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.