No Heat In 89 4.0 Mj

[1989 Eliminator]

Okay so here in Michigan the temps have been close or below zero for most of the month. My issue is that sometimes my heat will not work. What happens is when the truck warms up fine to operating temperature, I turn on the heat and cold air blows out.

 

What I have done to try and fix this is back flush the heater core. Which did help because when the heat does work the air is a lot hotter than before. There was quite a bit of crud in there.

 

What I assume now is that:

 

1) I am not getting enough vaccum to open the main air flow gate which either lets ambient air or heated air through.

 

2) The main gate is frozen somehow

 

3) vaccum leak somewhere...?

 

After the temperatures outside increase a little and the inside of the truck warms up a bit, the heat works fine. It is only when the mornings are arctic cold.

 

Am I right with my assumtions or is there anything else I should check?

[yxmj]

Have a look at this thread.....http://comancheclub.com/topic/41624-heater-core-valve-hub-delete-25/

 

If you have that valve look at the way i describe to by-pass it ( a couple of hose joiners and 4 hose clamps is the fastest)....That way you know you are getting hot coolant all the time. If that does not solve it then look at your blend door.

 

 

 

A tip on heater core flushing.......Just picked up another XJ for my youngest and heat was a problem (lately here it has been getting down to -40)......Leaned on a chance experiment i tried years ago..... most modern cores can still have water pass even though the heat passages are blocked....Grabbed myself a $7 jug of CLR (Yes CLR....I don't care what is says on the bottle....I am not trying to shine the core to a polished finish......i am trying to remove deposits) and a $2 jug of vinegar.

 

Since it was so cold went down to a local car wash...Removed heater hoses at the T-stat and water pump let them drain in a pail and cool....Then I blew threw the top hose till just air came out....raised both hose ends up above the level of the core and poured CLR in the BOTTOM hose until it dribbled out the top hose......Then I went and vacuumed the interior....Came back put a coin in the slot set the washer to hot rinse and put it in both hoses one after the other....lots of crud.....Puffed up my cheeks and blew it out again.....repeated with vinegar...twice.....filled and let set for about 5 min each time....after the 4th coin the core made a "BURP" and let loose a puddle of mud you would not believe....one more rinse until the water ran clean...blew out the core...hooked up the bottom hose.....filled the top up with coolant....hooked it back up....took off the rad cap ran the tuck to temp....added fluid to the rad....put the cap on and topped up the reservoir.

 

HE HAS HEAT YOU COULD WELD RAILWAY TRACK WITH   :thumbsup:

[1989 Eliminator]

Yes, I could re-flush my core using CLR and vinegar. Could make a increase in heat. But I did flush it a few times back and forth with just water and air which seemed to do the trick.

 

If I were to get rid of that stupid valve though would I run a hose from the pump to the pressure bottle, then bottle to heater core, then heater core back to the pump?

 

I am unsure because I still have the closed coolent system. Either way, both hoses are getting hot and going through the heater core. I am not sure where to check in the HVAC where the main gate door is and if its working

[yxmj]

HA HA

Here is Cruiser's answer from another site

 

http://www.jeepforum.com/forum/f11/1989-cherokee-blend-door-heat-air-1151030/

[1989 Eliminator]

haha is right. Well ill check that out!! :thumbsup:

[gogmorgo]

After the temperatures outside increase a little and the inside of the truck warms up a bit, the heat works fine. It is only when the mornings are arctic cold.

 

This hints at a bad thermostat. Number one fix for slow to build heat in any vehicle. Symptoms of a bad t-stat could be really long engine warm-up times (or just never getting to temperature) if the thermostat is stuck open, or constant overheating if the t-stat is stuck closed. Sometimes they just get sticky and slow to open or close, which results in wildly swinging temperatures. A new t-stat and gasket are cheap (less than $20 up here where things are typically 50% more expensive than they are in the states) and really easy to install. A few of the members on here will recommend going with a genuine Mopar t-stat. I never have and haven't had an issue with parts-store t-stats, but I could just be lucky. Either way, you'll want a 195°F thermostat.

 

When deleting the heater valve, just pull it out and put the lines together, and plug the vacuum line to it. It doesn't really matter which direction the coolant pumps through the heater core, although some members have stated preference for one or the other.

[1989 Eliminator]

Yeah that makes sense to delete that valve.

 

But I bought a quality t-stat not a crappy one less than a month ago. So that isnt the issue. The truck is warming up fine.

 

I am just sometimes not getting heat through my vents. So its either: my main vent gate gets stuck sometimes when its cold as crap outside, or I have an intermediate vaccum leak which isnt opening the main gate sometimes. Which I think the last one isnt correct because if I had a leak the vents wouldnt work at all. It could also be that the vaccum diaphram is frozen?

[HOrnbrod]

Make sure each end of the blend door cable is anchored with the clamps. If one or both ends are loose around the cable sheath the cable will not operate the blend door fully open and closed. Also pop the cable off the blend door, then move the cable in and out using the temp control. Also reach down and manually open and close the blend door where the cable connects to makes sure it travels to full closed and open. Would be a good idea to lube the cable too if it's hard to move.