Finally! I have cabin heat.

So I'm going to eat some crow here. Over the past 4+ years I've maintained that the design of the 2nd gen 4.0 V6 didn't have enough coolant flow through the heater core when idling, outside temperature is 25 degrees or lower, and blower is on high. The reason was because I've changed out 2 thermostats and blew out the heater core twice.

Heater core: I got good flow of compressed air blowing it out backwards. I got good flow flushing it out forward and backwards with a garden hose setup. I tried some Prestone flush in the core only and let it sit for 1-2 hours. I watched the engine move coolant to out a disconnected inlet hose at the same rate as it moved coolant on the exit of the heater core. (At least I thought, read on).

Thermostat: Engine would always come up to full temperature quickly even at idle in freezing temperatures outside. I replaced the thermostat anyway. No change to the cabin heat. I tested the thermostat in a pan of hot water in watched it open and close just fine. One day I saw Cody posted this thread about a custom thermostat housing that you can install a Stant thermostat. This allowed me to put in a 195 degree thermostat instead of the OEM 180. I did that and while it was a little warmer when running, the heater went cool/cold at idle when blower was on full.

As a last ditch effort I decided to get more serious. I was scared to go crazy with CLR since it eats aluminum and wasn't even sure if the core was the issue. I did some more research and ended up with a profession product online called Irontite Thoro-Flush. It's safe for aluminum. I then bought a cheap Harbor Freight transfer pump you hook a drill to, a bunch of 5/8 clear hose and garden hose fittings with claims. The exhaust from the pump went into the outlet of the heater core. Another hose went out of the heater core to a 5 gallon bucket. Another hose went from the bucket to the suction side of the pump. With 3 gallons of clear water running through the setup, I wasn't getting any dirt or cloudiness. I then added the Irontite and let it run for 10 minutes. I notices white scale starting to float on the circulation bucket of solution. After 10 min. I blew out the solution with compressed air, flushed out with clear garden hose water, and then blew out the water with compressed air. The crud in the bucket of used solution was surprising. There was a film of crud floating on top and lots of black particles sunk to the bottom.

I put everything back together on the truck, filled up the coolant, got all the air out, and holy crap do I have serious heat at idle. If you've tried Presone, Zerex, or other WEAK chemicals with no success, try some Irontite Flush.

As to the 195 thermostat, the air coming out of the left vent almost burns my hand now. I switched back to a 180 degree yesterday and all is well.

I hope this helps someone with a stubborn heating problem.

 

We have a Pump system that Honda made kinda like that...i dont know what kind of pump it is but it puts out some serious pressure...put CLR in it and run for 2hrs will
usually do the trick

 

I've always gone the OTC blast vac route with CLR and let it rip.

 

Saving for future reference. The thread title needs work tho. If I may request that

 

Interesting. Luckily I've never had that issue. I do notice below about 10-15*F idling isn't enough to keep the coolant fully up to temp which reduces HVAC heat output

Do you know the history of the coolant in your truck or why it was corroded?

 

This was my issue.

History: Yes, I've owned it since new. I currently have 178,000 miles. According to my maintenance history that I keep, I did a radiator drain and fill at 72,000 miles. I was too lazy to drop the skid plate on a cold garage floor and couldn't unscrew the drain petcock. Each time I changed out the thermostat at maybe 110K, 150K, and 170K, I would push the bottom hose down below the radiator and drain it. I never have done a proper flush. I plan on taking it to an independent mechanic next week because he has a proper evac and fill machine. That way I hope it will physically clear out the system and get out the sediment. I'll give him 3 gallons of Toyota coolant to put in. My coolant isn't horrible but isn't as translucent as the stuff from the jug.

 

You are more brave than I am. I didn't dare use CLR and surely for 2 hours. It stats not to use on aluminum. Now I do see that CLR has gone "green" (another word for doesn't-work-as-well) with the container saying made with plant based ingredients. Maybe it's not as caustic as the old stuff was. I do not want to replace the heater core. That's on the high end of DIY or $2,500 as a mechanic.

 

I've replaced heater cores, it isn't that bad, if you can paint by numbers or solve a picture puzzle you can remove the dash and heater box. But yeah once in a while you can save yourself the hassle on a beater and pour some crap in and hit er with the pressure.