No heat when idling. Help diagnose.

I have a heater problem that I just noticed. I was at a fishing lodge this weekend that was 25 degrees in the morning. I started the truck to get it warm as I scraped the windows. It idled for a good 15 minutes as we packed up the truck to head home. I turned on the heat to full hot and hit the fan. Only cool air blew out. I moved the temp dial back and forth from cold to hot and it got really cold then back to cool. I thought this was going to be a bad 400 miles back home through the snow and ice. As soon as I started driving the heater started blowing warm air. I made it home without icing and staying comfortable inside as long as the engine RPM was faster. Here are my thoughts.

1) The truck never over heated and the temperature was in its usual 1/2 way up position. Could I still have a stuck thermostat? What is the chance this is the problem? Would I have other cooling issues?

2) It has 131,000 miles with new coolant at 30,000 and 100,000 miles. Always used Toyota brand fluid replaced by the dealer. Never had an issue during all last winter. I am not losing any coolant in the radiator and reservoir. I could have an air bubble in the core but don't see how. I haven't replaced any hoses or drained any fluid. Does air get into the system with non-use like over the summer?

3) I don't know why I would have this problem since I've been good with maintenance but is it possible that I have a blockage in the heater core? It looks like the core entrance and exit are near the top of the firewall just left of the brake fluid reservoir (correct ???). Should I flush it out both directions with a garden hose and give that a shot?

4) Since I'm not losing any fluid, no water out of the exhaust, and see no bubbles in the radiator fill spout when running, I'm not sold on it being a head gasket or water pump. Please tell me no but is it possible based on no leaks or loss of coolant?

Any advice is appreciated on what to do first.

Thanks.

Update 10/15/2018:
So I bought a 10' of 5/8" ID clear plastic hose and a female 5/8" garden hose repair with clamp for like $14 at Home Depot. I cut a 2' and 8' sections. I then put the hose connector on the 2' section. I then took off both top and bottom heater core hoses. Using a infrared thermometer the top was hotter so I assumed that's the inlet. I microwaved a glass of water to boiling, put the ends of the tubing in it to soften it up and slid the small hose with garden hose connector to the bottom (outlet) and the long hose to the top (input) into a clean white bucket. To start with I GENTLY blew a little air into the bottom and blew out the coolant (this reverses the normal flow of coolant). I saw about 10 small flecks of black but nothing serious. I then hooked up the garden hose and slowly let water flow through the core until clear. I reversed the hoses and went back and forth about 5 times. I then mixed up some Prestone radiator cleaner+flush in a heavy concentrate but didn't dare use it straight from the bottle. I filled up the core and let it sit for an hour. I then flushed it out both ways twice. I blew out the core with light air and filled it up with Toyota coolant. I reattached the heater hoses.

Since I already had a mess I said "screw it" and just replaced the thermostat with a genuine Toyota OEM one ($46). I think the old one was still good since I put it in and out of boiling water and it still opens and closes. I topped off the fluid and reservoir after starting the engine and doing my best to get air out. We have some pretty steep streets where I live and I parked for a minute uphill and reved the engine up and down a few time to try to get air to the radiator cap. Whatever I did now works. I have hot air at idle and it's 38 degrees outside.

 

What did your temperature gauge look like when you went to turn the heater on? Do not try to put a hose to flush anything and I mean anything on your truck.

From my limited experience with second gens Ill throw some guessing at you

1) it was just cold af and it took longer than expected to warm up the coolant
2) thermostat got stuck open from the cold which made warm up process longer
3) there are small “doors which open and close depending on if you want hot or cold air. These could have been having an issue.

Like I said I don’t have a second gen but I’ve seen these happen to other people in the automotive world. Hope it helps

 

Wait so you’re saying it only blew hot air at mid to high rpm?

 

Air pocket in system.

 

Air bubble. Bleed the system and enjoy...
lots of times when coolant drain and fills or flushes are done, bleeding the heater is overlooked. When I do them I use a spill proof funnel with the engine at operating temp I rev the engine a bunch to force the air bubbles out. Also helps if the front end is jacked up so the radiator cap is the tallest point.

 

The temp gauge was at normal 1/2 up. Plenty of time to heat it up. I drove it again today for 20 miles and it was blowing hot. I stopped in the driveway and just idled. In and 30 seconds it started blowing much cooler air. When driving and warm air coming out the blender door worked just fine. I could change the temp.

I know that the cooling system runs about 15 psi and a garden hose is around 50 or more. You have to be careful or even gravity feed the back flush. You can blow the core.

 

I’ll try that but you would think 30,000 miles would work that out.

 

Yes. Cools back down at idle.

 

You would be surprised. Fords give me the biggest headaches with heater bleeds.

 

If no air pockets are found, the heater core could be getting restricted. This is a common symptom on VW cores when they get some scale/sludge in them

 

hmm thats weird. well if ur coolant is up to temp then I'm out of the game here. These guys above seem like they have some pretty good idea of what it could be past my knowledge. The air bubble seems like a good place to start.

 

The VW Beetles cores don't get restricted, & have, by far, the biggest air bubbles. Just my opinion

 

Asking for a few to help with this experiment:

I'm asking someone to try this on their 2nd generation. It won't take but a minute and you can do it after a few miles of driving.

If you live where the ambient temp is in the 30's or lower run the truck to full temp (gauge 1/2 way up) through normal driving, just turn on the heat to full hot, adjust the air to vent (dial to full left), fan on full, recirculate button off, and leave the truck in idle. Do not even touch the gas pedal. Do you notice the air getting cooler in about 45 seconds? If you rev up the RPM to say 1500+ do you notice the air getting warmer? Maybe I never noticed this before.

It makes sense that it cools down since the fan is running cold air over the heater core faster than the water pump pushes in hot coolant at idle. BMW's even have an electric recirculation pump that turns on when idling to counter-act this condition.

 

Could it be that your water pump is eroded and the impeller is not moving enough coolant at idle?

 

That is possible but since it's second most intrusive and expensive fix after a head gasket I don't want to just jump into that. What I did last night (see the update in the OP) increased the heat at significantly. Even at that I'm just asking if a slight but noticeable cooling occurs when the fan is on full blast and the truck in idling. When the fan is less than full speed the air comes out pretty darn warm. I'm guessing is normal but I wouldn't notice it because once the cabin is warm I never run it on HI. This is livable and wouldn't spend the time/money to replace the pump if it's not overheating.

 

Also I have never seen an actual 4.0 water pump but all the videos and OEM pump pictures show that the impeller on these are metal and not rubber/plastic. I think the only thing that could go wrong with a water pump would be the seal or bearing, not the impeller.

 

I just tried your requested experiment...It's only about 40 deg F outside currently. At idle, max temp setting, max fan setting with air out the front dash vents, mine will drive you right of there. Very warm, no decrease at idle...tried for about 3 minutes. No perceptible difference at increased rpm either. This is a '15 DCLB Sport.
Jeff

 

Good. It sounds like you got the air out of the system.

 

My ford truck does what your talking about when the water gets low in radiator

 

No heat at idle or stop lights but fine when you are moving? I came across this problem on my 2004 4Runner... Of course after reading all the opinions here I was ready to tear this thing apart and prepare for the worse .. but being a retired N.A.I.S.E fully certified mechanic I decided to stop and follow the first rule. Always start with the basics first Here are my thoughts and final answer to what seems a rather puzzling problem and how you will more than likely be able to fix it

1) If you are getting hot air out of your heater core while driving but no heat at stop lights...then your heater core is fine - move on.

2) Does your car have signs of actually overheating? If not, its not your head gasket. Trust me you would know if you had a bad head gasket. A bad head gasket leaks hot exhaust gas into coolant chambers inside the head (or possibly water in to the oil - visible as a white sludge under your engine oil cap)....making the coolant extremely hot and your car would definitely overheat quickly. Besides, imo a blown head gasket would have no little to no direct effect as to lack of heat. - move on.
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3) A quick cycle through your vent positions should quickly let you know if a vent actuator is bad. Is the air going where you direct it in the different positions?.. If so, no problem with vents - move on.

4) Does your vehicle start up and reach normal operating temperature on your temp gauge in a few minutes? If it does than forget about the thermostat. Assuming your temp gauge is working, a thermostat stuck closed would peg your temp gauge and your car would be overheating. If the thermostat stuck open your engine would take much longer to reach operating temp - but that's all. Your temp gauge may remain a bit lower but you would get the exact same lower amount of heat regardless of speed or if stopped. If its in the normal temp range on your gauge with no signs of overheating your fine. Leave the thermostat alone and move on.

5) Without any visible signs of overheating you can probably forget about that water pump.. At idle or stop lights your engine would need its greatest cooling requirements met as there is no air blowing through the radiator. Again, your vehicle will quickly overheat at some point without a fully functioning water pump..never mind your car heater not blowing hot air. - move on.

6) Now - for the final solution which will apply in 98% of situations where you get heat when you drive but none when you stop or idle at a light. Fill the radiator. Yes, that's it. Fill the radiator. Your plastic reservoir tank can appear full... and your radiator can still be quite empty. Remove the actual radiator cap, start the vehicle and add antifreeze until its full...run until minimal air bubbles appear (don't worry all the tiny ones will eventually purge themselves.. Make sure your heater is on when filling....when you think you have most of the air out cap it and enjoy the heat at stoplights again It's not rocket science...just common sense