Melrose 4r

Its been 20s and 30s in the mornings this week and i've been using the 4runner to commute with since my daily driver had some issues.
i replaced the thermostat with an 88*C aisin unit about 2weeks ago and the gauge always points at the "P" in TEMP, about 1/3 up. But the air coming out of the heater is not very warm. I am beginning to think the heater core may be restricted in flow. Hey, its 31 years old possibly, right?
I am thinking about backflushing it but afraid i might damage it if it's weak. Any advice?

akwheeler

Make sure your heat control valve on the inlet to the heater core is opening properly and that the doors in the heater are moving properly as well. I am assuming your fan is blowing well on high? Back flushing the heater core shouldn't pressure it up and if it starts leaking afterward you just found out sooner rather than later that it needs replacement. better to find out in your driveway or garage than to have it suddenly blow out on the highway (fogs up the windshield really well).

Melrose 4r

yes the fan works well on high. I guess if the core fails, it needed replacing anyway! I dont really plan to drive the truck through the winter but will drive it now and then if the roads aren't salty. The coolant itself looks really fresh, i think the truck got a timing chain job and new radiator before i bought it.
i just changed the thermostat to see if it would change anything and because i didn't know how old it was.

RAD4Runner

Agree 110%.
That's exactly what I do when I flush my cooling system during the rinsing phase. I disconnect both hoses from the heater core at the firewall or at the engine block depending on accessibility. Then I flush with garden hose both directions. I rinse after draining old coolant, add flushing solution, then again after draining flushing solution / before re-filling with new coolant.

Melrose 4r

I was able to backflush the heater core with the garden hose the other day. Some discolored water came out but no "goop" or anything that looked alarming.
Using a laser theremometer I was able to see that the air temp coming out the vents rose from about 160 to 200F when the truck was held at higher RPMs for a couple minutes.
Then when dropping back to lower RPMs the temp dropped back also. All the while the water temp gauge remained constant, pointing at the "P" in "TEMP" about 1/3 up on the full range of the gauge.

The head gasket was done on my truck sometime in the last few years and I wonder if anything (gasket, RTV, gunk) could be restricting flow through the pipes that run from the H20 pump to the heater core. Also wonder what effect a bad fan clutch will have on the temp of the engine or rate at which the engine warms up in cold weather. Planning to change my clutch soon.
Anybody know the correct Dorman Help p/n for the studs and nuts that mount the fan to the H20 Pump? Some of mine are bad.