Hello all, I have a bit of a stupid question, but will ask nevertheless. I got a 91' 4Runner with the 3vze. It went through rebuild and since I have been driving it around 12k miles. However, I went through several radiator caps, ending up with the OEM cap. The radiator have been replaced with a new (not OEM) as well. The thing is the OEM cap does not fit as tight as the 3rd party ones. Moreover, I check the coolant level every morning and notice that the cap is slightly unscrewed every time. That means everytime the engine gets to operating temp the cap unscrews a bit, maybe like a 15-30 degress. Is this normal? Is this bad?

One more question. The spring kicks in and temps getting up these last days and I notice in the morning, when I open the radiator cap the coolant level is on the edge, and today even dripped a bit outside. Can this be because of outside temp, sitting in the morning sun (around 50-60 degrees F)? Hope its not an early stage of HG

Thanks for your time, this is by far the best Yota forum out there!

 

Any chance the hood is hitting the cap? My 4runner has an aftermarket radiator possibly for a v6 ( installed before I got it) the cap is off to the side as opposed to down the middle and the hood was hitting it causing it to turn a little.

 

Is it normal? NO!
Is it bad? YES!
You need a cap that will seal the filler neck properly, and allow the system to pressurize. It's designed to have 13-15 PSI at operating temp.

When the system cools down, it draws fluid in from the recovery tank. As it warms up, the metal parts the water flows around and in expand. Since water isn't compressible, it has to go SOMEwhere. After the pressure builds to whatever the cap is rated at, usually 13-15 PSI, it starts venting "overboard", into the recovery tank. IF it's tightened properly! The cap has a relatively heavy spring on the "exit" portion of the cap. The large rubber seal with a metal backplate on the bottom of the cap. Please note that any air that has gotten into the radiator/cooling system will vent as the engine heats into the recovery tank as well, but it bubbles to the top, and is vented out of the tank. Thus, as the whole system cycles warm/cool/warm/cool, it will "burp" the system, removing any air, and replacing it with fluid.
This why, after working on the cooling system, like replacing the hoses, flushing the coolant and putting new in, whatever, you should park it nose high on a hill, and run it about a 1/2 hour to get all the air out. Air does NOT cool things. That makes sure the radiator filler cap is the highest point in the system, so the air goes there, and then overboard. Then, after it cools again, you need to make sure the recovery tank and radiator both have the correct amount of fluid in them. The system, after getting the air OUT, will draw fluid IN from the recovery tank, and can drain the recovery tank completely. Air likes to hide in little nooks and crannies, and will usually bubble out over time. It's best to get it out as soon as possible.
Once the truck is shut down, and starts cooling off, the metal parts shrink back down, causing a "vacuum" in the cooling system. Since the caps are designed to allow the water to flow without restriction INTO the radiator, the small metal valve in the center of the larger rubber seal with a weak spring holding it closed, it draws the fluid previously expelled into the recovery tank back in.

You seeing the radiator full to overflowing in the morning is perfectly normal. As it cools at night, it draws from the recovery tank, and as the heat starts to build up, ie: the temperature rises after sunrise, the system will build pressure, so if you pull off the cap it may well overflow a bit. If you were to start the engine, it would overflow more, as the engine starts to warm up. See all my rambling above.
I believe that cap that unscrews itself if either bad, ie the metal ears that clamp it onto the filler neck are bent open, OR the filler neck is damaged. Get a new cap, that designed for THAT radiator. They aren't THAT expensive. See if that stops the "unscrewing" problem. Make sure you tighten the cap ALL the way down. There are two "levels" of tightening the cap does. Make sure you tighten it all the way down to the second "level" of tight. You might have to press down against the caps spring, and twist while pressing, to get it all the way tight. I use my palm to do it.

Finally, always use the RED Toyota coolant, and DISTILLED water. NOT just tap water! It's important. It's designed for the iron block/aluminum head setup in Toyotas, and prevents corrosion in the engine. The green actually causes corrosion. Rust. BLECH!

I hope something in all this is a little help.
Pat☺

 

Thanks Damian812, 2ToyGuy!

To sum it up so far:
Nothing wrong with the hood and the cap, I checked that. I tried 2 third party caps, they both were sealing better, but the water hose seemed harder when hot, compared to the OEM cap. Otherwise yeah, I know all the stuff about getting rid of air in the coolant system, I use a diy bottle to make sure the rad is the highest point. I did this last week, just to make sure everything was fine, and indeed it was fine. No bubbles when cold, meaning no HG problem. That was a relief. Actually there was nothing unusual and I am happy about that. Now I am waiting for another OEM cap to be delivered at the local parts store and will try it on. Hope it will seal a bit better

 

Well???

 

Waiting for a new OEM cap, which I will get next week or so (they dont have it on the shelf).

 

New OEM cap came, stays in place and not loose as the old one. I think everything is fine now. Thanks for all replies!