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.
The picture for the 4.0L looks like this in my FSM though, if it matters. It’s nearly identical.
It just struck me that these are not complete! They don’t show the lines to the heater or the vacuum reservoir.
Ive been looking at this diagram all day. I don’t understand how it helps me find the leak.
I did the propane bottle/rubber hose check again today. Checked every inch of this system and no change in idle. Checked around injectors and manifold. Even shot a little gas behind he heater controls in the cabin.
I put in a new blower motor about a year ago and cleaned it out in there with scrubbing bubbles. It made the heater work again that winter. Everything worked. I don't know exactly when it happened. But the thing is stuck on defrost again.
How do I check the blend door and that I didn’t mess something up when I replaced the blower motor and the other thing that had fried with it? Some kind of control circuit or something. I forget the name.
And mine is a 93 so do I have the option of going in through the glovebox if I have to replace that?
I think what Cruiser54 meant is that the vent doors (dash, floor, defrost. recirc) are all vacuum operated and the control sets which vents distribute the air. The Blend door is cable operated and only control the temperature by diverting the air either through or around the core.
Blowing out the defrost only is due to the lack of vacuum to the HVAC controls. All of the vent servos "default" to Defrost only in the absence of vacuum. The upper right corner of the diagram show the vacuum source and reservoir.
Here's how it works. At idle, with a small vacuum leak, the reservoir can hold enough vacuum for the vacuum actuators to work. If you go drive it and are accelerating, the vacuum goes away under the load and the reservoir can't keep up.
You have a very simple and common issue. It happens all the time. I'm betting there is a compromise/leak somewhere between the vacuum source at the intake manifold and the reservoir under/behind the front bumper. It's usually around the battery.
It sounds like this is all obvious to you guys. But that doesn’t make any sense to me. To climate control? I blew into the hard line on the other side of the check valve at the manifold. The one that runs into the firewall. That seems to be what people say goes to climate control. It was blocked as if I were trying to blow into the wrong way of the check valve.
I blew smoke into the other one too, that the diagram says goes to the heater. Though I didn’t have anyone sitting inside that time. Though they'd have a hard time seeing a little smoke coming out behind the dash.
Nobody seems to be able to tell me what the vacuum source is either. This whole thing is very strange.
I’ve checked the reservoir and line running to it and it looks fine. Yes, the line under the battery looks fine. Cant see any cracks. When I unplugged it, it went pop, whoosh.
I’d happily replace that line too, just to rule it out, if it weren’t some kind t mystery what size tube would replace it.
The vacuum goes from there to the reservoir and then back up to the firewall on the passenger side. It then goes through the firewall to the heater controls.
The vacuum goes from there to the reservoir and then back up to the firewall on the passenger side. It then goes through the firewall to the heater controls.
Thank you for your patience and for explaining the layout and where the source is.
But I still can’t wrap my head around how knowing this is helpful or how it tells me where to blow or suck in where to find out where a leak might be.
I’ve blown smoke in this entire system three times now. Ive run propane over every inch about 5 times now. I tried carb cleaner and soapy water.
Nothing has yielded any results. And, for some reason everyone online is cryptic about finding vacuum leaks. Everyone thinks it’s so easy and obvious. I’m baffled.
I need this truck to be road worthy and reliable so I just ordered a complete vacuum hose replacement kit and a new vacuum canister. That should narrow it down.
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.
05-12-2024, 09:26 PM
