93 FZJ80 Heater Problems

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wbrad99

wbrad99

Joined
Sep 20, 2019
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San Diego
Done a lot of research and looking for advice regarding my heater.

‘93 FZJ80 - My front and rear heaters blow hot for about 10 seconds then become lukewarm. A/C blows cold and all blowers, modes, and vents work. Heater control valve on firewall is new and opens / closes properly. Back flushed heater core (I have not noticed the passenger floorboard get wet). New thermostat. Pulled most the dash to replace air mix servo motor, no improvement. The cable attached to air mix servo motor moves when I change the temp. Inlet and Outlet heater hoses are hot after running heater, so I would like to assume the heater core is not clogged. I’ve also burped the cooling system and ensured coolant levels are good.

Looking for any advice or direction for where to look next. Any tips are appreciated!
 
How long did it take you to change / get to the servo motor?

Have you checked the FSM that is specifically for diagnosing the AC? Its a separate document.
 
RC51 said:
How long did it take you to change / get to the servo motor?

Have you checked the FSM that is specifically for diagnosing the AC? Its a separate document.
Click to expand...
Took a few hours over two days, wasn’t fun but not as bad as I expected. I checked it out but did not find much addressing my issue.
 
Both front and rear heaters you say?

I would get an infrared thermometer gun and measure the temperature of the heater pipes on the firewall. Make sure it is hot on both sides of the valve.

To me, if both heater cores are doing this, this is either the new valve on the firewall, or something odd is happening at the water pump. There is blockage some where other than the heater cores.
 
NeverFinis said:
Both front and rear heaters you say?

I would get an infrared thermometer gun and measure the temperature of the heater pipes on the firewall. Make sure it is hot on both sides of the valve.

To me, if both heater cores are doing this, this is either the new valve on the firewall, or something odd is happening at the water pump. There is blockage some where other than the heater cores.
Click to expand...
Thinking you may be onto something with the water pump. Heater pipes both get warm / hot to the touch. I have never had any overheating or reason to suspect cooling issues, but not sure where else to look besides the blend doors, which I would love to avoid.
 
perform a block test to see if the head-gasket is leaking and putting exhaust gas in the cooling system. A small leak can do things like this and still run OK, not overheat. The factory water pump has a metal impeller, I don't think it's possible to slip. If it's a aftermarket water pump anything is possible.
 
So,
First off crawl under PS dash look up and find the lever which should move according to where you set heat/cold. This needs to stay where you decide. You are looking for the lever and threaded aluminum rod.
Picture of lever is laying on your back looking up...
20240128_084515.jpg
20240128_084528.jpg
 
Looking for any input on the blend door / flapper for the heater core. I suspect the problem with my heat is either poor circulation (water pump?) or the flapper not closing fully. I can not see the flapper, but the servo motor attached to it moves as it should. I am not against tearing the dash apart, but want to exhaust any potential solutions before I start that job. Any input is appreciated!

And if anyone knows what other toyotas have a compatible water pump or t stat for FZJ80
 
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If it's both your heaters acting up then it's 100% not the blend door. Sounds more like air or exhaust pocket in the cooling system, waterpump is unlikely unless the belt is missing or slipping, maybe if you had an aftermarket one with a plastic impeller it could be bad.
 
TheBussman said:
If it's both your heaters acting up then it's 100% not the blend door. Sounds more like air or exhaust pocket in the cooling system, waterpump is unlikely unless the belt is missing or slipping, maybe if you had an aftermarket one with a plastic impeller it could be bad.
Click to expand...
Cooling system has been flushed and I burped the system several times since changing fluid, no change. I have read some 80s require a lot of burping to get the air out, so I plan to keep trying. Thanks
 
wbrad99 said:
Cooling system has been flushed and I burped the system several times since changing fluid, no change. I have read some 80s require a lot of burping to get the air out, so I plan to keep trying. Thanks
Click to expand...
Steep hills are your friend when trying to get all the bubbles out of your cooling system. Any time I mess with cooling systems, there is a hill near my house that is routinely shut during icy weather - super steep. I make sure I'm topped off in my overflow bottle, get fully up to operating temp and go slowly cruise up that hill as a final check. I started doing this back when I had a Hilux with a body lift that put the heater core up a kinda high making it a challenge to purge all the air. It just became a habit after that, but it's a pretty easy extra step to be sure.

Not sure if air is your problem or not, but this should at least eliminate it as a variable.
 
Marvelicious said:
Steep hills are your friend when trying to get all the bubbles out of your cooling system. Any time I mess with cooling systems, there is a hill near my house that is routinely shut during icy weather - super steep. I make sure I'm topped off in my overflow bottle, get fully up to operating temp and go slowly cruise up that hill as a final check. I started doing this back when I had a Hilux with a body lift that put the heater core up a kinda high making it a challenge to purge all the air. It just became a habit after that, but it's a pretty easy extra step to be sure.

Not sure if air is your problem or not, but this should at least eliminate it as a variable.
Click to expand...
I have been jacking the front end up but will take the advice and find the steepest hill I can, I want to exhaust every option before I tear back into the dash!
 
wbrad99 said:
I have been jacking the front end up but will take the advice and find the steepest hill I can, I want to exhaust every option before I tear back into the dash!
Click to expand...
Did you figure it out? Currently have the same issue. I think I just need to burp more. My front end sits a tad lower than my rear end.
 
Same problem here. Do the block test and confirm your HG is good before you chase your tail any more.
 
Could also be your heater valve not opening or closing all the way.

The Rock Auto version does not close all the way. Toyota does.
 
It is doubtful in the extreme that a bad water pump, bad thermostat or bad head gasket would/could cause these symptoms. They would definitely cause other bigger problems.

Just off the top of my head, I am gonna suggest that you have very low flow through the heater stem. When there is no air passing through the heater core to pull the heat out, it warms due to whatever restricted flow is making it through. When you turn the heater on and air begins flowing through the core, initially it will be warm, but the air flow quickly pulls more heat than the restricted water flow can replace and the heater output drops.

You mention that you have "back flushed" the heater core (core not cores?) If all you did was run water or over the counter radiator cleaning mix through the core, you have done little. If the heater core(s) are internally clogged with rust or mineral deposits (or worse "radiator stop leak" goop) it is going to take a more to actually accomplish anything.

Acid washing heater cores in situ is a bit of a PITA, but it is easier then replacing the cores.

Mark...
 
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