take the mounting screws off and slide the HVAC controls out a bit (don't disconnect anything except the fan switch if needed) in order to get a better look at the vacuum harness and connections.
At some point, you may need to invest in a vacuum gauge to measure the vacuum rather than rely on "hearing" it, and/or a vacuum pump to actually test it without the running the engine. You can then test the individual components as well like the servos and check valves.
My'88 will loose all HVAC vacuum if it sits more than a few hours. As soon as I start the engine I can hear all sorts of movement of the doors. I don't know how long it should hold vacuum but it's long enough. I don't loose vent control accelerating onto the freeway, or climbing up big hill.
Oh gawd. At least this is a clear procedure to follow. Thanks I’ll try it.
Now that the vacuum source is finally revealed I can try the vague directions I got from the jeep master, cruiser54, too. But without a gauge I’ll have to try sucking and sticking my tongue in the hole.
Maybe I need to have a vacuum gauge to go with this jeep though.
Ao I wasted a hundred bucks on new vacuum hoses? Ugh
I don’t even have to try sucking on the “source” at the manifold. Every time I blew smoke in there (into the intake manifold)it came out the throttle body shaft and the airbox.
So obviously that wouldn’t make a vacuum.
And everyone said that the throttle body is supposed to leak, except one dude online who refurbishes them with o-rings from home depot.
So how about this? Start the motor. HVAC controls all OFF. Does it idle OK? Blip the throttle a few times. This helps build up vacuum. Shut the engine down. Then pull the line off the "T" that feeds through the firewall to the HVAC controls. Do you hear the whoosh of air relieving the vacuum? You can repeat this if unsure. Also do this again, but instead of pulling the line right after shutdown, let it sit for 15-20 minutes before you pull it. Do you still hear the whoosh? Is it the same or a bit weaker?
With the HVAC controls off, all vacuum ports at the control should be closed. If you're able to build and maintain vacuum, this would indicate that the lines from the intake port to the reservoir are all good. Waiting 15-20 minutes (you can wait longer), will help determine if there's a very small leak or bad check valve. If no "whoosh", you have a big leak. If weak whoosh after a brief waiting period, you have a small leak.
If all good so far, start the engine and build up vacuum, HVAC controls OFF. Then shut down. Then quickly slide the controls from off to Vent. You should be able to hear the vent doors operate. You can then try sliding over to heat. Again, you should hear the doors moving. You won't get much more than 2 maybe 3 setting changes before you run out of vacuum. If you get no movement, and you're sure you have vacuum at the "T", then you have a leak somewhere in the dash. You can try removing the radio to see the HVAC controls below it. Then take the mounting screws off and slide the HVAC controls out a bit (don't disconnect anything except the fan switch if needed) in order to get a better look at the vacuum harness and connections.
At some point, you may need to invest in a vacuum gauge to measure the vacuum rather than rely on "hearing" it, and/or a vacuum pump to actually test it without the running the engine. You can then test the individual components as well like the servos and check valves.
My'88 will loose all HVAC vacuum if it sits more than a few hours. As soon as I start the engine I can hear all sorts of movement of the doors. I don't know how long it should hold vacuum but it's long enough. I don't loose vent control accelerating onto the freeway, or climbing up big hill.
Test results:
Started it. HVAC controls off. Runs a little rough. Not purring like it does when it works well.
Vents still stuck on defrost. (Had to check). Had to move it over in the driveway a spot. HVAC controls off. Tapped gas a couple times. Turned it off.
Disconnected T and big whoosh.
Repeated by starting and running for a minute and tapping the pedal twice again. Waited 15 min.
Slightly smaller whoosh. Hard to tell.
Repeated and waited 1 hour.
Slightly shorter whoosh maybe. Hard to tell.
I’m thinking it might be worth $30 to me to just go get a vacuum gauge if it means making this easier to notice.
They sell some kind of hand pump thing with a gauge and a hose. Is that what I need?
I have. And I talked about it each time. It went pop whoosh the first time I unplugged it after the jeep had been sitting for a day. It looks new, the parts I can see. And I pumped a lot of propane in there all around it and on the connection.
Nothing. But a new one was only $16 so it’s oncthe way, just to rule this out. The new vacuum lines will at least replace all the rotting hard lines and odd elbows. Tired of wondering what ancient plastic part will break next.
After running it again earlier I left it for about 5 hours and checked that T fitting. Same whoosh as earlier.
So maybe driving it that little bit to park it made the first vacuum stronger this morning.
Looks like it’s holding vacuum at that spot so tomorrow morning I will be moving on to the next part of the procedure, taking the radio out and checking behind the climate controls.
If all good so far, start the engine and build up vacuum, HVAC controls OFF. Then shut down. Then quickly slide the controls from off to Vent. You should be able to hear the vent doors operate. You can then try sliding over to heat. Again, you should hear the doors moving. You won't get much more than 2 maybe 3 setting changes before you run out of vacuum.
I got movement on vent. Got movement on heat. Turned it off and heard movement again!
So I started it again and the a/c works fine on max, norm, and bi-level again.
I let it warm up to operating temperature and it never went back to defrost. I was scared to try heat while it’s on. Don’t want to break it. Ha
So does any of that make any sense? I’m going to bed shaking my head.
A few years ago I chased a vacuum issue with my HVAC controls. Everything seemed to test fine, at first. I blew through the check valve going into the fire wall and it only let air through one way. Chased my tail trying to solve the issue without success, eventually returned to the check valve and tested it with a vacuum pump and found it wasn't holding vacuum as strong as one would expect it to. Replaced the check valve and it cured my issue.
Super strange, don't know why or what happened, but they're cheap. Throw a check valve at it just to rule that out.
Well, sliding the temp lever to anything but full cold should open the heater valve when the mode selector is in any position except OFF. At full cold, the valve is closed. Are you sure the heater valve is connected? If so, unplug the vacuum line and cap it. If it all still works, then perhaps the heater valve diaphragm or vacuum line is bad.
Reconnect it and note the position of the arm when "Heat" is selected and temp selector is slid to WARM (valve open). Then turn everything off and note the arm location (valve closed). Now turn on the AC, move the temp to full cold (valve closed). Finally, while still on AC, slide the temp over to about the middle (valve open). Here's the table of setting with the valve and door positions. When make sure you have the fan on as well, any setting.
If the valve is bad, Jeep ditched it entirely in '97. Many simply plug the vacuum line as the valve defaults to OPEN. Many more remove the valve (it's plastic) and simply use 2 barbed unions to join the lines. If the hoses are different diameters, you'll need "adapter" unions.
Well, sliding the temp lever to anything but full cold should open the heater valve when the mode selector is in any position except OFF. At full cold, the valve is closed. Are you sure the heater valve is connected? If so, unplug the vacuum line and cap it. If it all still works, then perhaps the heater valve diaphragm or vacuum line is bad.
Reconnect it and note the position of the arm when "Heat" is selected and temp selector is slid to WARM (valve open). Then turn everything off and note the arm location (valve closed). Now turn on the AC, move the temp to full cold (valve closed). Finally, while still on AC, slide the temp over to about the middle (valve open). Here's the table of setting with the valve and door positions. When make sure you have the fan on as well, any setting.
If the valve is bad, Jeep ditched it entirely in '97. Many simply plug the vacuum line as the valve defaults to OPEN. Many more remove the valve (it's plastic) and simply use 2 barbed unions to join the lines. If the hoses are different diameters, you'll need "adapter" unions.
Thank you. I will start trying to figure out what the heater valve is and where it is located.
A few years ago I chased a vacuum issue with my HVAC controls. Everything seemed to test fine, at first. I blew through the check valve going into the fire wall and it only let air through one way. Chased my tail trying to solve the issue without success, eventually returned to the check valve and tested it with a vacuum pump and found it wasn't holding vacuum as strong as one would expect it to. Replaced the check valve and it cured my issue.
Super strange, don't know why or what happened, but they're cheap. Throw a check valve at it just to rule that out.
Oh yeah the heater control valve! I’ve been wanting to bypass this since the last time it leaked.
I just got the two new check valves and the hoses to bypass the HCV.
But it’s suddenly running so nice and blowing a/c out the front vents, I don't want to mess with it.
These kinds of mysteries are no fun. But I’ll be replacing all the vacuum lines in the engine bay along with the Ts and check valves soon. And I’ll bypass the HCV when I replace the pushrods and rockers soon.
I just pray it’s not having issues behind the dash.
This thread got confusing. It seems to have been combined with different threads or something.
I was trying yo get actual proper directions for finding a vacuum leak in my original thread. That title made more sense if we finally figure out how to explain this to lay people, like me, here.
So the closest thing to proper directions sounds like what our jeep master, and I mean that sincerely, cruiser54 had to say.
Originally Posted by cruiser54
It's very simple and nearly the same across all models.
Identify your vacuum source at the intake manifold. Got vacuum?
Go to the reservoir. Got vacuum?
Got vacuum going into the plastic line at the firewall? If you pinch off the vacuum at the intake, what happens to the vacuum at the firewall? Does it go away? If so, the reservoir isn't holding vacuum.
So I finally got myself a cheapo vacuum gauge/pump tool to try to decipher what the guru has said. I’m still not sure if the source would be the end coming out of the manifold or the end going into the firewall on the drivers side.
The new vacuum ball holds a vacuum. But, how much pressure? All the way to 22? Somewhere in the middle between 16-22 like I did here? And how long to let it sit to know if it holds?
Edit: Sorry, some of this is googleable. Now that’s a word. Ha.
“The correct vacuum at idle for the 4.0 is between 16-20Hg”
I imagine a proper test would be to the maximum. 20Hg
How long? Wow the google AI is rocking it today!
“To check a car’s vacuum lines, you can attach a vacuum gauge and start the engine, then let it idle for a couple of minutes”
05-14-2024 | 01:01 PM
