This is a long story so please bear with me. The heater in my '85 4Runner SR5 2.4L seems like its never gona work.

The Problem: For a long time the heater hasn't worked. What happens is the truck warms up pretty fast. I'll get very weak heat for a little while. Then nice hot heat for a little bit, and then the temp gauge will spike to just under max and start blowing cold. I'll pull over and pop the hood. There will be so much pressure in the radiator. I'll crack the cap a little and pure steam will come out. IT SEEMS LIKE THERE'S SO MUCH PRESSUE BUILLT UP! I will let the pressure off, then make it home quick

The Situation and What I did.

I went on the forum and typed in heater and heater core and did much research. Believe me, I learned a lot. I learned that I should try back flushing the heater core. I learned that the valve bypass by the firewall may be stuck (which it was).

So today I back-flushed the heater core, installed a brand new radiator, brand new water pump and brand new premium thermostat. The same thing still happened. As soon as the heat started blowing hot, the truck overheated and then graaaaadually cooled off. When I got home I manually moved the valve to bypass the heater core (because the cable that does it on the dash is broke) and the temp instantly went down and the top radiator hose( which is usualy hard as a rock) instantly went softer and squeezable.

I don't know what gives.

Theres only two other factors that come into play that I can think of:

1. my rear heater line started leaking years ago so I by-passed it and ran the input that goes to the rear heater to the main input tri-splice in the valve on the firewall.

2. the heater core has had a small leak onto the driver's side floor for a while. Not a major leak but a puddle will form under the floor mat in about 2 weeks. I put some stopleak though the system a while ago and maybe that clogged the heater core, but when I did the back flush today there was plenty of flow.

3. And I don't hear the gargle of death inside the cab. I'm fairly sure that my head gasket is okay. It was recently replaced

I plan on replacing the heater core, but since theres still flow, wouln't the heater still work a little?

What gives? I could really use some input from some forum experts here, because the problem is bugging me so much, I've thought about selling my Runner ( I don't want to), or moving to a warmer climate lol.

Friend had the same issue in his 86 Bronco, we replaced the heater core and no more issues. I would start there and replace it since its still old and the rest is new.

Ive never heard of a plugged heater core causing the cooling system to pressurize and blow steam.


Sounds to me like either you have a massive air pocket in the system, or your headgasket is blown and is pumping combustion gasses into the cooling system.

You can do one of two tests to see what is going on. One is a chemical test on the coolant to see if you are getting exhaust gas into the coolant (NAPA sells a test kit for this). Or you can run a pressure test on the radiator and see if the pressure is building with engine revs. My '85 had heater symptoms like that when I had a head gasket leak.

I will look into the chemical test from NAPA. But why would the pressure seem to only build when the cooling system is passing through the heater core? Like I said last night, I bypassed the heater core by closing the valve, and the pressure seemed to disappear?

I also have another question. The cable on the inside that controls the temp is broke so I have to manually move the valve on the firewall, but how come when I move the temp from either hot or cold it still makes a difference even thought the cable is disconected?

Got a NAPA part # for this test kit? I looked for such a test kit at NAPA last year, and all they could come up with was a test kit for antifreeze strength.

According to this thread:
- http://www.thesamba.com/vw/forum/viewtopic.php?t=330015


NAPA BK700-1006

Thanks for the link. Looks like they had ambiguous results with that NAPA tester....

I was wrong, the pressure builds up in the cooling system even when the heater core is by-passed, I can't even drive 7 miles without it happening. I'm afraid you guys might be right about the head gasket. But what I don't understand is that the truck runs perfect. The headgasket was replaced only about 5000mi ago. Can they fail that fast?

Sorry to say, headgaskets will last 200,000 miles if they're installed with the proper preparations. I recently bought an '87 4Runner from a guy who did his own headgasket change, and it lasted him a week before it blew again. He got disgusted and bought another truck. He hadn't cleaned out the head bolt holes in the block, and the head bolts were bottoming on oil/antifreeze in the holes. Even though his torque wrench had indicated the correct spec, the bolts weren't exerting proper clamping force on the head gasket. When I took it apart, some of the head bolts were barely more than finger tight.