Brandon Trevino

I recently replaced my radiator, thermo and coolant on my 1990 v6 4runner. Everything seemed fine beside now I do not constantly blow hot air. I can turn on the heat and it will only blow hot air for a couple of seconds until it goes back to outside air temp. My truck decided to overheat today on the 15 minute drive to work in 40 degree weather... By the time I pulled my truck in the parking lot, the temp gauge almost made its way to hot! I'm wondering, do I just need to do another burping of the system, or has something gone worse like my heater core going bad? I'm fairly new to this truck, (owned for about 8 months now) so any information is good information to me.

13Swords

It does sound like trapped air in the cooling system to me. Several things/ideas:

Exactly how did you "burp" the cooling system? Sometimes you need to elevate the front end more to allow any air in the system to rise up & out of the system. Ramps are great for this but I have seen it done with just driving the front wheels up onto a 2x4. You are running it up to full temp, heater on, radiator cap off, while trying to burp the system? I usually let it run for a few minutes past when the thermostat opens, then give it some throttle for 30 seconds at 1500 RPM, several times, until I don't see anymore dropping of coolant level in the rad. Top off as needed. Make sure you have your overflow tank filled.

New radiator but you didn't mention if you got a new radiator CAP for it or not. Best to just get a new one for it, they are cheap insurance. If the relief spring in the cap is stuck closed this could also lead to overheating, and the system not being able to "self-burp" any remaining air in the system to the overflow tank.

You might also consider reverse flushing the heater core. The truck is 30 years old and the heater core may have a fair amount of "stuff" in it that a regular system flush might not get out. You didn't mention if you flushed the entire system or not.
Here's a good video on reverse flushing the heater core:

Co_94_PU

iTrader: (-1)

Start with verifying the thermostat is opening. When the engine is up to temp and the guage starts to move out of its normal area is the radiator tank/inlet area hot or cold?

You mentioned your heater stops blowing hot air, this leans towards a more serious issue. Is the pump failing or is the core blocked? Given that the engine guage is going high it's probably not just a blocked core since the heat exchanger (core) doesn't play a large role in cooling the engine. This means a major blockage in the engine a slipping or otherwise failed water pump. With the radiator cap off warm up the engine and judge the flow across the upper tank of the radiator, it should be relatively vigorous (Swift Creek vs lazy srtream).

If you have a thermostat that opens reliable, a pump that flows, the next check is the fan. There is a spring loaded valve on it which when heated makes the fan stiffer to spin by hand. You can test this by warming up the engine and engine bay and comparing it to cold, it's faster to apply heat directly with a hair dryer or heat gun just hard to get to.

As for getting air out. The is a tool for this, a lisel spillfree funnel, attaches to the radiator cap location. You fill the system and add enough extra to fill the funnel about half way which makes it the high spot in the water column. Now run the engine up to temp and let it purge air while you keep the funnel filled enough to stay the high spot. Once it stops bubbling you insert the funnels plug and can drain the extra.