Help me fix my other car (long)

I'm not an expert, but i play one on TV. have you tried changing the radiator cap?
if i read it right, your overflow fills after any driving, but your t-stat and fan are working.
you know there is a leak in the pressure test, but the only place you see fluid is in the overflow.

Scott, I would start with getting the rad fiixed, pressure is being releaced, similar to a faulty cap.

Second, the t-stat openimng would make the hose soft, it may be faulty as well.

Since it didn't overheat before you worked on it, I would go back and recheck the intake gaskets. A gasket slipped or ??? Could you have installed the intake gaskets backwards? The coolant openings should be at the front of the intake by the thermostat, not the rear. Most intake gaskets use a restrictor or block off the rear coolant passage at the back of the head.

This isn't really your problem but, I once used Felpro intake gaskets on a Ford 4.2 V6 and they didn't seal at the rear very well. Water would seep out the passenger head at the back as coolant pressure built up. But there was no overheating. Installing OEM Ford gaskets fixed it.

After reading this I question the thermostat, can you visually verify coolant flow with the cap off?

A bad cap would not get pressurize.

But, I'll throw a new thermostat in there anyway.

Pressure only comes from 2 places,heat or head gasket  failure.

If the head gaskets are bad the engine will pressurize before it heats up.

Shoot, sorry to hear that, Scott.  Good luck.

Did you test to see if there was pressure prior to the car heating up?  (just going off of Team Infinniti's post.)

Buy one of these and run your engine with it filled halfway. If it starts burping exhaust after the thermostat opens, you'll know for sure. Plus it's the handiest damn coolant funnel you'll ever buy. I highly recommend it to anyone who ever has to put coolant in a car. Jack up the front end, fill it halfway, run the engine until the thermostat opens, keep the funnel half full, and you'll have no trouble bleeding the coolant.

http://www.google.com/products/catalog? … CMQ8wIwAg#

That thar's the missing clue.

Pressure comes from Combustion or exhaust - not going to come from intake gaskets unless you are turbo/supercharged.

You have a wee-small crack in a head, and as they are alu, probably between the steel seat and the head at an exhaust valve. It might be between an intake and exhaust, but that wouldn't make a difference in diagnosis.
~ it opens up when combustion pressures go high, as in under loaded driving conditions.
   You won't see pressure build up idling.
   It won't release and fill a bore because it is small and only opens loaded.
~ your distributor gets removed when you do a manifold, yes?
   the 'change' you made when replacing gaskets was a very small bump in ignition timing.
   This causes compression pressures to go high at load, just enough to open a crack
   OR - a head gasket is weak and the head studs are stretching enough for similar result
   --but this latter will get much worse rater rapidly, and usually be found with the fizz-tester.

Getting heads "pressure checked" sometimes will not show the size crack you are likely to have! A good head-man can spot this sized crack visually with a magnifying glass, but if they were to get sandblasted or aggressively cleaned before a check-out it might be missed. Pressure-test with Dye-Glo will usually Not miss this...

~said~ now if not that, then it IS a head gasket just barely failing, at the sealing-ring to a waterway, and again, timing and hot-run combustion pressures are doing the deed - and will get rapidly worse if you drive it more.  A crack will take longer to get worse (unless you run it dry)

ON the T-stat backwards: they work backwards, but during warm-up and non-hard driving you will see a spike in temps then a cool-down, the gauge will go up-down-up-down noticeably as the t-stat bulb lags instantaneous temps in the water path. Not great for long-term life of heads. The initial system warm-up can generate steam-high temps on the upstream side of the t-stat, your temp gauge may not indicate these depending on where in the stream it is.
~here's the here: the t-stat senses temp in the bulb - put one in backwards and the area of water not flowing on the 'behind-the-door' of the t-stat only warms by conduction. There can be a significant lag in time between closed and open dependent upon the initial starting temp. If your t-stat has a itty-bitty purge-port (1/8" hole with a dingus in it) and it's installed backwards this effect will be lessened to a slight degree (enough to make the gauge readings not fluctuate wildly).
--net effect, on warm-up in winter months - could be overheating critical places like the exh valve seats and the weakest part of head gasket clamping during warm-up. Not enough to notice, but after repeated times enough to matter.

Sorry to say - heads off, inspect, replace gaskets at least...