Is there a check valve in vacuum line?

[N8N_99xj]

Noticed last night that when accelerating hard HVAC shifts to defrost mode and then back to where controls are set afterwards... is there a check valve between manifold and reservoir and if so where is it physically? Thanks...

 

[Ecomike]

Sounds more like a small vacuum leak in an old vacuum tube or hose, possibly near the passenger side firewall. Found mine by killing the engine, while under the hood, and listening for the leak.

 

[WB9YZU]

No check valve.

X2 on Mike's analysis.

Sounds like a vacuum leak. Vacuum bottle deletion can also cause you not to have good vacuum on acceleration.

 

[Shorty]

assuming you have the vacuum reservoir still attached behind the front bumper (passenger side), the most common place to find a leak is below/behind the battery. The vacuum lines for the reservoir run through the radiator core support right there and tend to deteriorate from battery acid venting and physical damage from battery replacements.

 

[O-Gauge Steamer]

Actually, depending on the year, there is a check valve. My 98 has one as did the 97 I had.

But the most likely candidate for the problem is a leak. Get a vacuum gauge form NAPA or any where and go hunting for it. Personally, I like to start at the manifold and work my way back with the caveat that on XJs I look at the reservoir connection first. Time and temperature tend to "cook" out the elastomers in the rubber allowing each connector to release it's hold on the device or line it is connected to. Easy to repair, just take a little time.

In my case, the reservoir on the 98 had micro-fractures in it. I gave it a coating of fibreglass cloth (one wrap of 2oz cloth) and picked up 2" of mercury for the effort. This, after replacing the connector at the reservoir.

 

[Pelican]

My '85 had check valves in each of the two reservoirs (one for the cruise and one for the HVAC). I had the default to defrost problem when one of the valves failed. I don't really see how the system will work effectively during acceleration without a check valve.

 

[Shorty]

I don't really see how the system will work effectively during acceleration without a check valve.

I don't either, unless there's a check valve built into the reservoir on some, or a small hole in the line between the valve and the reservoir.

 

[Saudade]

Right out of the '99 FSM

"VACUUM CHECK VALVE
A vacuum check valve is installed in the accessory vacuum supply line in the engine compartment, near the vacuum tap on the engine intake manifold. The vacuum check valve is designed to allow vacuum to flow in only one direction through the accessory vacuum supply circuits.

The use of a vacuum check valve helps to maintain the system vacuum needed to retain the selected heater-A/C mode settings. The check valve will prevent the engine from bleeding down system vacuum through the intake manifold during extended heavy
engine load (low engine vacuum) operation.

The vacuum check valve cannot be repaired and, if faulty or damaged, it must be replaced"

 

[Ecomike]

OP YEAR?????

I have never seen one, other than the brake booster vacuum check valve on the older jeeps, 1987- 1989. Perhaps they wised up in later years.

 

[Pelican]

Well, they understood the concept in 1985 and I can't imagine this bit of arcane knowledge was lost until 1999 but stranger things have happened. I haven't had a problem with my '93 so haven't looked for the valve but I guess I assumed it was an integral part of the reservoir. Has to have been somewhere all these years: silly thing won't work without it.

 

[N8N_99xj]

Ok, armed with above responses i found check valve and sadly (at least by highly scientific suck 'n' blow test) it appears to be working properly... so either i'm looking at the wrong line or there's a leak somewhere. Great. The line i tested connects to the manifold directly below the rear connection to the valve cover. There is another line below the TB that does not appear to have a check valve.

 

[hubs97xj]

This is a later manifold on a 97, so the location isn't exactly the same, but this is the "non-existent" check valve, installed just after the elbow. This is why you don't hear a hiss as you lose vacuum each time you shut your junk off...



Now that you know the check works, start there and work your way back to the canister.

 

[N8N_99xj]

Yup, looks the same on mine. Don't have a mityvac handy though. Guess i need to finally buy one?

 

[N8N_99xj]

Aaaand, I took it for a spin to meet a friend after work, and it doesn't seem to be exhibiting the problem any more? my heep is possessed...

 

[Saudade]

Well, maybe you blew out some debris from the check valve. Doesn't take much to make it leak.

 

[Ecomike]

Well, they understood the concept in 1985 and I can't imagine this bit of arcane knowledge was lost until 1999 but stranger things have happened. I haven't had a problem with my '93 so haven't looked for the valve but I guess I assumed it was an integral part of the reservoir. Has to have been somewhere all these years: silly thing won't work without it.

They understood how to get to the moon and back in 1969 too. The 87-90 does not need the extra check valve, if there is no vacuum leak in the lines near the AC vacuum lines that go through the fire wall on the passenger side.

 

[Saudade]

OP YEAR?????

I have never seen one, other than the brake booster vacuum check valve on the older jeeps, 1987- 1989. Perhaps they wised up in later years.

Nope, got 'em on Renix too. In fact, the CAD on the older rigs have check valves too.

 

[Ecomike]

Nope, got 'em on Renix too. In fact, the CAD on the older rigs have check valves too.

I have never seen one on a Renix, where is it (are they)?

If there is one, it did not solve the OPs problem on my 87 or 89 when I had the same problem, it was either a bad actuator valve (leaking diaphragm) or vacuum line leaks (the 85) or a hose leak near the firewall on the Vacuum source lines going into the firewall that supply vacuum to the AC system controls on the dash on mine.

FWIW, I think my 85 has about 6 or more of those check valves on the hoard of solenoids it had on the original gas four banger. The vacuum lines on that OEM rig were a nightmare come true.

But now that I think about it, my AC system damper doors still function after I turn off the jeep, I can switch the damper door (vacuum operated), from the dash to outside vent, or no vent, so there must be a check valve working somewhere.