View Full Version : Heater Temp Problem

11-27-2014, 09:44 PM
91 Celica ST, 4A-FE automatic, 262,000 miles. Heater always worked great. All temp knob cables connected and levers move with temp knob. Coolant is the Prestone Extended Life yellow (5 years, 150,000 mile type), last changed 3 years ago. Engine temp gauge in normal spot with engine at operating temp. This year, heater air temp is warm (knob set to max hot) with engine running fast, cool when engine runs slow (not hot like it used to be).

Where should I look first? Clogged heater core? Water valve problem? Clogging heater hoses? Water pump impeller worn (original water pump, I believe)? If clogged heater core, why now with good coolant in it (checked coolant with test strip, 50:50 mix, pH is where it should be)?

Any common problems with the heater on this particular model that would help me test the most likely causes first?

11-27-2014, 10:59 PM
The fact that by increasing rpm the heater temperature rises makes me feel like it's a flow issue. Higher rpm means more flow from the water pump. I'd check that the valve (mounted on the firewall about center I think) is opening all the way. Remove the cable from it and manually actuate it. If not that I will assume the core is restricted. A restriction can occur simply from the natural breakdown of metal materials in the cooling system, coolant is corrosive.

11-28-2014, 06:02 AM
Engine gauge isnt reliable. It has a wide range of temps it will stay in "normal" under.

You also could have a bad thermostat cap that isnt holding pressure, or you could have air in your lines somewhere, it could literally be anything. Id just try a good old fashioned bleed.

11-28-2014, 07:43 AM
my 92 had a similar issue. Coolant plenty hot, checked the valve for flow, and flushed the core. none of those things fixed it. Turns out the blend door was obstructed and not actuating all the way to the hot side.

I agree however that your issue sound like a flow/core issue.

11-28-2014, 02:18 PM
Thanks for your inputs. Sounds like there isn't one particular common cause on these 5th Gens for this problem - which I was hoping to uncover but didn't really expect. I'm not convinced of a thermostat problem given what the temp gauge is telling me, but I did notice the temp gauge takes longer to reach operating temp than it used to, which is a hint something isn't quite right there. I think I will try a back flush on the heater core and hope that knocks out some of the problem, and will replace the thermostat at the same time and put in fresh coolant. I'll also check the flow valve at the same time, but I know the cable moves the flow valve lever the full range of travel, and it is in the open position when the temp knob is all the way to hot. Doesn't mean internally it is opening though, but will check that. I'm just hoping I don't have to remove and replace the heater core - I believe that is a very difficult process on these 5th Gens. Has anyone successfully replaced a heater core on a 5th Gen? If so, what was you take on difficulty? What else broke during the process of replacement? Just want to be prepared for this, just in case.

11-29-2014, 01:04 AM

my 92 had a similar issue. Coolant plenty hot, checked the valve for flow, and flushed the core. none of those things fixed it. Turns out the blend door was obstructed and not actuating all the way to the hot side.

I agree however that your issue sound like a flow/core issue.

12-05-2014, 09:30 AM
I changed the heater core on my AllTrac.


I guess Joey said that it can be done without removing the dash, but I couldn't manage it.
Disconnect battery, wait 10 minutes.
Remove steering wheel, airbag, guage cluster.
remove center stereo surround, stereo, disconnect heater/AC controls
(Attach a fish line to the ones that go through to the engine bay *on the engine bay side* or you will have a bitch of a time getting them back through)
Remove glove box.
remove dash vents and defrost vents on sides.
remove center console.
remove all retaining screws for lower dash.
Remove the driver and passenger side A pillar covers.
Start from driver side and remove all 10mm and 12mm bolts and retaining nuts going into crash bar and firewall.
Work over until you get over to the passenger side. Make sure that the screws holding in the air box
(in under dash area) have been removed and all cables are out of the way and plugs are unplugged)
Start from the bottom and when you remove the last ones from the top, the entire dash should move more freely.
Check again to be sure that you have removed all retaining bolts (nuts)
Make sure that the heater control valve cables are disconnected and that the heater core clamps and lines are completely disconnected with no damage to the heater core tubes. (THIS ONE IS IMPORTANT)
Slowly begin removal of dash with another person helping you. There are big "C" clip things that hold the dash to the crash bar. only like 2 or 3 I think. Just one quick yank gets them off of there no muss no fuss.

Once it is out, send it to the radiator repair shop you called before starting this project so that they can boil it out, pressure test it and rod it out/clean it. They should be able to braze any leaks if they find them.

Hopefully, you were diligent and used sandwich baggies and labels and sharpie to mark and sort all of your bolts along with their locations so that you don't go insane trying to get everything back together correctly.

There are a few little pads of (now) compressed felt along the inner edge of the dash where it meets the windshield. Go buy some new felt to replace this, as it stops the dash from squeaking. Trust me on this one.

Put the whole mess back together. The end?

Total time for me with no help and no workspace was 9 hours start to finish.

Cost for heater core to be cleaned, pressure tested and repaired (mine needed some brazing) was about $50-60.

I hope this helps you if you do end up needing to do this, but I don't envy you the job if you do. ;)

12-05-2014, 04:17 PM
When I did this in my 1st gen Neon it took me about a week....
As an inexperienced kid figuring it out on my own.