89 heat is lukewarm at best

[88monteSS]

Working on a friend's uncle's 89 with the heat only option that's cable driven, so no vacuum/electronic actuators to fail.

We're unable to get heat out of the vents unless sitting still for a period & even then it's not really hot, more lukewarm. As soon as it starts moving again it goes back to lukecool.

To start with the motor wouldn't warm up, so after 3 thermostats we got a good one & the motor now comes up to temp. Heater core is too hot to touch. The blend door feels like it's closing correctly (stuck a hand in through the blower motor hole & moved the lever). All the moving parts seem to be moving correctly & we get good flow out of all 3 positions, so we think that eliminates obstructions (although we checked as best we could without completely dismantling the crumbling dash.

At this point we're stumped, all we can think of at this point is the cable moving the blend door is slightly stretched from 30 years of use & just isn't closing the door all the way. However we've been unable to find a replacement cable, none of the parts stores/sites seem to even know the manual version existed. Anyone got any ideas or a source for the cable?

 

[Eveready]

If the cable is stretched it might be that you could shorten the end at the dashboard control and thus move the door more. You may have to find the shop manual for that year to get the detail on the manual version. Good luck and be sure and report what you find. If the heater core is hot you are most of the way there. When I first read your query I immediately thought "thermostat" but you have done a good job of eliminating that part of it.

It may be possible to find a generic replacement cable and cut to fit.

 

[RichLo]

Also make sure there is no airflow around the heater core, air has to pass through it to heat the air. while your in there, make sure its not plugged with dust/dirt/leaves/etc.

 

[88monteSS]

Friend finally found the culprit yesterday, I had to wait until today to get an explanation of the pic he sent me. The plastic where one of the heater core mounting tabs screwed in broke & allowed the core to drop just enough for air to bypass it.

Most likely it broke when he changed the core a few weeks ago, but with the crappy weather it's been hard to get the truck for more than a day. His uncle has some serious health issues & trusts it to get him to the hospital more than his newish Jeep. TBF tho, I'd trust a 30 year old TBI over that crappy 3.7l any day.

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[TechNova]

I just had a lack of heat in my 89. I had to back flush the entire system, someone else told me they had to pull the block drains and clean with a wire. The rust swirls and plugs the core.
FYI for others with heat problems.

 

[Schurkey]

I spent two years trying to figure out a "no heat" condition. I was wearing a snowmobile suit to get to work--35 highway miles away. I had a 205 thermostat in the engine at the time.

Turned out that the foam seals on the heater box joints had rotted away. The heater worked fine--but the seal between the metal cowl and the plastic inlet to the blower fan had turned to jelly, so cold air POURED into the cab uncontrollably. The heater couldn't keep up.