Excess pressure in coolant system-gage will peg Hot.

The air/fuel ratio that is capable of burning in an engine is fairly limited - too lean and the engine won't start and won't run, too rich and an engine will probably start but won't run smoothly and will soon choke and stop.
A cold engine needs a richer than normal mixture and acceleration also needs more fuel but unless a hot engine receives the right amount it won't run smoothly or reliably.

The useful life of a fuel injector is fairly limited and it is downhill after 70/80,000 miles. In the case of bad running, reduced power or excess fuel consumption it is best to check the fuel pressure and delivery volume and pull the fuel injectors for testing before moving on to other checks and adjustments.

From the reserve tank, one hose should go to the radiator and the other should be for overflow. Sometimes that one goes to an accumulator, sometimes its left dangling cause its just there to drain any excess fluid from expansion in a hot system. Depends on the vehicle. If its in an enclosed system or plugged you would get an excess of back pressure which would further heat up the coolant and inhibit flow. Just leave it there. If it pukes coolant when hot then its doing its job. Don't overflow your reserve tank.

Have you already confirmed that the coolant overflow tank has at least the minimum coolant in it?
Does the water level in the coolant overflow get higher when the engine is heated up than when it was first started?
At what speed does this happen? Idling only, or also when driving 35+ miles an hour.
A quick test - when the engine is warmed up, touch the radiator. If it is still cool, the problem is likely the thermostat - it probably isn't opening when the engine gets hot.

Can you explain your problem more ?
The thermostat should be at the end of the upper or lower radiator hose to block flow to the radiator. The spring must be on the engine side to get the engine heat to open.

the lower radiator hose is a return hose and there are no vapor lock issues. If your coolant level is full when you start your vehicle and when it heats up and the system pressurizes your coolant expands with heat and then that excess pressure will go through the overflow outlet to the reservoir and the reservoir has a relief opening near the top to relieve pressure or for excess fluid to come out. If the lower hose is just warm then your water pump may not be circulating properly (be sure to check belt for tension no more than 1/2 inch of play should be present). One messy way to test water pump would be to first drain coolant if it is good to reuse it, then remove the lower hose, open the radiator cap and put a water hose in it and turn on half way. Look at the volume of water coming out and have someone start the vehicle and idle at about 2000 rpm if that volume has not doubled your pump is bad

Your car's heater is a life saver in cold weather. Before attempting to repair your car heater, you should understand how your heater system works. First, on this page is an explanation of how the car heater system functions, further down the page are some pointers on heater system trouble-shooting. To warm the passenger cabin of your car, the heating system makes use of excess heat from the engine internal combustion process. A car's engine, in fact, when in operation produces so much excess heat that if the excess heat is not removed, it would destroy the engine. It is your car's cooling system that removes that dangerous excess heat. Some of the excess heat is released through the exhaust. Most of the excess heat is absorbed by a circulating liquid coolant, which is a mix of water and antifreeze. The heated liquid coolant is carried from the engine through hoses to the radiator, which transfers the heat from the coolant to the outside air. That heat transfer lowers the temperature of the liquid coolant, which is then circulated back to the engine to absorb excess heat again. Whereas the radiator is located at the car front grill, the unit that transfers heat to the passenger cabin is located inside the dashboard. This unit is something of a mini-radiator and is referred to as the heater core. Heated liquid coolant circulates through tubes in the heater core and a heater fan blowing across those tubes, as well as through little fins encasing the tubes, directs warm air through heating vents into the passenger cabin. Because your car's heating system works off of its cooling system, heating system malfunctions are often caused by problems in the cooling system. The heater core in your car is similar to the radiator in the front of your car; in fact it looks like a small radiator. The difference is the heater is mounted inside the car and air is blown through the fins of the core. The heater hoses transfer engine coolant from the engine to the heater core, this allows the heat from the engine coolant to be utilized and warm the passenger compartment. When a heater stops functioning determine what type of failure has occurred to execute a repair. We have listed the most common problems below:

Heater Core
Troubleshooting Procedure

• Step 1: Check Engine Coolant Level - Most cars have coolant reservoirs with opaque white plastic tanks and measuring lines allowing you to see if the coolant is at the recommended level. Coolant expands when heated, and for that reason, most coolant reservoirs have a Full-Cold and a Full-Hot line. If there is too little coolant in the reservoir, you should be alerted to the possibility that there could be a leak somewhere in your car's cooling system. Remove the reservoir cap and add coolant to the Full-Cold line, if a very short time after you refill the coolant reservoir the coolant level is low, there is a leak in your car's cooling system. Engine coolant is used to heat the heater core and if the vehicle is low on coolant the heater core will stop working. The heater cannot produce heat due to the lack of heat from the engine coolant. Even if the engine is not over heating the coolant level can still be low. Inspect the engine coolant level in the coolant reservoir tank; coolant level should be between the hot and cold marks. Always check the coolant level when the vehicle is cold, preferably over night. Check for engine coolant leaks and repair as needed, then refill with coolant and recheck the system.
  
  Radiator Cap
  
• Step 2: Inspect Heater Control Valve - The heater control valve is used to shut off hot coolant from entering the core when the heater is in the off position. A heater control valve can be actuated by either a vacuum line or a cable from the vacuum heater control valve to the heater core. This hose should be warm; if not the heater control valve is stuck and needs to be replaced. After the repair has been made refill the coolant level and recheck system.
  
  Heater Control Valve
  
• Step 3: Check Vacuum Feed - Most heater systems are operated by engine vacuum. This vacuum is used to actuate servos to move the temperature adjustment doors inside the heater plenum. If engine vacuum is not present the heater will not operate properly. To test for this condition remove the vacuum feed line from one of the vacuum servos. With the vacuum feed line removed insert a vacuum test gauge. If no vacuum is present, open the hood and inspect all small vacuum lines from the engine to the firewall and repair as needed. If a hissing noise is present under the dash when the engine is running a vacuum leak exists and must be repaired to restore heater function.
  
• Trouble Shoot Heater/Air Conditioner Blower Motor. An electrical motor is used to push air through the heater and air conditioner system. If this electric motor fails it will not force air through the system. Ground a test light lead to a good ground source, like a metal brace under the dash, seat mount bolt or under hood brace. Turn the key to the on position; check the fuse in the heater or blower motor fuse panel with a test light. Then lightly touch the service port at the top of the fuse on both sides. If the test light illuminates on both sides of the fuse, the fuse is ok and working properly. If the fuse lights the test light on one side it is blown and needs to be replaced.
  Power Distribution Center
  If you are unsure of the location of the heater fuse consult a repair manual. If the fuse condition is ok, position the blower speed to the highest setting. Connect the ground end of the test light to a known ground source. Then probe both ground and power feed wires of the fan motor. The test light should illuminate on only one of the wires (fan motor failed). If the test light illuminates on both wires the ground source has shorted. If neither of the wires illuminate the power source is shorted. To trouble shoot this style of problem a wiring schematic is needed.
  

check the coolant level, if your blow motor works and your not getting heat the coolant is probably empty, the heat work when the engine gets hot and coolant starts to get hot, from there the blower, blows the excess heat into your car, low or no coolant means no heat

Your car's heater is a life saver in cold weather. Before attempting to repair your car?s heater, you should understand how your car?s heater system works. First, on this page is an explanation of how your car?s heater system functions, further down the page are some pointers on heater system trouble-shooting. To warm the passenger cabin of your car, your car?s heating system makes use of excess heat from the engine?s internal combustion process. A car?s engine, in fact, when in operation produces so much excess heat that if the excess heat is not removed, it would destroy the engine. It is your car?s cooling system that removes that dangerous excess heat. Some of an engine?s excess heat is released through the exhaust. Most of an engine?s excess heat is absorbed by a circulating liquid coolant, which is a mix of water and antifreeze. The heated liquid coolant is carried from the engine through hoses to the radiator, which transfers the heat from the coolant to the outside air. That heat transfer lowers the temperature of the liquid coolant, which is then circulated back to the engine to absorb excess heat again. Whereas the radiator is located at your car?s front grill, the unit that transfers heat to the passenger cabin is located inside the dashboard. This unit is something of a mini-radiator and is referred to as the heater core. Heated liquid coolant circulates through tubes in the heater core and a heater fan blowing across those tubes, as well as through little fins encasing the tubes, directs warm air through heating vents into your car?s passenger cabin. Because your car?s heating system works off of its cooling system, heating system malfunctions are often caused by problems in the cooling system.

There are many many reasons for car heater not coming ON or not working properly:-- i will explain you the procedure to check this possibilities Your car's heater is a life saver in cold weather. Before attempting to repair your car’s heater, you should understand how your car’s heater system works. First, on this page is an explanation of how your car’s heater system functions, further down the page are some pointers on heater system trouble-shooting. To warm the passenger cabin of your car, your car’s heating system makes use of excess heat from the engine’s internal combustion process. A car’s engine, in fact, when in operation produces so much excess heat that if the excess heat is not removed, it would destroy the engine. It is your car’s cooling system that removes that dangerous excess heat. Some of an engine’s excess heat is released through the exhaust. Most of an engine’s excess heat is absorbed by a circulating liquid coolant, which is a mix of water and antifreeze.

The heated liquid coolant is carried from the engine through hoses to the radiator, which transfers the heat from the coolant to the outside air. That heat transfer lowers the temperature of the liquid coolant, which is then circulated back to the engine to absorb excess heat again. Whereas the radiator is located at your car’s front grill, the unit that transfers heat to the passenger cabin is located inside the dashboard. This unit is something of a mini-radiator and is referred to as the heater core. Heated liquid coolant circulates through tubes in the heater core and a heater fan blowing across those tubes, as well as through little fins encasing the tubes, directs warm air through heating vents into your car’s passenger cabin. Because your car’s heating system works off of its cooling system, heating system malfunctions are often caused by problems in the cooling system.
The heater core in your car is similar to the radiator in the front of your car; in fact it looks like a small radiator. The difference is the heater is mounted inside the car and air is blown through the fins of the core. The heater hosestransfer engine coolant from the engine to the heater core, this allows the heat from the engine coolant to be utilized and warm the passenger compartment. When a heater stops functioning determine what type of failure has occurred to execute a repair. We have listed the most common problems below.

please click on this link directly for more detailed help
http://www.2carpros.com/first_things/heater_failure.htm
check the diagrams and parts in the link to troubleshoot your care heater
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sounds like a blown heat gasket, very common on a 4.6