'93 FZJ80 overheating when the heat is ON (1 Viewer)

[Seth_O]

I have been battling an overheating issue on my rig for a LONG time, since I bought it. So far I have done the following:

1. Raventai gauge mod (~3yo)
2. replace the radiator (~3 yo) - found clogs in it
3. new OEM thermostat, oriented correctly (~3 yo)
4. new blue hub fan clutch, modded with 30k CST (~6mos ago) - it was old and manky
5. re-did the PHH bypass (~2mos ago) - found a leak
6. replaced the heater control valve (~2mos ago) - it was leaking
7. checked for leaking exhaust (that ones for you @NLXTCY)
8. checked for exhaust gases in the coolant (~2wks ago) - negative
9. pulled my plugs - they look carbon fouled consistent with the miles on them

Rig is running Toyota red + distilled water. The only other potentially related issue: I occasionally have issues where the heater goes from blowing hot-hot, to lukewarm, to cold (ambient) and then a few seconds later gets hot again.

I am STILL overheating. I get heat soak headed up hills that doesn't really ever fully recover. Its gotten remarkably worse in the last 30 days. It will quickly climb to outrageously hot (assuming my dash gauge is accurate) and then slide back down to the middle of the gauge. The entire process takes maybe 60 secs.

I had an epiphinal moment yesterday that it happens much more frequently/worse when the heater is ON. In fact, since the last round of work (~2mos ago) I cant think of a time when it happened when the heater was NOT on.

The only thing I can think of would be a partially obstructed heater core? Is there anything else that could be causing this? Can I flush the core while en situ? I just recharged my AC system and do not want to pay to do that again, nor do I want to deconstruct me entire dash.

Open to thoughts from folks.....

 

[mct75]

A wonky thermostat can shut due to cooler water returning from the heater core and reaching the thermostat. The 22R's are bad about this, they call it "temp gauge overshoot" but it's normally a one-off thing when you start driving.

 

[Seth_O]

A wonky thermostat can shut due to cooler water returning from the heater core and reaching the thermostat. The 22R's are bad about this, they call it "temp gauge overshoot" but it's normally a one-off thing when you start driving.

Tks, unfortunately mine starts out fine, I don't get into issues until after its been running for a while and am at good operating temps, and it will bounce back and forth. But I'll hold this in my back pocket...

 

[gummycarbs]

Get one of those temperature sensor guns and check to see if the water pump inlet is actually as hot as the temperature gauge claims.

Did you flush your cooling system? If your radiator was clogged, then I'd think it likely that you have crud in the block and heater core(s) too.

What about the water pump?

 

[Seth_O]

Get one of those temperature sensor guns and check to see if the water pump inlet is actually as hot as the temperature gauge claims.

Did you flush your cooling system? If your radiator was clogged, then I'd think it likely that you have crud in the block and heater core(s) too.

What about the water pump?

I did replace the pump, forgot to list that, tks. Good call on flushing the heater core, I did NOT do it when I replaced the radiator. Honestly I didn't even think about it. Its on my list to do. My question is really: could anything ELSE be causing my problem that I need to look at as well? And am I safe to flush it while in the truck?

 

[gummycarbs]

And am I safe to flush it while in the truck?

Well, I'm taking about flushing the entire system, but that includes the heater core.

I did at least three Prestone "3 day" flushes over the course of about a week and a half on an old Jeep 4.0. It made a huge difference. The radiator was so blocked that the fan never even got up to temperature, something that became obvious once I hit it with the temperature gun. The vehicle now runs cool and the heater is much more effective.

 

[Seth_O]

Well, I'm taking about flushing the entire system, but that includes the heater core.

I did at least three Prestone "3 day" flushes over the course of about a week and a half on an old Jeep 4.0. It made a huge difference. The radiator was so blocked that the fan never even got up to temperature, something that became obvious once I hit it with the temperature gun. The vehicle now runs cool and the heater is much more effective.

Cheers, I'll give that a go!

 

[FJP971]

Could it be an air bubble restricting flow? Maybe pressure finally overpowers the bubble when it gets hot enough and then circulates cooler water that was sitting in the radiator? Just brainstorming here.

 

[Seth_O]

Could it be an air bubble restricting flow? Maybe pressure finally overpowers the bubble when it gets hot enough and then circulates cooler water that was sitting in the radiator? Just brainstorming here.

At this point I'm game for anything. I have bled a couple of times and willing to bleed/burp again, just can't imagine a bubble getting so 'stuck' that the water pump cant overcome it?

 

[flintknapper]

Bypass your heater core (temporarily) and go drive the snot out of the Cruiser. IF you don't get a rapid rise in temperature...then look to your heater core as the 'main' culprit (clogged).

I don't like to do full system flushes IF I suspect one or more components may be obstructed (radiator, heater core, oil cooler). Better to flush any suspect part(s) individually, so you don't move a bunch of crud though the whole system.

Then do full system flushes (every few years) as 'maintenance'.

 

[Formulaonehd]

+1 to the posts above. I just went through this recently after replacing the rad (cheap chinese knockoff) never do this i fell for a scam (advertised as a Koyo) any major air bubbles have the potential to blow your top tank pretty easy. Better quality systems can fend of air bubble pressure for longer
The engine cooling system flush is an absolute winner. i did 3 flushes draining the engine block everytime.

Take everything back to basics as you have with overheating issues you may have found all the crud in your block and lines may have worked back into your new radiator. Are you using alot of coolant?

 

[cruiserdan]

It could be the head gasket even though the test did not indicate it.

 

[BILT4ME]

It could be the head gasket even though the test did not indicate it.

Now, if @nukegoat said that, we'd all ignore it. But YOU saying that.....we all listen. Kinda like E. F. Hutton.

 

[MoJ]

Good advice on bypassing the heater core and then evaluate. When you do flush, flush like mad. See the FAQ by e9999.

 

[Seth_O]

It could be the head gasket even though the test did not indicate it.

That was just rude.....

 

[Seth_O]

I drove home tonight from work - same route as every other night - but with the temp slider pegged to "cool". The temp gauge never got above the middle.

I'm going to isolate the heater core and back flush it really well, then I'll do a flush/cleanse on the whole system. Hopefully that takes care of it.

And I'm going to pretend @cruiserdan didn't say anything just yet....

 

[scottryana]

Check the temp sensor?

 

[LS1FJ40]

It could be the head gasket even though the test did not indicate it.

Don’t worry about Dan. He’s getting a little long in the tooth.

It’s sad to see a legend deteriorate this way. Like watching Gehrig or Ali in their final year...

 

[cruiserdan]

Wow,

Oh, I'm really sorry about your head gasket

 

[LS1FJ40]

Wow,

Oh, I'm really sorry about your head gasket

Come on! Don’t do that! I beg forgiveness and mercy!!