My truck has started misbehaving. mpg drop was the first sign.

Now idle is acting strange.

At startup idle is 1500 and quickly drops to 900 as warming up. The cold fast-idle system has been working fine.

After it's been driven hard and fully warm, it will eventually start rising the idle to ~1300-1500 and I hit the fuel cut threshold when hitting the brakes. Very annoying at stoplights when warm.

I figured out if I unplug the pcv valve from the valve cover, plugging it w/ my thumb, leaving the hole in the valve cover open to atmosphere, idle drops instantly back to 900. If I hold my hand over the hole in the valve cover, it feels like steady exhaust coming out. The pcv valve itself works fine and I bought a 2nd to test, just to be sure.

Shouldn't there be vacuum at the valve cover at idle, not exhaust pushing out?

I did a compression test today and it was 175psi on 1,3,4 and 160/165psi on 2.

Burned exhaust valve? Plugged exhaust? Blown head gasket?

My oil/coolant both look clean.

I was going to check my valves tomorrow.

thanks