oldnslow

87 Cherokee, HVAC system only blows from the defrost, no problem and I know why. Problem is still no hot air flows from the defrost vents. Today i flushed both directions, the heater core. Brown water came out but it eventually flowed free and clear, both directions of flush. The vacuum heater valve has been previously eliminated. I fill the system, start the motor, warm it up to temp. Both heater hoses are very warm, fan is blowing on high, but only cool air flows from the defrost vents. I now suspect a physical blockage (leaves, debri, mouse nest, but I dont smell the tell tale smell of mouse ****) of air flow to/across the heater core. I have removed the fan and found no obvious blockage. I have the air conditioner thing blocking any further visible inspection beyond the fan mounting hole. I cannot see the heater core.

MY QUESTION... is there enough clearance in the heater box/duct work, to allow the forced air from the fan to bypass the heater core and make its way to the defrost vents with out ever passing through the heater core fins? (go around the heater core?) Thus, giving me the airflow, albeit some one restricted sounding, through the vents?

Saudade

Check your blend door. It's cable actuated. Look under the glove box. Most likely the cable slipped off the post. If you move the temp lever back and forth, you should be able to see the cable end.

oldnslow

Thanks, the blend door is working as it should. I can see it move and hear it close and open.

Saudade

Go look at the pics in this thread (please don't bump it). The blend door blocks airflow through the core. You can see the opening to the chamber where the heater core sits and the blend door positioned about 4 o'clock. Moving the temp lever to "cold" swings the door to block off the core. That's the "bypass" as you phrased it.

On "defrost" your AC kicks in too. Supposed to dehumidify the air before blasting it onto a cold windshield. Assuming your AC works, try disconnecting the compressor and see if the temp rises.

Since you no longer have the heater valve, coolant will flow through the core as long as the engine is running. With your engine up to full temp, those hoses should be very hot, not warm. Did you flush the hoses?

https://www.cherokeeforum.com/f2/hea...e-help-255469/

oldnslow

Thanks again. What I describe as very warm, most would call hot. I work in a weld shop and handle hot metal all day.LOL . The heater hoses are hot. When i move the temp selector to hot, the blend door rod moves, and I can 'hear' the blend door making contact with its stops inside the box. Also, when I move from 'cool' to 'hot' with the selector, the air flow from the defrost vents is reduced by about 50 percent, leading me to think that air from the fan is not flowing across the heater core as it should, because perhaps the surface area of the core is blocked; but the fan driven air is somehow being forced around it it, even with the blend door in the 'hot' position.

Does the blend door normally fully close, forming an airtight chamber? Or does it have a small opening designed in it from the factory, and the air flow i am getting is from that small normal opening?

Saudade

Did you look at the pics in the link above? Did you flush your hoses too?

Jeepwalker

You might be able to snake an inspection camera down through the upper hole beneath the cowl cover at the base of the windshield (where the fresh air comes in) ...and from the heater and possibly see if there is a nest or obstruction. I *thought* there'd be a nest in mine ..I have mice all over my Jeep, but in fact the problem was the foam on the blend doors had rotted off.

What happens a lot on these Jeeps is the foam on the blend doors degrades simply from age and just 'falls off'. Mine was good up to a point then after not driving it for a while I turned on the heater and it blew tons of foam out. Then the door's don't seal like they should and you don't get the airflow where you should get it, nor the heat.

I completely R&R'd my Jeep's heater box with new foam, new (Copper) heater core ...not aluminum. Aluminum is a low-path-of-resistance but inferior solution on these Jeeps IMO. You can find new high-quality copper/brass cores out there just like the OEM's. But if yours isn't leaking I'd probably remove, put some drain cleaner (Lye) in it (be at room temp so it 'works') ...and after a good 'cooking' flush it out. Might do it twice. Don't let it sit overnight or anything. It won't hurt the noble metals but it could attack the solder. But in a short time it won't. Then, get some 1/8" high-temp foam from McMaster Carr ...and maybe some 3/16" (I guess I don't remember all the thicknesses I used, and cut out and re-adhere them as gasketing material to the blend doors like is designed to have. Note: I bought 1/8", 3/16, 1/4" & 3/8 ..just to have around.

I also put wire cloth with 1/4" square holes across places where mice 'could' possibly intrude into the heater box. Even the drain. Finally, always turn the controls to Vent and Cold when you park it. It closes it up as much as possible. And put some dryer sheets down by the heater outlet (mice hate that smell and won't get near it).

It's a paint to pull the dash partly apart to do the job, but given the line of work your in, you can handle it. I've done far worse and more complex jobs and I'm sure you have too. It's not that bad. The pay-off is a great operating heater at the other end. Mine could heat a house now!! LOL


Best of luck.

Jeepwalker

Note. If you DO have mice pi$$ in your heater core (or vehicle) ...the sure-fire solution to neutralize the odor it is the following recipe:

Mix a solution of 10 ounces of 3 percent hydrogen peroxide, 3 tablespoons of baking soda, and 2-3 drops of dish washing detergent. Apply. Let soak for an hour. Rinse with water.

Um, that's going to work better than any commercial product. You can scale the amount up/down as needed. I got that recipe from Mythbusters who said it worked better than all the commercial products. Supposedly works on Cat and other pet urine odors. AND... I had mice smell in my Land Rover SO BAD (years ago), yer eyes would water riding down the road with the windows open at 35* F!! LOL. After spraying the inside of the heater core (w/o removing from the vehicle) with the above solution, you cannot even smell a hint of mice urine. Not even after sitting with the doors closed for 2 weeks on a hot summer days. And it's been about 10 years since I did it. So it works. In fact, this fall I took everything out of my Jeep XJ and sprayed the whole entire interior with the same solution 3x, using a garden sprayer (lol). Everything except the new headliner I put in. Let it sit, then hosed the interior down with water and shop-vac'd it all out. It's great in there now!! He he. Use dryer sheets so mice won't continue to come in. Esp under each side of the rear seat. It's Jeepwalker-tested!!

...And put your heater controls on Vent and Cold if you're not going to drive it a while.

.

oldnslow

Well, after re evaluating my diagnosis, I found out I didn't understand the hose routing for the heater core, and i was not flushing it at all, I was just flushing the coolant tank! DOH! So, then i tried to flush the core properly and found it to be plugged solid.

I read and re read my Haynes manual on heater core removal, and I was able to get the core out without removing the dash. Just had to remove the lower dash panel, unhook all the wires, vacuum hoses and various stuff, then unbolt the dash on the passenger side, behind the kick panel, then have a helper pull/pry the dash about 2 inches, and the fan box was able to be rotated down and out! I'm not so sure how easy it is going to be trying to squeeze the fan box back in place, but i am deleting the AC unit, so the box will be substantially lighter and easier to wrestle around.

Its a slow process, a few minutes and hours at a time.

I may have heat by springtime!

Saudade

Live and learn!

Go to https://cruiser54.com

Lot's of tips and getting our old Renix rigs running right! (that's a lot of RRRRs).

Then go here (same site) and get the real deal guides. Save the haynes for firestarter. https://cruiser54.com/?page_id=365

oldnslow

I got the new core in, and got the fan box put back under the dash. Wasnt nearly as big of a deal as I thought it would be. I used a ratchet strap from the roof rack to the corner of the dash. That held it up plenty far to wiggle the fan box into place.