I've done some researching on here, and haven't found anybody else that has had the same problem. I'm looking to diagnose and fix this problem. Of course with your help and maybe a walk through, depending solution to the problem.

So the problem is that the vents only blow out "lukewarm" air.

What's happening is when the heat is initially turned on, it will blow out hot and has a strong air flow, just like it should.

Literally one second later, the air flow isn't as strong and it's just warm, and I can hear a change. Not sure if it's a change with the blower motor or direction of air, but it's a great difference. I cannot feel air coming out of any other vents, aside from ones that it's set to come out of.

I must add that when the control is set to off for a couple of seconds, and then flipped back on, the full power heat is restored for another second.

The only things I've done is changed water pump six months ago, and verified that both heater hoses in engine compartment are hot.

I read on some other site that there's a vacuum line under hood by wipers that could cause a similar problem, I'm going to go check that in a little bit.

I really do think the problem lies in resistor/blower or blend doors, but I want to make sure before I really start to tear into anything.

Any helpful advice would be greatly appreciated, especially with winter coming.

Edit: Vehicle application is a 98 Dakota with a 3.9

 

Ok, just wanted to do an update, and I still want opinions...So, I can hear the blend doors moving when I move the **** from one side to the next. I also read that if it's a vacuum problem, then defrost is the automatic, and non of other vents will work. Ok, well, my other vents work. I also get excellent heat, it's just a matter of how long, so I've decided it almost has to be the resistor or blower motor.

If anybody else has anything else to add, I would appreciate it. Otherwise, I will just update later.

 

Is your blower motor working in all three speeds? If so, that would eliminate the resister. If it's working at all, that would eliminate the blower motor. Wouldn't it?

Does it work okay in the other vent selections (like the dash vents you would use for a/c?)

I think you have a leaking blend door diaphragm or a leaky vacuum line. When you first switch to the heater position, it sets the blend door, but then the vacuum leaks off and the door opens back up. At least that's my logic for the situation.

 

I had a somewhat similar problem; I had only lukewarm air coming out, too. But it was always lukewarm, it never started out hot, as you are reporting.

My solution was: I had the heater core backflushed. It fills up with gunk and sludge, and backflushing took that away. Afterwards, the heater blew out hot air.

Supposedly you can do this yourself with a garden hose; there are YouTube videos showing this.

 

A clogged heater core can give you good heat that slowly gets colder. It heats up from the coolant but there isn't enough coolant to keep it warm so it gets cold.

But, it shouldn't change the airspeed. The blend door is cable operated, not vacuum. The mode doors are vacuum operated.

 

Thanks for all the replies. Just to verify, the A/C works perfect and it goes through the vents that it's set to use, same goes for heat. So where do I go from here? What are some tests that I can do to determine exactly what the problem, or where the problem is? And bonus question is, exactly how do the blend and the mode doors work?

 

Blend door is cable operated, if you turn the dial all the way to cold you should hear the door "stop" basically hit the wall in the box. If you turn it back to hot, you should hear the same "stop" sound as it hits the wall the other direction.

The mode doors are vacuum operated, if there is a leak in the vacuum line it will default to the defrost position. The vacuum solenoid is located behind the lower plastic panel below the passenger air bag key switch and your 12V lighter plug. It is toward the back (front of the cab nearest the firewall).

So I understand correctly, you can change between all settings of the mode door ok and your blend door is operating correctly. There is one small adjustment you can do with the blend door. Turn the **** so it is pointing straight up (midway between cold and hot) Slightly pull the **** out, to where it is about halfway off the post, then turn it all the way from cold to hot, it will go past the point where the blend door should close off access to cold air. Then twist the **** back to the center between hot and cold and push it back in place. This is basically a quick way to check if it is closing all the way on the hot side and automatically adjust the cable (ever so slightly to where the range ends when the door closes completely).

If after all of this you have good adjustable airflow on all three speed settings whether hot or cold, and you can switch between all modes, you can rule out the blend door and mode control.

Is your truck warming up properly? A stuck thermostat could cause a loss of heat as well.... and is cheap and easy fix.

At that point I would flush the heater core. The procedure is pretty easy. There are two lines to the heater core. You can separate one on the passenger side valve cover, and the other on the driver side valve cover.

Connect a hose fitting you can screw your garden hose into (Home Depot or Lowes is your friend)... similar to the connection on the end of the hose you connect to your faucet, and put the end of the other heater core hose into a bucket and turn on your water.
Run it until it is flowing clear water. (If you see chunks of stuff you are on to something)
Then shut off the water and switch the fitting to the other hose and repeat.
This time you are pushing water through in the other direction.
When you are done reconnect the hoses and top off your radiator.

One important point to note, you will likely have an air bubble in your cooling system at this point.

If you can elevate the front end of your truck (ramps or park on a hill) when you refill the radiator and leave the cap off while it is running (take cap off when truck is completely cooled off), you can basically burp the system by letting it completely warm up so the thermostat is wide open. Then rev it up to about 2k RPM and drop back to idle. You should see the coolant rise slightly in the neck of the radiator to indicate it being pushed by the water pump (get someone to help/watch for you if you can). Continue revving up and back to idle for about 5 minutes to ensure all air is out of the system. You can carefully squeeze the upper radiator hose (wear a glove, it's hot!) between the thermostat and radiator as well to help force any air out. Turn the truck off, top off coolant if needed, check the overflow bottle as well, and replace the cap and see if your heat is improved.

 

I have a '97 3.9 that has tepid heat, but everything else works fine. I am tempted to flush the heater core, even though I had the coolant flushed a year ago when I bought the truck. My concern is that I will flush a leak and then end up with the dreaded heater core replacement. Is that likely just using a garden hose as described? My guess is "yes", so I've left it alone for now....

 

I suppose it could be possible, but what makes you think you had a leak to start? Have you had moisture on the passenger floorboard? If not, you may just have some blockage and that would help. However, if you had the system flushed a year ago I would look elsewhere first. Check simple things like the blend door, thermostat, etc. before flushing again. It is possible a flush would be beneficial. Kind of up to you at this point.

 

The only leak I ever had was from a corroded intake manifold bolt. That was expensive enough to get repaired. I have heat, it's just not real hot. I think I'm just going to live with it because if I flush the core and it then starts leaking, that will be an expensive fix.