intermittent heat

I noticed something on the way home tonight. I had the heater on and adjusted so it was coming out the vents. When I hit the accelerator, the amount of force of the heat that comes out the vents is drastically decreased. The blower is still working strongly. I can even turn the blower on higher and hear it in the dash. The vents feel will feel hot but it's almost like someone just closed them all. When I let off the pedal, the heats back on. I don't have to completely romp on the throttle to get it to do this. I didn't notice any problems while sitting at a stoplight in town though.

I found some stuff dealing with the heater valve in the archives but I don't think that would present an intermittant problem. Any ideas?

-river

96 w/ 4 liter, auto, 120,000.

when the volume of air went down did the air start coming out the defrost vent?
if so there is a problem with the vaccum supply most likley the vac. check valve is
failed they are aviable at the dealer for $5ish
pretty common

justin

I can't say. At first I just heard something different when it happened. It's entirely possible though. Now that you said it...the noise I heard almost sounded like what you'd hear if you switched it from heat to defrost. Thanks bigazxj....I'll search for that vaccum check valve and see what it entails.

OK...I see tons of stuff online dealing with a tech service bulletin for Grand Cherokees that have the exact symptoms. The solution is to install a check valve. I see nothing for the Cherokee. Do I have one of these that is bad? Would this be something in the vaccum canister?

First thing I'd do is check for loose, unattached, missing, misrouted, hardened/cracked, or otherwise damaged vacuum lines. Under the dash and under the hood. If you don't find anything there, check under the dash to see if the heat door is closing when you accelerate. Could be a weak vacuum dashpot. or low vacuum which can have a multitude of causes.

If you, or someone, has (that you know of) recently done any work on the dash, such as a stereo install, for instance, or work under the hood, a vacuum line could have been damaged or knocked loose or something might have occurred to lower the vacuum available at that heater door or somewhere in those controls.

The heat valve will only be opened in bilevel, heat, and defrost.

When you have no vacuum to the HVAC, it defaults with the heat valve opened and the vents set on defrost.

Update:

I got some starter fluid and hit all the vaccum lines and their connections and found no change in idle. I checked all lines from the duty cycle solenoid, evap canister, all on valve cover, air cleaner, as well as intake. The line coming off the front part of the evap canister heads towards drivers and eventually down the firewall and back to the tank. It's a metal line along the frame rail that turns to rubber near the tank and T's off. That rubber line and the T were inside a block of ice but after removing it and spraying it with fluid, there is no change.

The line that comes off the intake that is closest to the firewall is supposed to be for HVAC, cruise and 4x4. That line heads over to the duty cycle solenoid and one small line enters the cabin (for the HVAC no doubt). It looks like it eventually ends up under/behind the passengers knee board and connects to some sort of diaphram. The heat flapper at the bottom of the center console does indeed work when the controls are switched from vent ot heat.

I don't have a vaccum gauge but maybe I'll see if I can borrow one from the NAPA.

One last thing. There looks to be 3 places for lines to come off the evap canister. I already talked about the one closest to the front of the vehicle. The middle one runs right up and connects to the duty cycle solenoid. There is a third spot for a line to connect to closest to the firewall but there is no hose coming off it or hanging anywhere. Can anyone verify if this is supposed to have a line coming off it?

Also...I ss no check valve.

Check the black vacuum line coming through the firewall on the passenger side. Take off the L connection, snip the tube maybe a half inch, slide the L connection thing back on (not TOO Far) and then connect it back up to the larger main vacuum line.

I had troubles and it turned out the vacuum line got crimped right inside the L connection once.

You need to look at the vac lines going to the FRONT of the vehicle.Not emession lines.
Vac lines run under the battery and down to the canister/blimp/football,ect behind the bumper on the pass side.With all the ice&snow in your area the lines could have been pulled off by that,or battery splooge could have rotted them.
Wayne

I checked all the lines Wayne. The vaccum cainster in the bumper was relocated months ago. It now sits in the engine bay. All those lines appear to be fine. Starter fluid did not reveal any leaks.

Check the main vacuum line to the HVAC. It's the black line beside the white/tan which runs to the heater valve!

riverfever said:

I checked all the lines Wayne. The vaccum cainster in the bumper was relocated months ago. It now sits in the engine bay. All those lines appear to be fine. Starter fluid did not reveal any leaks.

Click to expand...

Ok,The lines are pretty small and a good ways away from the motor,I wonder if a leak would flow enough to change engine rpm enough to notice?
After a quick check of the FSM The check valve may be bad.
Valve should work at 8"Hg.
Wayne

Neither of my manuals show a check valve so I'm not sure where it's at. Can you tell me where to look?

riverfever said:

Neither of my manuals show a check valve so I'm not sure where it's at. Can you tell me where to look?

Click to expand...

FSM says "The valve is located on the(black) vacuum supply hose at the intake manifold"
My 96 XJ is not here at the moment(son has it) so I cannot do a visual on it.
Wayne

I bet it's the white portion in the line closest to the firewall.

The white line goes to the heater valve. And if vacuum to that white line is disrupted, the heatever valve defaults to open.

Blaine B. said:

The white line goes to the heater valve. And if vacuum to that white line is disrupted, the heatever valve defaults to open.

Click to expand...

Blaine are you talking about the white line that goes through the firewall near the A/C lines?

Dealer says they don't show a check valve for the 96 XJ. Told me to go the boneyard.

riverfever said:

I bet it's the white portion in the line closest to the firewall.

Click to expand...

Thats the check valve right above the intake--beside the green clamp.
Wayne

The dealer doesn't sell just the check valve but the entire line for 19 bucks. I ordered one so we'll see if that fixes things when it comes in.

If the new line doesn't fix things then maybe I'll see about getting another main vaccum canister at the yard. Ya know...I probably should just say screw it...I am getting hot air regardless if it comes out the defrost or the vents. The cruise works. Damn OCD. I say I'll just let it go and then it'll just fester inside until I storm out in the garage and tear the dash out.

I did find that the black line going through the firewall operates a flap for the A/C. The white line (like Blaine said) must have something to do with the heat but in order to get to it and see if it's cracked or disconnected...the dash has to come out. It's killing me. MUST........HAVE EVERYTHING...........PERFECT. Raaaaarrrrrrrr!