Engine is losing antifreeze. New radiator, thermostat and head gasket. No water/antifreeze in the base pan. Pressure tested and ok.

Finding a small mysterious coolant leak can take time. If your engine is loosing coolant it can only leak three different ways. Externally, on to the ground. Internally, into the engine oil. Lastly the head gasket, head or block could be to blame. When this type of failure occurs you may see coolant build up on a spark plug, coolant system over pressurizes, pushing coolant into the over flow tank or cotton white smoke out the tail pipe. Regardless of what the pressure test yielded, there is a leak. Look around the radiator and engine for white dry coolant trails/stains. If none are found check the oil pan. Water will sink to the bottom. If that passes I suggest purchasing a "Compression in Coolant Test Kit". I the leak is very small you may need to run this test twice. One when the engine is cold and one when the engine is hot. Be very care of hot antifreeze. You could get badly/terribly burned removing a radiator cap from a hot pressurized coolant system. Any questions please write back. Good Luck.

Pretty much same answer as I gave for 87 Reliant see below. Shops have equipment to do cylinder leakage test. Do basic stuff below before doing last test ...cylinder leakage...which costs $$$.

Main point you are losing antifreeze/water. Check to see if you have water on the dipstick mixed in with oil. If so...probably a head gasket. If oil looks normal..... Before you do any test, tighten every hose connection. Check for loose connection into antifreeze reservoir. Sometimes there are small cracks on top which lets out pressure from cooling system. Look for leaks on water pump and radiator. Look at spot where you park the car...any liquid on floor? Not sure?..Put piece of cardboard under car to see if liquid is dripping from car. No leaks?...Start with a pressure test of the system. The pressure test is simply equipment that replace the radiator cap, a hand pump connected to it then air is pumped into cooling system. You watch the gauge to see if pressure is dropping. If pressure does not drop ..problem is most like a worn radiator pressure cap or stuck thermostat.. Do not buy the kind with button on cap to push down and release pressure. get original type and correct pressure. If overheating still, thermostat could be stuck, replace. Overheating of engine causes vapor lock, bubbles in fuel, which causes car to stall and kill. When cools off, fuel cools down and fuel will flow through fuel system again. Large amount of oil disappearing with overheating also sounds like head gasket or cracked head....Tighten up all bolts to gaskets, look for oil leaks on garage floor or driveway. If engine has developed a miss when running, pull plugs out and look for wet fouled plug. If antifreeze leaking into cylinder...plug tip will be whitish, if oil leaking into cylinder, grimey oil fouled. Do compression test on cylinder that is fouled and that will pinpoint if bad head gasket or cracked head. When doing compression test, radiator cap is removed and you listen to hear bubbles in radiator from the cylinder leaking into cooling system. 87 Reliant not worth pulling engine apart if head gasket or cracked head. Best oil treatment to reduce oil consumption is "Engine Restore" Used on my 58 Impala 348 tri power for 28 years. If I did not add it after oil changes it used oil and smoked....I swear by it for ANY engine after 100,000 miles.

the water pump is likely shot

The only place, oil and water share a gasket is the cylinder head gasket, oil and water galleries run through the engine block and head via the head gasket. The cylinder pressure will be OK, it's the oil pressure that's putting the oil in the coolant, the gasket has gone between the water and oil galleries or there is an internal crack (unlikely). A coolant pressure test should show it up.

You have a head gasket problem with the engine generating abnormal pressure, and sending all the antifreeze to the reservoir, thereby leaving the engine running dry; it is dangerous for the water pump to operate with no water.
Drill 3 11/64 or 3/16" holes in the thermostat side facing the engine, making sure not to touch the opening mechanism; this will keep antifreeze always running thereby, compensating for the abnormal pressure regenated by head gasket leak(s).
Your thermostat was never your problem.
In cold weather, 5 degress celsius or under, the thermostat takes longer to open and more antifreeze will be sent to the reservoir leading to an overheating.. Partially cover the radiator with a sheet metal piece to make the thermostat open faster and stay open, as the antifreeze is not being cold that much by cold air. You're fooling the cooling system into warm weather driving.
Don't waste your money buying radiator stop leak or liquids to repair head gaskets; this will only corrode and make your radiator to eventually leak.

You have an air bubble at the top of the engine.
The antifreeze spewing out of the overflow happens due to your perfectly functional radiator cap
passing the overheated coolant/air mixture out of the system into the reservior (in an attempt
to relieve overpressurization of the cooling system)
The sudden nature of your overheating comes from the boiling of the inadequate amount of
coolant/antifreeze in your engine, since it cant properly cool the engine, so it boils (overheats quickly), and pressure rises immediately within the cooling system.
The Fix is free: locate and open the (air) bleeder port at the top of the engine, and add the usual
50% coolant/50% water mixture to the radiator until the radiator is full to the level indicated in
your owner's manual (or the "Full" line). Then close the bleeder port and replace the rad (radiator)
cap.
The air bleeder port is normally located near the thermostat, and should be opened each time
that coolant is added to the radiator. The air bleeder port need not be opened when adding
50/50 coolant to the reservior, since the rad cap is going to be closed while adding coolant
to this part of the cooling system.
Warning: driving beyond the point where your engine temperature gauge indicates overheating will
destroy the car's engine - it'll seize up due to overheating, and the only fix for a seized/burned-up
engine is replacement (of the engine). Seizing an engine is really easy with an air bubble in the
top of the engine - you could easily burn up/warp the head, necessitating replacement of the head
and head gasket.

Sounds like you're working with an engine with severe internal problems (head gasket etc). Chances are it may not be worth fixing but first, do some basic testing such as a compression or cylinder leakdown test and a simple radiator pressure test. Then decide upon the appropriate course of action. If you do decide to replace the engine don't forget to put the parts you just replaced on the new one!

thermostat should be replace on over heated engine, the spring loses tension, probaly not problem, most like head gasket if over heated severly, if you have a air pocket & wornt purge thats one sign,press test on cold engine let sit at start up run slightly rough may see & smell coolant at tail pipe, could also pull plugs may have one wet all crude checks, there is also chemical you put in coolant & use a a stripe for combustion I think napa or snap on sells, alsoyou could do tear down & inspection wish you luck not sure what engine or car you have grand am or grand prix my guess a GP with poss 3.4 more likely then 3800 engine

problem could be heads bolts . i put new bolts in put a little sealer on them . make sure your bolt holes is clean free from debris. put a lillte sealer on intake gasket on top and bottom where coolants travel. if all was done . drain engine oil in clean container .set it aside. take radiator cap off .you a coolant pressure rise kit .put a pan under oil pan with drain plug still off .pressure your radiator about 10 psi. make sure you have antifreeze in radiator and engine . when you pressure up to 10 psi look under the car if coolant comes out oil pan. you got leaking head gasket. if not .put oil pan plug back on . put oil back in car . check your freeze plugs also.

REMOVAL & INSTALLATION
2.4L Engine
See Figure 1

1. Disconnect the negative battery cable.
2. Place a large drain pan under the radiator drain plug. Allow the cooling system to sufficiently cool down before opening the drain plug to avoid personal injury. Drain the coolant to below the thermostat level.
3. Disconnect the upper radiator hose at the thermostat housing.
4. Remove the thermostat housing bolts and coolant outlet connector of the thermostat housing.
5. Remove the thermostat assembly from the vehicle and discard. Discard the old thermostat gasket.

Fig. 1: Thermostat installation-2.4L engineTo install:

1. Clean all gasket mating surfaces.
2. Install the new thermostat in the correct position. If equipped, align the air bleed valve on top of thermostat to the vent recess in the water box (engine side) of the thermostat housing.
3. Dip the new new gasket in clean water and install on the water box surface of the thermostat housing.
4. Install the thermostat housing over the gasket and thermostat, making sure the thermostat is in correct position.
5. Install the thermostat housing bolts and torque to 20 ft. lbs. (28 Nm).
6. Reconnect the radiator hose. Tighten the radiator hose clamp.
7. Connect the negative battery cable.
8. Fill and bleed the engine cooling system.
9. Pressure test for leaks.

3.0L, 3.3L and 3.8L Engines
See Figures 2, 3, 4, 5, 6 and 7

1. Disconnect the negative battery cable.
2. Place a drain pan under the radiator drain and drain the cooling system to just below the thermostat level. Close the drain.
3. Remove the upper radiator hose from the thermostat housing, then remove the housing.
4. Remove the thermostat and discard the gasket.

Fig. 2: Thermostat installation detail-3.0L engine Fig. 3: Thermostat installation detail-3.3L and 3.8L enginesTo install:

1. Clean the housing mating surfaces.
2. Dip the new gasket in clean water and place it on the water box surface.
3. Center the thermostat on the gasket, in the water box.

Fig. 4: Remove the radiator hose from the waterneck by loosening the hose clamp ...

1. Make certain the bolt threads are clean. Threaded bolt holes exposed to coolant are subject to corrosion and should be cleaned with a small wire brush or correct size thread-cutting tap. Install the housing, making sure the thermostat is still in the recess, and tighten the retaining bolts to 105 inch lbs. (12 Nm) for 3.0L engine or to 21 ft. lbs. (28 Nm) for 3.3L and 3.8L engines. Reconnect the upper radiator hose and tighten the hose clamp.
2. Connect the negative battery cable.
3. Fill and bleed the engine cooling system with a clean 50/50 mixture of ethylene glycol antifreeze and water.

Fig. 5: Lift the waterneck housing off of the lower intake manifold, then ... Fig. 6: ... remove the thermostat out of its mounting flange

1. Make sure the radiator is full and start the vehicle.

CAUTION Do not remove the radiator cap once the vehicle is warm. Coolant is under pressure and may cause scalding or personal injury.

1. Run the vehicle until the thermostat opens. Check the coolant level in the overflow tank and fill if necessary.
2. Pressure test for leaks.

Fig. 7: Make sure to clean the waterneck before installation