I just replaced my motor last week - the old crank bearing finally gave out and threw a rod out the side of the block. I found a 60,000 mile 5.0 motor, and since the old motor had everything new or gone through from the heads up, we stripped the new one down and built it up with my newer stuff.

Anyway... that was a week ago, and I noticed that the temp guage runs around the O, but sometimes hunts up toward the M, and then back down. This never happened before (it ran higher on the old motor but stayed steady), which made me wonder. I checked the coolant after a couple of days, and it was a touch low. Thought it worked a few bubbles out and refilled. Checked again tonight, and it's low again. Not alot, but down to where you can see the top of the fins. I'm thinking if I have to check the level every couple of days, something is wrong.

The truck sat all evening, was cool as could be, and I decided to let it idle with the cap off and see if it would burp any more air out. As soon as the motor started, coolant started to spray out of the filler neck. Not hot, but spraying like it had a fan behind it!

I've never seen such a thing before - didn't find anything searching old posts either. Any clues where to start?

 

Sounds like the thermostat is sticking.

 

my old 5.0 used to always stay around the O and i checked and saw the fluid was low i just kept checking and adding and it eventually stopped dropping but it took a full jug of antifreeze just out of curiosity how many miles did your old motor have

 

I'm thinking...Air is still trapped in your cooling system, when you started the engine with the cap off, you gave some of the air a way out, but in turn lost water.
You don't have an additional problem because of this! That's normal if air is trapped.

Continue adding water when radiator is cool, the air will work it's way out eventually. Air gets trapped in the upper areas inside the engine/cooling system during cool down.
With cold engine not running, by removing the radiator cap and adding water/coolant to radiator, this procedure will finally displace the air, it may take a week of driving, cooling down and adding coolant to get all the air out.

There's probably a correct way to bleed trapped air from the cooling system by opening a fitting / bleeder valve / ? at a high point on the engine, but I don't know for sure.

 

Thanks guys!

I'll keep driving it and checking the level for a bit, and if worse comes to worse, replace the thermostat. It sure is in a bad place to get to, but I've done it before... at least if it is sticking, it's sticking open!

 

this is a really big indication that the headgasket is gone

 

He just did the head gaskets. Hopefully that's not it.

 

he's building pressure in the cooling system without heat

 

It just can't be the head gaskets... knowing my luck, it wouldn't surprise me though. I'm not seeing any oil in the coolant or vice versa, and I'm amazed at how strong/smooth it's running.

I'm hoping more for the air working out of the system theory!

 

i've seen head gaskets do the exact thing you're explaining. fords do it this way mostly, instead of burning the coolant or mixing oil and coolant it just leaks the compression into the cooling system some how. i've actually seen it on a catapiler engine when i was in the army. did you check your heads to be sure they weren't warped? kinda sucks if its the head gasket since you just did the work, but i'm pretty certain thats your trouble