Ive got a 1992 bronco with a 5.0 that is having some heater problems. Truck warms up and runs fine but doesn't blow hot air out of the defrost or floor vents. When I switch to the dash vents I get slightly warmer air...but not how it should be for the hvac control being full hot. Any ideas on what could cause the temp to be mild at best and much colder on the defrost and floor side? Its got plenty of air flow...just not much hot air. Heater core is fine as well; Im thinking its a vacuum leak somewhere in the dash but not sure where to start.

Thanks!

 

When you switch to defrost, floor, and vent, does the air blow out of each of those areas reasonably well?

A vacuum leak would normally prevent the air from coming out of certain areas, and not necessarily prevent the air from getting hot. It definitely wouldn't be a bad idea to check for vacuum leaks, but I'd probably check under the hood near the passenger side firewall. There are some vacuum lines that hook into the HVAC system there that would be worth checking out. Usually when there is a vacuum leak on the HVAC controls, the airflow defaults to the floor position.

I'm guessing your heater core may be clogged or on it's way out. I'd check under the dash on the passenger side and see if there is any wetness. That would be a for sure sign that the heater core is bad. If there isn't, then it's still highly possible your heater core is clogged with debris so that it can't transfer heat well at all.

Also, I'd check on your coolant quality/level. If your coolant is in bad shape, it could mess with the heater core as well. If it looks muddy or anything, I'd try and flush it out REALLY well and fill with ONLY distilled water and antifreeze (I use the cheap green stuff, full strength if you're mixing with distilled water.) Well water/tap water/drinking water will all eventually cause corrosion/rusting of your coolant system.

On top of checking the coolant, do you happen to know what temperature thermostat is put in the Bronco? If it's lower than a 180 degree thermostat, I'd replace that and put in the proper one. If you don't know, then it might not be a bad idea to replace it when doing a coolant flush (if your coolant is in bad shape?) with an OEM rated thermostat.

 

Thanks for the detailed response! Everything is getting good air flow...defrost, vents and floor. Just no real heat; this is my second bronco...fist one was an 82 and would run you out of the cab it got so warm. I'll probably go ahead change the heater core and thermostat with a flush for good measure. I occasionally get a whif of antifreeze from outside the truck but no noticeable leaks. Ive also noticed that the temp gauge will climb pretty high before the thermostat opens and brings the temp down to a more "middle of the dial" reading. Not sure what thermostat is in it though...ive read other threads that mentioned a 190somthing, but not sure.

Ive also noticed that the previous owner zip tied the vacuum actuator that controls the vent/recirculator door...looks like they tied it off in the recirculate position.

 

You can use your hands to carefully feel if the heater hoses get hot. They should be hotter than the top radiator hose after the thermostat opens. But they should start to warm up as the engine warms up.
There is also a cable to the blend door behind the glovebox. If the heater hoses are getting hot but the air is not, the cable adjustment might be off.

 

I had a similar problem with my 1990 5.0L. I tied the door shut under the engine hood, that helped a little bit. Then I moved from Montana to Las Vegas, problem solved, AC works good.

 

My 94 had an issue with the cable to the blend door being stretched. The door is behind the ashtray area and you can access it by removing the glove compartment. I would turn the **** into the red all the way, but the door would never close tight, always allowing a little cold air in. I zip tied the door shut until summer, but eventually ordered a new cable from Ford. Solved my issue.

 

Thanks for all the input! I still havent had any luck fixing the issue though. Changed the heater core today with no change. Checked the blend door and cables while i had it pulled apart and all looked tight. Heater hoses were all hot, strong air flow across everything, topped off coolant ...running out of ideas.

 

...starting to get frustrated now. Literally have no idea what else could cause this issue. Everything in the hvac system appears to operating normally...even the original AC will freeze you out of the cab.

I have recently noticed that my temp gauge tends to fluctuate quite a bit. Engine warms up normally and will hold temp in the middle of the read out. Eventually the temp will shoot up towards the end of the register, hang out there for a few seconds and then come back down...usually while driving. Sometimes the temp falls below normal and then comes back up, but tends to swing high.

Any chance this could be related?

The heater hoses are plenty hot to be feeding the core, unless they're supposed to be way hotter. Its not something i want to hold on to for more than a few seconds.

Any thoughts?

 

Robert , apparently there was a problem when you purchased the vehicle because you mentioned the previous owner used zip ties. http://www.supermotors.net/clubs/sup...ry/media/71652

 

Hmm well let's see. You've replaced the heater core so that can't be the issue now. I assume you've changed the thermostat as well? If not, I'd look into that for sure in case it's too low or it might be sticking somewhat. Based on your fluctuating temperature issue, it sounds very likely that your thermostat is acting up, or that you have air in your coolant system (both could contribute to not a lot of heat from the vents.)

However, if the thermostat (and coolant temperature) and or coolant level isn't the problem, then the flow to the heater core, or the blend door have to be the issue.


I know you said your blend door cable is tight, but I'd try and see if it moves with the position of the **** on the HVAC controls. It's possible the cable for the blend door is nice and tight but the **** on the HVAC control panel isn't moving the cable itself.

 

I was going to say I believe this may be because he probably had a vacuum leak to the HVAC controls for his recirculate door. I know it's a common problem on the 92-96 Broncos for the vacuum line to deteriorate and break off to the valve that controls this door, so I think it's probably what happened on his truck as well. I don't believe his blend door is operated by vacuum, it should be a cable.

Still not a bad thing to look into though.

 

Seems you may be mixing cold air with warm air .Some where cold air is entering your system.Possible he used zip ties to accommodate an initial problem? TR

 

My 92 had the white vacuum line deteriorate years back and the only thing that happened was vent to floor functions changed occasionally,(Vacuum leak)but still got a good flow from where ever it came out.Frustrating I Know , its has to be a blend door issue( Allowing cold air to enter) and yes the

 

Blend door is cable actuated .If the seal is shot I wonder if its allowing enough cold air in ? Possible ! Good luck TR

 

Thanks for the input everyone. When I changed the heater core I also checked the blend door operation against the control **** position...everything looked good to me and the door was able to fully cycle between the temp range. The only part that has been zip tied is the vacuum actuator in the motor bay that controls the recirculator door. Since the truck is equipped with factory AC the old owner just zip tied the arm that controls the recirculator door in the recirculate position since thats where it would normally go with the AC on. I havent done the thermostat yet but am planning on it since its a pretty easy change out. The coolant was a little low so i topped it off, but it made no difference. Ive thought it might be poor flow, but would the heater hoses still be getting hot if flow was the problem?

Im beginning to think is an air leak somewhere since the amount of heat produced seems to depend more the outdoor temperature than the engine temperature. On a day where temp is in the 20s or 30s its like the temp **** is sitting right in the middle, but on days where it's in the 40s or 50s its way warmer. Any ideas on what kind of leak could cause that much difference?