No heat at idle - Heat gets warmer when above 1500 RPM

2016 TRD Off Road - 6 spd Manual 121K
Truck has had every recommended service done by the book at the dealership I purchased it at.
read through forum and haven't found what I'm looking for...other then one worst case scenario

A month ago I was waiting for some one with my truck running and all of the sudden I noticed my truck heat was blowing cold air. Thinking that was weird I adjusted the dial hi and still nothing. started driving and boom hot air started blasting. Not thinking anything of it weather warms back up and a seems fine, even had a few dustings of snow and let the truck run for about 10min and hoped in and it was piping hot in the cab.

Well last week the previous symptoms came back, no heat at idle regardless of the fan speed and if you keep the engine rpm above 1500 it heats back up.

- I started keeping an eye on the dash temp gauge it never went above the middle where it always sits

So I started with the basics and popped the hood I noticed the coolant res tank was down half of the way - I filled it the full mark - no change

Thought maybe the system could have air in it. parked the truck on an incline with the passenger side higher then the drivers side both elevated over the rear. used my no spill funnel and ran the truck for about 20min and seamed to get some air bubbles out but not that much. - no change and the temp gauge remained the same except once for about thirty seconds climbed to about 3/4 the way still not getting near the red H but in between half and the second to last mark. It then came down once I raised the idle. Still no change with the heat.

Heater core hoses - both are hot, but one is hotter then the other one

while driving and the heat is working - heater temp gauge works as should along with heat direction (floor vs. vents vs. defrost)

At this point I notice upper rad hose is warm as it should be, lower rad hose ice cold

At this point I'm thinking maybe the t stat is sticking and causing temp gauge to do weird things and the fact that lower radiator hose is always ice cold.

So I go down to Toyota get a Water inlet housing and install. use the same coolant bleeding process as used above I get the same results - however this time as I was going up the hill for a test drive i heard some gargling behind the dash then it went away. Upon my trip back the temp gauge started climbing back to 3/4 and a little higher while driving with a very light foot. if I increased throttle it started going back down. but stayed hotter then I have seen it run in the past.

still no heat at idle

I tried today to attempt another air bleed with the lisle funnel and same results some initial gargling behind the dash and then the temp seems to climb when driving with a light foot. noticed before this air bleed attempt my coolant res tank was almost empty.

At this point I've done all i can do so its probably going to a shop, I have never seen the temp gauge climb past half since new so that part worries me.

Oil is clean, no leaking from the motor, no leaking or coolant smell inside the cab, no sweat smells coming from the exhaust. All fluids had just been flushed for the third time at a 100K

Sorry for the long thread but I'm at a loss

Cheers

 

There's a lot to this, but this is how I'd start.

Check for heat to the heater core. Two hoses connect to the firewall, they should both be hot to the touch at operating temp.

If they are both hot, there is no coolant issues.

After that I get on my back and cycle the heater control and watch the actuators on the heater core box. Scan tool can watch the commands but you need to physically see them move.

Once those 2 are verified I look at oddities like stuck cooling fan clutch, or other issues.

 

I had my OR putting out cold air once also and I found out I had the temp set at colder than the ambient temp in the truck . Once I raised the temp with the dial ( dual zone ) the air turned hot . I hope it is that simple for you .
Edit , I raised the temp setting and not the fan speed .

 

Might be the radiator cap needs to be replaced as this will affect system pressure. I had car several years ago with a heating problem and a $10 radiator cap seemed to fix it. Worth a try.

 

Idk why but it seems like a water pump issue? My gut could be wrong tho.

 

I compliment you on a very organized post -- often we see something more like "my truck don't heat, what's wrong?".

I have had older cars acting similar that would have an air bubble in the heater core.

This is easier with a helper:

Wear safety glasses or goggles!
1) with the engine cold and off loosen the uppermost heater hose at the fitting on the firewall
2) have helper start truck, make sure controls are set for maximum heat, speed engine up to 1500 rpm
3) briefly pull the hose off the fitting with the engine running to burp any air out - usually you will see it
4) put hose back on when coolant flow is solid
5) shut off engine, tighten clamp and refill system as needed

If you have an old blanket (disposable) you can put it under the truck to catch any mess.
Please update us on your progress & good luck.

 

Thanks for some of the feed back so far everyone, I am out of town and will be back for next week to get back to diagnostics.

does anyone know if the waterpump on these have metal or plastic fins?

 

Me the opposite of the op. I had heat but the fan would hardly blow even on high. I thought maybe some snow on the top of the windshield would have anything to do with it but no. Then I lowered the temp down to the coldest then up again to heat and the fan came on full blast. Weird thing cause it worked fin before this incident. I read the 3 Gen have a lot of small little annoying things happening. Still have no clue why.

 

Having my first tacoma in 2020 i found the cooling system different from my gen1 tundra that it takes a long time before getting heat. Another weird thing in cold weather below freezing temps after the truck has fully warmed up, and traveling with the temp gauge in the middle, then coming to a stop, the temp gauge drops to 1/3 warm. Soon as i take off its back to the middle. None of my older toyota's have ever acted like this. I asked the service dept last year to check it out, but they told me thats how these trucks are.

 

I have had all 3 Generations over the years and never had any issues with the heater fan. This new one I think is too hi tech and needs a lot of experimenting to figure out why it acts the way it does especially in the heating and cooling section. I am still experimenting with the dual zone and especially the auto button on it. When I push the auto it jumps into AC and REC phase which to me is not normal especially if it is cold outside then I have to manually turn the rec and ac off to get heat.
It says in the manual to leave the Heat system on auto for faster heating/cooling in the cab when remote starting it and that does not seem to work as when I get in the truck after starting it up the auto is off.
Like mentioned still lot to learn.

 

It is very likely that the plastic pipe at the rear of the engine connecting the two cylinder heads is cracked and letting air enter the cooling system. The air rises to the highest point (the heater core) and cuts down the heater efficiency. I had the same problem 2 years ago and solved it by putting two bottles of Subaru cooling system conditioner (which is really a leak sealer) in the radiator. Cheaper than the $1000 or so it costs to replace the pipe with another plastic pipe that will probably crack again. The Subaru sealer works better than most sealers because it is (was) factory installed in most Subarus due to potential head gasket failures. I had a 2002 WRX and noticed the dreaded heater failure after I had the water pump replaced; I had insisted that the shop put the sealer in but it turns out they did not because they did not believe in leak sealers. I then put in 2 bottles of the sealer and solved the problem. I finally traded the car in (with 247,000 miles) on my Taco with the original head gaskets.

 

That would suck! I am hoping for something dumb. there is still 4,200 miles on my warranty but who knows what that means. I am starting it again right now and seeing if it will stay at temp or start overheating

 

Sounds like air in the coolant system to me. I use an airlift to fill cooling systems and it works well.

 

Just ran the truck for another test, I did not drive but just let run for 30 min
- heat on defrost, on hi, and only 2 clicks of fan
- Temp gauge reached the middle and stayed there
- continued to blow cold air...no heat

- heater core lines are warm to touch as well

The minute I brought the throttle to 1,500 rpm heat instantly

honestly I know all vehicles have issues but its a tacoma... Id go back to my second gen any day! 268K fluid flushes, brakes, and wheel bearings only never had a single problem...

 

hopefully that's what it is and not catastrophic issues like suggested from others

 

My 2005 did the same exact thing when I lost one of the Head Gaskets. Driving I had heat and when I would stop at a red light it blew cold air, as soon as I was back on the gas pedal I had heat. While the truck is cold. Take your radiator cap off. Start you truck and then look down in the radiator. Watch for small bubbles coming up to the top of the radiator. Give it a little time. If you see small bubbles you have a problem.

 

You want AC on when heat is running. That allows any moisture to condense on the coils as the air passes through.

Auto also starts blowing air slowly depending on the temperature, but a fix for that is changing the setting to defrost.

 

Thanks for the explanation on the auto I just learned something. I still would like to know why I had heat and was not using the Auto but fan was not blowing on HI till I lowered the temp down to lo and back up to hi and the fan started to work normal.

 

First with it cold remove the radiator cap check level in the radiator. Next you stated the heater core hoses seemed different temperatures could be a clogged heatercore. Check the cabin air filter for mouse nest. If core can be back flushed that may help. I have noticed on my 16 /19 SR manual temp settings it would heat a lot better than my temperature setting heater. Lastly Chrysler had uses stop leak from the factory on the 200 and they would plug up.

 

You should get a little heat fairly fast because we have the PTC electric heating element that triggers before your coolant gets warm.