I think before you start replacing parts or doing anything maintenance, get a thermal gun or even use your hand on all the coolant hoses and see if you are feeling cold spots. If you can isolate the cold spots, it may help to narrow down the issue.

 

we did check the hoses while we were working on this issue over the weekend. Flex hoses would get very warm to hot to the touch. We did use a infrared thermal gun on the lines/hoses going to the heater core, though what those read outs were, I dont recall right off.

 

Hello all, Son here,

Just got done with a 1 hr road trip from Denver to Colorado Springs. I inserted a probe thermometer in the air duct to monitor temperatures while driving.

The air coming out of the vents would stay around 60 degrees, but there were brief spurts where it would reach 80; But not for more than 2-3 minutes before dropping again.

When I reach Colorado Springs, I had 2-3 errands to run before going home for the night, if the vehicle was moving 30mph or less, the temp coming out of the vents would get to 90-100 degrees.

We tested the heater in a 2007 toyota 4runner sr5 while we were attempting to fix the FJ over the weekend. That vehicle is well maintained and in excellent condition. Temperatures coming out of those vents was hovering between 130 and 140. So Im taking that as a benchmark of where we could/should be able to get.

Pop and I are both at a loss... Any guidance would be welcome. (Weather in colorado this time of year is on the chilly side)

 

There's another recent thread here with a poster's 2008 FJ exhibiting the same problem. It's highly unlikely that the heater core is blocked. My personal running theory with this early engine is that #6 cylinder has the start of a failing head gasket and is throwing a bubble into the system only on cold startup, which migrates to the system high point in the heater hoses and interferes with coolant flow through the heater core. Cylinder #6 on these 2007 to 09 engines is notorious for a failed head gasket after a few hundred thousand miles, but usually people get sudden engine overheating issues with this problem. The poster is going to let his FJ sit for a few days, pull the spark plug and look for coolant sitting on top of #6 piston to see if he has this problem. His piston in that cylinder has a weird combustion soot pattern too. So far, he hasn't reported back.

https://www.fjcruiserforums.com/posts/9620725/

The other theory is that a heater blend door is not fully opening. The servos that operates the doors can partially fail or break. As for testing it, that might be difficult since there are several levers that operate several doors and I'm not sure which one may be at fault. You'll also want to check the cabin air filter behind the glove box for debris and dirt blockages as well. Read through the thread above for more info. Maybe FJtest can advise which blend door and lever to check.

 

OH JEZZZ! Head gasket?

 

Just a theory. I'd check the cabin air filter and operation of the heater blend doors first. If you have an infrared thermometer, crawl under the dash, point it around the heater core and see what the temp is at full engine operating temps. We still haven't verified what the issue is with the other poster's FJ yet either. Personally, I'd love to get it figured out, as does he.

 

With two similar vintage FJs now exhibiting exactly the same lack-of-heat-from-the-heater symptoms, and no 'obvious' fault that would cause this symptom, it's time for a very detailed fishbone diagram that would identify every possible fault or condition that could affect heater output temperature, and what type of diagnostic test would need to be performed to eliminate each fault from consideration.

Tomorrow is likely to be extremely busy as the first real workday after the holiday, but I'll start on a heater-with-no-heat fishbone.

 

That'll be interesting. Thanks.

 

Scott, fishbone? You mean logic tree?

 

Yes.

 

As a point of reference for the heater coolant temperatures of a healthy FJ, here is what I measured on my 2013 Canadian spec FJ tonight. I used a fast acting K-type thermocouple on a temperature meter to measure. On a 2013 FJ, the aluminum heater coolant pipes are on the drivers side and run vertically up the firewall, just to the right of the gas pedal.

With the engine just barely warmed up so that the thermostat is just starting to open (181ºF on the ODB2 reader), the heater core inlet pipe surface measured 165ºF and the outlet was 161ºF with the engine idling and fan off. With the heat at maximum and fan on high, the inlet pipe was 160º and the outlet pipe was 124º. The ODB coolant temperature was down to 176ºF with the fan on high.

If it is cold outside, the heater can pull enough heat to start cooling off the engine at idle, even with the thermostat closed. The ambient temp was about 44°F. If it is colder outside, if the engine is just idling and the heater at maximum with the fan on high, the engine will get even colder. With the outside temp at 12°F on a windy day, idling outside with the heater maxed out, I've seen the coolant get down to 152°F from 182 after a long time. The engine doesn't make much heat at idle, it is too efficient.

With the fan on low and the selector to bi-level (floor and vents), the center vent air temperature reached about 144ºF. With the fan on high, this air temp dropped to about 122ºF. The outer vents are a bit cooler with the heat loss in the ducting. This is at idle where the heater can take more heat than the engine can make at that temperature. If I were driving so that there was load on the engine to keep the coolant temp up then the vent air could get hotter.

If you try to use a infra-red temperature gun instead of a contact thermometer or thermocouple, then make sure that the emissivity setting is set correctly for the surface that you are pointing at. Otherwise there are large measurement errors. Bare aluminum has very different emissitivity than black plastic vent material.

 

FJ -
Your heater core inlet temperatures are interesting - how did you make the physical connection between the thermocouple and the heater inlet pipe?

I'll make some comparative measurements on my US-spec '14, except with ambient temperatures in the 60-70F range. I have a tablet permanently set up displaying OBD parameters (including coolant temperature) via Torque Pro, and I have some low-mass type K thermocouples.

I would expect that our measurements would be very similar.

 

Kaiju I have been following the thread you linked above since my son discovered he had no heat. Lots of good info in that thread. I did see the mention of the head gasket but since I have no history of the engine maintenance and since in general the vehicle appears to have been well care for I was chasing that idea yet. From that thread I got the idea/thought that there could be air trapped in the system. I don't have a vacuum draw down tool nor are they available to rent at the parts store. So I did what I thought might be the second best option. Empty the radiator then raise the front of the vehicle and refill using a no spill funnel. That is when we finally got good heat, not great but improved. Our plan was/is to install fresh coolant after we solved the heating issue. With 150k miles on the engine and not knowing any history of the vehicles prior maintenance this made sense.

FJ-6MT thanks for your detailed response and the info regarding measuring temps. We did use a heat thermal heat gun but it is a cheapie, it has no adjustability. I will see if I can locate a surface contact measuring device.

Appreciate any and all help

 

I'd like to know the specifics of that as well.

 

If you look in the driver's side footwell, just to the right of the gas pedal, you can see the two aluminum heater pipes running vertically together. I used a low-mass thermocouple that had a stiffener tube near the end, but had the actual themocouple weld joint fully exposed. I just pushed the end of the thermocouple right against the pipe perpendicular to the pipe. The bare aluminum is not slippery so the probe is easy to hold solid against the pipe with fingers well away. For the air temp measurements, I just inserted the probe an inch into the vent louvers.

The posts above had temperature measurements, but since it isn't clear what the temperature should typically be, it is hard to know if the values are good or bad. I was curious to see what the temperatures actually are in my FJ where the heater is very strong.

 

To keep this alive and to keep you guys that have responded updated. Yesterday (1/6/2025) son had small amount of heat in the city (90 out of the vents). He had an appt about 60 miles from his home. During that drive he saw temps close to 110. I didn't talk to him last night so I do not know what the performance on return trip home was as of yet.

After reading everyone's comments I am beginning to lean towards the blend/mode door being and issue. His FJ doesn't have the typical clicking that seems to be associated with failed motor so I am not 100% sold on this thought yet, but it seems more logical than I was thinking (plugged core) i.e. he does have some heat sometimes.

I will definitely be checking the heater core with either a thermal gun or try to locate a thermocouple contact type device before I proceed with the Blend/Mode door actuator replacement.

Kaiju said above he wasn't familiar with how to tell if the Blend/Mode door was/is failing (besides the common clicking noise)

Anyone familiar with a method we could use to confirm the blend door isn't functioning at 100%?

Thanks again everyone for your insight.

 

One more question since the blend/mode door isn't making noise and seems to redirect the air when needed is it possible that the problem is the "Air Mix Control Servo" instead?

 

Here are the 2 servos. They operate the blend door and mode control door. They are located to the left of the glovebox if you remove it. You are probably having issues with the air mix door servo or the door not moving properly, which is the lower down servo on the HVAC housing, but I would check the operation of both to make sure the mode servo is moving to the proper "heat" position.

 

Brief Update: Son's FJ will be here this weekend and we plan to attack this problem. He has had mostly intermittent low heat all week. I will update this thread with what I discover or fix I find.

Kaiju thank you very much for those pictures.

 

Thankfully, I am not having any of these problems, but I appreciate from those diagrams how much easier Toyota engineers designed it than the fools at Jeep. Some Grand Cherokees have one of the blend door servos located between the HVAC housing and the firewall, behind everything under the dash.

 

Update: We had a chance to work on this “Low to No heat” this weekend. First we confirmed that the all 3 servos were functioning completely. Following "Friday Night Cruiser’s" advice we started checking the plumbing going to the heater core. Sure enough the inlet was hot and the outlet was much cooler. First we hooked up a garden hose to the heater core backflushed and then a forward flush. Lots of larger particles in that flush. Repeated that 3 times with less and less particles in each flush. Now we have heat, but it is hottest on the driver vent (about 140) progressively less across the cab with the passenger side nearly cool (about 65-70). Next we did a distilled water flush followed by Blue Devil Radiator flush. Ran that about 1.5 hours. Drained and did two more Distilled water flushes. Still have heat, only partly improved on the passenger side. Since it was going to freeze in Denver Saturday we decide to add coolant and attack the problem again another day. Son took the FJ to Home Depot that night and heat quit.

Sunday morning, we started blowing out the heater core (garden hose), backward then forward. Did the cycle 2 to 3 times. Lot more stuff in the first pass. Took the FJ for a run and heat worked again (very slight improvement on passenger side). Heat fluctuated on that run, came home and repeated the flushing steps on the heart core, again a lot of particles in the first flush. Out for a run and again slight improvement on passenger side. Repeated these steps about 6 times today. Each time we would get a lot of particles on the first flush of each session. The last blow out was late in the day and there were less larger particles than previously but still too much.

So, we know something in the block or the radiator is floating around and getting into the core and blocking it. What we don’t know is how to more effectively get all that crap out.

I am very mechanical but not familiar with the cooling system design in the block. Would it be possible to blow water thru the block via the heater hoses?

Anybody wager a guess/suggestion for what this might be (see pics). The white is crunchy maybe a calcium deposit? The black stuff isn’t as crunchy.

Got a call from the Son on his way home, about halfway home the heat stopped. So, I know there is more stuff coming out of the system.

Using the Torque app the cooling system is running dead on 187-188 constantly. Also has a new thermostat.

Suggestions for how to get this stuff out? Is there a more professional/powerful method to clear the block. If it needs to go to a shop then we will consider that.



Thanks for any input/suggestions.

 

That stuff appears to be corrosion particles from using improper coolant in the system at one time. Do you know the history of your FJ's coolant changes? Have you reverse flushed the cooling system and the heater core separately?

 

We have not back flushed the cooling system. We were unsure if that was safe or not.

We have drained and flushed the cooling system several times by draining the radiator and engine block, filling with Blue Devil treatment mixed with distilled water as instructed, then we flushed with just distilled water twice, then filled with new Toyota premix 50/50 coolant.

We have back flushed the heater core in excess of 15 times over the weekend, but not back flushed the block.

We are unsure how to proceed with getting the rest of the debris out. I think our current plan of attack (unless discouraged otherwise) is to:

1. drain the coolant system,
2. recapture what coolant we can,
3. back flush the engine block,
4. back flush the heater core (separately),
5. flush the radiator,
6. clear the radiator hoses,
7. capture what debris can be captured,
8. repeat steps 3-7 if we think it necessary based on debris coming out, Possibly multiple times.
9. fill with Blue Devil and distilled water again
10. run vehicle for 1-3 hours,
11. drain system,
12. flush twice with distilled water
13. inspect coolant for debris
14. reuse coolant if possible as it is currently in the vehicle less than 72 hours and its about $40 a gallon.

If someone thinks this is unwise, or has a better idea please share, we are completely stumped.

We worked on it last Saturday, this Friday, Saturday and Sunday and have yet to solve the issue for more than a few hours.

 

Your plan should be fine.
I agree with your thoughts: that there is so much of this mineral crap in your system that although you've been back flushing the heater core there is so much more in the engine that it's re-plugging the heater each time it is run.

Someone put tap water in your cooling system, mineral deposits settled out and it is amazing that the only symptom was low heat from the heater core (goes to show the main cooling system has a nice margin that saved the engine from overheating which could have been catastrophic).

One suggestion: fill the system with 50/50 distilled water / vinegar and run that warm and then let it sit for at least 15 minutes / 30 minutes. The acid in the vinegar will melt the calcium deposits, the warm temp will help speed that up and the short duration / low acidity of the vinegar means the metal won't be negatively affected.

Try putting some of your deposits into a cup of vinegar and see how they dissolve into a sort of a jelly like substance - much easier to flush out.

 

Excellent Idea with the vinegar test
Glad we saved some

 

OK, I spoke with my mechanic friend yesterday and the new (SLIGHTLY) revised plan may now be:

1. drain the coolant system,
2. recapture what coolant we can,
3. back flush the engine block, radiator, radiator hoses and heater core separately with tap water
4. capture what debris can be captured,
5. Fill radiator and heater core with CLR and let sit overnight
6. Drain and flush Heater core and radiator with tap water
7. flush twice with distilled water
8. inspect coolant for debris
9. reuse coolant if possible

 

Brief Update:
I put some of the debris in a 50/50 solution of vinegar & water, I also put some debris in CLR. After 24 hours neither of those chemicals melted the debris. I think the particles are less crunchy now but neither solution melted the particles completely.

Thanks everyone for the help/insight

 

How old is the radiator? They typically have a service life of 10 years and that part may be holding a lot of that junk. You're having the same luck dissolving minerals as I do in toilets and sinks. Vinegar and CLR don't really dissolve the stuff effectively and you don't want something stronger that would damage the system. Unfortunately, you'll probably never get all of the stuff out of the block, but several reverse flushings should get most of it. Be sure to remove the driver's side coolant plug (I know, it's a PITA) to flush out that side of the block. You can try removing the radiator to perform positional flush to clean it out too, unless it's old, then I'd replace it. If you have petcocks in the block, completely remove the units to get a larger hole for flushing.