Heater blows cold air

[WJune]

Have not run my 90 Dodge truck much this winter do to sub zero weather and short trips. Last 2 times the heater blows cold even though engine temp is warm, although one time it started blowing warm after 9 slow miles and pegging temp gauge? When thermostat opened gauge got normal and heat started. Today 5 mi trip around town, gauge in middle, no heat. Parked truck, after 30 min, started truck up an had heat. Vacuem controlled valve in heater hose, has no vacuem, yet where small line comes off brake booster, there is vacuem. Shut engine off and I can hear heater doors? closing. Must be getting vacuem there? My book does'nt have much info on heater, and at 10 deg I don't feel like pulling my plastic dash apart. Any idea's ?

Anti freeze was changed in fall and had heat after that. Assuming vacuem should open control valve, I have it propped open with piece of wood. Help ??

 

[Bob Beauchaine]

Make sure your coolant is topped off. Low coolant can cause your heater to not function quite right. Don't depend on the over flow bottle as a level indicator. When cold, remove the cap and the level should be right at the top. If the over flow bottle is at the correct level and the radiator is low your cap is bad.



Bob

 

[Tippin Spotted Cow]

Definitely low coolant. It takes longer for the thermostat to open when you have low coolant. Your heat gauge will peg at the top for a while, then all of a sudden it'll drop, and you'll get heat. If you're real low, the thermostat will close again while sitting at a stoplight and you'll get blasted with cold air. I have a leak, and the same thing happens to my truck when it starts to get low. I'll drive it in 2nd to get the thermostat to open, then top it off then next time I stop.

 

[JLEONARD]

In addition to what Bob said make sure the opening in the bottom of the overflow bottle is not clogged with sediment as that will prevent the system from being able to refill.

 

[WJune]

A few weeks ago I could see the rad core with cap off, didn't over heat and heater worked. I tightened hose clamp an topped it off. Lately I check an rad is full, but no heat. Have not checked over flow bottle yet. Any idea's on the vacuem situation discribed ? It's bad enough with no heat, but can't be out at night with no defrosters !

Thanks for responses, keep em coming.

 

[Ctdchris]

The vacuum lines go through the firewall right behind your glove box. If you take your glove box out you can see them. If that valve is not getting enough vacuum your heater will never work, it shuts off the coolant to your heater core. A quick fix is to take a zip tie and push that plunger all the way in and zip tie it open. This won't affect anything until you need to use your A/C. That valve closes to make the A/C more efficient. I've heard stories of people taking that valve out and just putting in a straight line also, your A/C will still work, just not as well.



Chris

 

[WJune]

OK. . I figured the vacuem out. In my opinion it works backwards! With heater button pushed off the line has vacuem which pulls the vacuem valve closed! Pushing Heat or Defrost on stops vacuem allowing valve to open. So my piece of wood against diaphram was closing valve! However that does'nt explain why I didn't get heat the first time, before I blocked valve shut ? I removed an plugged vac line so I will remember which is open. May have to replace thermostat when it warms up, don't like it sticking at hot.

 

[JLEONARD]

Hate to say it but mine always worked so well I had to turn it down after a couple of minutes.

Would blast me out of the cab. LOL

 

[mysteryman]

vacuum controls, heater/ac

The vacuum control switch , the part that has the push buttons on it in the dash. . They go bad from time to time. The main problem that causes them to fail is a bad blower motor that is drawing too much current on the "Hi" fan position. .

This causes the electrical connections in the vacuum switch to melt. This often causes that vacuum portion to malfunction also...



The vacuum switch then gets replaced. Often times with the WRONG SWITCH... They all looked the same on the outside for many years.

But internally they were different. The difference is the mapping of the vacuum ports... For example, if you take a vacuum switch out of a K car and put it in a D350 . The buttons will not do the things they are supposed to do. .

Always refer to the part # inked on the switch. .

OK. . I figured the vacuem out. In my opinion it works backwards! With heater button pushed off the line has vacuem which pulls the vacuem valve closed! Pushing Heat or Defrost on stops vacuem allowing valve to open. So my piece of wood against diaphram was closing valve! However that does'nt explain why I didn't get heat the first time, before I blocked valve shut ? I removed an plugged vac line so I will remember which is open. May have to replace thermostat when it warms up, don't like it sticking at hot.

 

[WJune]

Up date: 9 deg. start truck, fast idle 10 min, needle just off cold, some heat blowing. Drive mile to town, several stops, needle 1/4 to 1/2 on temp gauge, now blows ICE Cold!

Truck sets walmart 45 min, start up, needle almost cold, got warm heat all the way home ? When it was blowing cold, felt all heater hoses an they were warm. If I had air in system from few hundred miles ago, would'nt it be worked out by now? How can cool engine = heat, warm eng = cold air? HELP... don't make me put my ctd in a Chevy. lol

 

[Ctdchris]

As erratic as it is, it screams vacuum leak to me, thats just the feeling i get. Do you use "heat" or "vent" on hot? Is it blowing out of the correct location? Did you get a chance to check that vacuum valve on the top of the engine? Which hoses did you check?



I hate vacuum lines



Chris

 

[WJune]

When it works, Heat blows heat and pushing Vent always blows cold. Heat and Defrost blow out appropriate vents. I have the vacuem unhooked from heater hose shut off valve so that it's always open. Never need AC in Wi. All hoses going in and out of heater are warm. If it was a malfunction on vacuem contolled heater doors, it would be constant I would think? Somewhere I heard how to bleed air out of system, but don't recall? May of been The Car Doctor show on Sunday morns? thanks for your reply.

 

[Ctdchris]

When it works, Heat blows heat and pushing Vent always blows cold.

The only way i can see this happening is if the controls are not working properly. Both "heat" and "vent" come from the heater core (obviously) and just get routed differently by the ducts. I don't see how one could be hot and one could be cold, other than "vent" getting routed from the condenser side, which would more than likely be a problem with the vacuum lines and/or controls.



Have you made sure all the lines are plugged in good and tight behind the actual controls? Also if you follow the lines from the controls back a little ways there is a male/male connector that connects all the vacuum lines together. My connector wiggled lose one time and my controls all started acting weird. Those are two pretty easy things to check, might worth your time if you haven't done so already.



I would have bet money that the valve for the engine coolant on top of the engine had a default position off closed, and it takes vacuum pressure to open it! I guess its been a while since i rebuilt my climate control system though, so i must just be remembering it backwards.



I wish I could be of more help!



Chris

 

[Ncostello]

Since its not getting warm anytime soon... take the valve off the heater hose in the engine compt. Install a straight fitting for heater hose and two clamps... easily available at Napa. This will ensure full heated coolant to the core. At least you'll be getting heat in the cab out of some vent. Whether the vents respond properly is another matter.

 

[WJune]

I seem to have reliable heat now. I changed the 180 stat with a new Carquest 180 that appears to be identical. The only other thing I did was follow directions an let engine reach temp BEFORE installing rad cap. Normaly I would put cap on and go. Thanks to all who chimed in.