Another test you could do would be to modify a gp or injector so that you can pressurize each cylinder with compressed air. The best way to really do this is to pull the valve covers and then loosen the rockers so that the valves are all closed. This would help to eliminate or pinpoint head gasket failure or cavitation.

 

Could be the water pump impeller spinning on the shaft or deteriorated badly. I can't think of how that would build pressure so fast though.

It would explain the heating up at idle and depending on how bad, not overheating at speed.

Pressurizing that quick does point to head gasket.

 

I have to bring this one to the top. Hopefully someone can help. I'm having the same problem after changing the heads with studs 2 years ago maybe 2000 miles since. Tried a new rad cap tonight. No white smoke. No water in the oil.

 

has the cooling system been properly "burped"? Had a similar problem with the 460 in my 77 F150.. would build pressure super fast, and fart out of the over flow tank...and I was thinking head gaskets, cracks, oh my....turns out it just had a S--- ton of air in the coolant...after I let it puke, and refilled it a few times it is fine now..you also mentioned the temp gauge going hot cold...another indication of air pockets in the cooling system..

Edit: just now noticed this was an old thread..did the OP ever figure out the problem?

 

I too wonder what would happen if the person would let the coolant system work any air out. Let it overflow all over the ground if need be, just make sure that when the T-stat opens you fill the rad the rest of the way and cap it off.

 

I let it run for 20 minutes last night, without the rad cap, and it puked on the ground, looked kind of foamy for a bit, i put the cap on it and watched the over flow tank, and no bubbles. I plan on doing that again this week or weekend, and i will let you know.

I'd love to know what happened with the original poster's truck

 

Bringing this back up.
Same problem but i'm having smoke, air intrusion i know for sure.
I put sniffer too it, and i'm going to run the sniffer tomorrow on it as well.
Sniffer said negative.
I put pressure on the system and no leaks. Holds 16 lbs easy. If i start it it spikes to as high as 70 psi, I immediately shut it down. But it would maintain 70psi. If its a bhg it shouldn't maintain. Right?
I'm going to burb the system tomorrow and flush the heater core.
I ran it for 2 hours, and it would gargle at idle, but going 70 it would do fine.
Ran cold too!
Let it sit and idle for about 15 minutes today and temps started climbing fast.

I'll let you know my result but i'm still needing some help

 

head gaskets or cracked head since the heads rarely crack its gaskets.