hey guys,

when re-filling an empty radiator (in my case because its a brand new one)... i've read that you should keep the cap off and run the engine, watch the coolant get 'sucked in' (circulate around) and cap off as needed.

How long can I run the engine with the cap off? I thought that is a major risk (Boiling coolant shooting out of cap, etc) -- and also when doing this procedure should the heater be running?

thanks much for any replies, i plan to finish this today.

This is a very good question. I just recently replaced my radiator and followed the service manual and it doesn't say anything about doing this, so I didn't. I would love to know if this is required too. From the best I can tell from what I researched, this may have been something you needed to do on older vehicles that didn't have an overflow reservoir. I am thinking we don't need to do it on ours, but would like to hear what some others think too.

I would recommend starting the truck with the cap off. Dont worry about the coolant shooting out... I imagine it will be cold since you just poured it in, plus the radiator will not have had the chance to build pressure.

heres how i would do it:
Fill the radiator and squeeze your radiator hoses to expel any air bubbles in the hoses. Then with the cap off, start the engine (heater on, good point) and watch the level. It will either drop and you can top it off again, or if it is full it will slowly start to overflow. if the latter occurs, put the cap on and let the truck idle for a few minutes or take it for a trip around the block a few times.

once the truck has cooled off enough to remove the cap, check your work and make sure the radiator is full.

Turn the rear heater on high (if you have one) and park on a hill to help dispel air.

it cant build pressure with the cap off, you can run it as long as you need. you may loose a little bit of coolant, as it heats up, and burps the air out of the system.
just top it off once its done.

sometimes they get air pockets trapped inside, and it takes several heating/cooling cycles to get all the air out.

thanks for all the replies guys.

i'll fill up the new radiator and overflow tank with the engine cold (Man, its 35 out in Denver today) and do as you guys suggested.

nevada, outside of doing this once or twice after my new radiator install -- do i have to keep doing it the next couple weeks a few times here and there for the air to get out?

No just check it a few times after driving, you should be able to see coolant at the top of the radiator by removing the cap. Follow the advice here and you'll be fine.

nevada, after reading this "it cant build pressure with the cap off" -- I feel like I need to take elementary school science again.

a huge duh on that one.

thanks buddy.

I would recheck after 1-2 days driving. and the again in another 1-2 days just to be safe.

also, were coming into winter, make sure too fill the res to the correct level with antifreeze.