85 4runner, 22re. First off I have the dual thermostat from toyota. When I drive my 4runner I can drive for about 15 minutes and then it will overheat. The weird thing is it will run a little cooler than it usually does then just shoot up. Thermo is about 3 months old. Could this be the water pump? or maybe the larger valve on the themo isn't opening? any help would be great, Thanks.

 

Did you check to see if you have an air bubble in the hoses or radiator? I did the same thing with my 85 4runner (replaced therm.). Turns out I had some air trapped in there.Just a thought..

 

What are the conditions that lead to overheating? Does the outside air temps make a difference? Does driving speed make a difference? Does load on the engine make a difference? Do RPMs make a difference? Did you test the thermostat in a pot of hot water before installing it? I always test t-stats this way before installing them, both the old one and the new one. On a few occasions, I've put the old one back in when I noted it worked better than the supposedly "new" unit. What is the condition of the radiator? Open up the cap and look down inside the cooling channels, are they all clogged up or can you see down them?

 

The thermo has been in there for about 3 months and was working just fine until about a week ago. I drove it to fresno with no cooling problems at all. When I put coolant in the radiator with the cap off, I get some big bubbles and it usually ends up making a mess from coolant coming back up, maybe something is clogged, the radiator looks ok when I look down it. It doesn't seem to matter what rpm it is at, just after about 15-20 minutes of driving. It wasn't too hot out either day. It did it in town once and on the freeway. I will probably take my thermo out and test it, that way it will get bubbles out and test it. btw Roger, your site is so helpful, thanks

 

What are the overheating symptoms? Does the temp gauge go up (only) or is it also boilng over? If so, does it boil over (blow out coolant) then get hot or does it get hot first then blow out the coolant? See if the lower radiator hose is soft and collapsing.

 

Is your guage working properly?
My new GDI radiator fixed my overheating problem, my old radiator look o.k. whem you looked inside but it wasnt.

 

Once the temp guage starts going up over half way there is no stopping it, it shoots right up. The coolant boils over and shoots out the overflow tank. I know the temp guage is working properly because it is overheating and overflowing. now that I think about it, the lower hose was feeling weak, It is at my other house, I will have to mess with it tomorrow. You think maybe it is sucking in the sides of the hose? I had to put a clutch in my pickup, so I didn't mess with it too much yet. I will let u know, thanks again, I will post tomorrow and let you know. thanks

 

Now that I think about it, maybe it is shooting out and the lack of coolant is what is causing it to get hot, I will drive it and check a few things after class tomorrow and let you know. thanks

 

well, bad news I blew a head gasket. I have water in the oil and all. I am in the process or deciding whether to rebuild it or not. The motor is worn, So if I am gonna have the head off it may be worth it to rebuild it. By the way, there was air under the thermo and that was the overheating issue, but I guess it is too late now. Thanks for the help