[bentos1]

I have not had proper heat in my 1997 4Runner since I've owned it. The heat has always been warm but never hot. The front and back are both the same temp which makes me think it’s the cooling system and not the front heater core or controls. I have read all the threads on this site but most fixes are: air in the system, thermostat, one of the HVAC cables was disconnected.

Months ago I had a high idle issue, 1500 RPM that would not go away. I decided to change my thermostat to see if it would fix the issue which had no effect. I wanted to test the original thermostat afterwards which appeared to be working and opened fine when testing with boiling water.
The fix ended up being my temp sensor.
While troubleshooting the above issue I had tried two thermostats, flushed the coolant and replaced with Toyota red.

My current no heat/warm heat issue (front and back are warm)

- I have attempted to burp the system with the front of the truck in the air. Have also used a hose from the rad to ensure it was the highest point. Squished upper and lower radiator hoses to help get air out of the system. No effect

- My coolant level is constant and reservoir level stays steady

- No signs of a leaking heater core, no damp carpet, no steam coming from vents, no coolant smell

- Coolant valve to heater core works with the HVAC controls and moves easily. Blend door under dash (by glovebox) is connected and you can hear the “thunk” when switching the HVAC from cold to hot

- Flushed heater core both directions 2 times. Upon flushing, the core was filled with coolant and everything flowed nicely. Some crud/particulate came out from the core but it has not changed the heating issues.

- Both hoses to heater core are warm, not really hot. I can keep my hand on there.

- The temp gauge in the dash stays constant at the halfway mark. I have never seen it get past half which I believe is normal.

- Top radiator house is hot but not extremely burning. Lower hose is fairly cold (The rad is doing a good job?)

- OBD scan shows coolant level being 87 C (188.6 F) after driving for 30 minutes. Only takes a few minutes to get up to temp when idling in the driveway though.


I have not bypassed the heater valve by the firewall as the warm heat issue affects both front and back. I understand that the 4Runner has a heater core for each.
The age of the radiator is unknown. There are not markings that I was able to see aside from “PA66-GF25” which pertains to the material.
• PA66-GF25 is a glass fibre reinforced injection moulding, it can withstand temperatures up to 310C, sounds ideal for use in the end tanks of car radiators!
Seems my truck would be overheating if the water pump was failing. No signs of this happening at all.

I would like to try my best to troubleshoot the issue instead of replacing parts hoping they fix the issue. I am just looking for ideas of what I should be troubleshooting and the best way to do so.

[RockyrT4R]

Under the dash on the passenger side there's a little servo (I believe it's SERVO 2/ it could be SERVO 1 but I don't think so) that controls how much outside (cold air) is combined with heated air to get the right temperature well if that servo stops working and the outside air vent was partially (or completely) open then it can't heat it up enough resulting in warm but not hot air, I took my servo out and unplugged it and just by hand moved the piece it turns until it was not pulling outside air in.
(Also my servo was making a clicking noise and wouldn't move)
I will attach a pic of about where it is...
Hope this helps!

[bentos1]

Originally Posted by RockyrT4R

Under the dash on the passenger side there's a little servo (I believe it's SERVO 2/ it could be SERVO 1 but I don't think so) that controls how much outside (cold air) is combined with heated air to get the right temperature well if that servo stops working and the outside air vent was partially (or completely) open then it can't heat it up enough resulting in warm but not hot air, I took my servo out and unplugged it and just by hand moved the piece it turns until it was not pulling outside air in.
(Also my servo was making a clicking noise and wouldn't move)
I will attach a pic of about where it is...
Hope this helps!

Thank you RockyrT4R. Just for clarification I have 1997 which uses all cables from what I can tell. The cable which moves this area is part of the HOT/COLD HVAC slider on my truck. This is connected and I can hear and see the parts moving via the cable.

It is possible that the part is not moving enough though.

Its my understanding that the front and rear heat have separate heater cores. This servo/mechanical linkage under the glove box only controls the front heater core, is that correct? I believe this actuates the blend door.

Would my rear heat be affected by this area?

[phattyduck]

The rear heater is 100% separate. You can see the lines (small) come off the main heater core lines in the engine bay, run to hard lines that go under the truck, then to the blower assembly under the passenger seat. The rear core is full-flow at all times, and the 'heat' adjustment operates a blend door that bypasses the core. All the switches, blower, etc. is separate from the main HVAC.

Your engine temps seem correct. You have the manual controls, so this is an odd one... not a whole lot to go wrong. I wonder if it something really whacky like a somewhat crushed hard line on the back of the engine?

-Charlie

[RockyrT4R]

Originally Posted by bentos1

Thank you RockyrT4R. Just for clarification I have 1997 which uses all cables from what I can tell. The cable which moves this area is part of the HOT/COLD HVAC slider on my truck. This is connected and I can hear and see the parts moving via the cable.

It is possible that the part is not moving enough though.

Its my understanding that the front and rear heat have separate heater cores. This servo/mechanical linkage under the glove box only controls the front heater core, is that correct? I believe this actuates the blend door.

Would my rear heat be affected by this area?

Yes the front linkage only controls front, although a pinched line would make a lot of sense , the rear heater has a manual cable line that lets more coolant flow through the little radiator type thing that it blows through and if you turn heat down it just restricts flow to that radiator thing, so I you had a main line that was pinched it would be a lot like the rear heaters function, and it still wouldn't let the truck overheat because it's just as if the heater was off

[bentos1]

Originally Posted by phattyduck

The rear heater is 100% separate. You can see the lines (small) come off the main heater core lines in the engine bay, run to hard lines that go under the truck, then to the blower assembly under the passenger seat. The rear core is full-flow at all times, and the 'heat' adjustment operates a blend door that bypasses the core. All the switches, blower, etc. is separate from the main HVAC.

Your engine temps seem correct. You have the manual controls, so this is an odd one... not a whole lot to go wrong. I wonder if it something really whacky like a somewhat crushed hard line on the back of the engine?

-Charlie

This is why I find it more difficult to solve. If the rear was poor heat or the front was only poor then I would have better idea of where to search.
The bit of heat I have feels the same front and rear at all times.

I have been leaning towards an air pocket but the tricks I tried to burp the system have not made any improvements.

Do you think blocking the rad with cardboard to attempt to get coolant temps up would help, or should my current coolant temp be good?

[nissanh]

You flushed the heater core but not the entire cooling system. Within few seconds again, dirt gets into the heater core.

I am sure until you do a complete coolant flush (remove Thermostat), you will not get hot air from the vents.

You'll even have to get a new radiator.

[alumarine]

If your coolant temp is really 188 then putting cardboard in front of the rad won't help.

I run at 190 and both my heaters blow hot.

[bentos1]

Originally Posted by nissanh

You flushed the heater core but not the entire cooling system. Within few seconds again, dirt gets into the heater core.

I am sure until you do a complete coolant flush (remove Thermostat), you will not get hot air from the vents.

You'll even have to get a new radiator.

When I replaced the thermostat I did have to remove lots of the coolant.
Are you suggesting to do this again and clean the block out this time? You are thinking this will cause tons of particulate that it will clock the existing radiator which will need to be replaced?

[bentos1]

Originally Posted by alumarine

If your coolant temp is really 188 then putting cardboard in front of the rad won't help.

I run at 190 and both my heaters blow hot.

Thank you for confirming that.

My OBD reader shows temp reaching 87 C. You can watch the temp rise as it warms up. I dont think I saw it go over 87 C though.

[spoonsports86]

i to have noticed my heat isnt super hot but more of a warm temp

but the other night it was hot as hell

[vt325xi]

I'm following this as well. My 1998 from AZ now living in VT is freezing this winter. Reversed flushed the core numerous times as well I let it sit for a few hours with CLR inside it to try and clear out the core. It looked like it was clean and flowing nicely so I put the piping back together and burped it as everyone has mentioned by raising the front end and running it to try and get rid of any air pockets. Still chilly in here this winter.

[bentos1]

@vt325xi is your rear heat the same temp as the front?

[vt325xi]

Originally Posted by bentos1

@vt325xi is your rear heat the same temp as the front?

Sometimes yes. Sometimes no. Like there is an airpocket in there or little to no flow. One of these days when it warms up again I will try to reverse flush both cores.

[bentos1]

Originally Posted by vt325xi

Sometimes yes. Sometimes no. Like there is an airpocket in there or little to no flow. One of these days when it warms up again I will try to reverse flush both cores.

you could try bleeding the air out like this guy: https://www.youtube.com/watch?v=lFA0tDZlm9Y&t=7s

I tried that the other day for 20+ min. Squished the hoses and had the front of the truck up in the air. Made no difference for me, but it might help you if there is air down there.