Uses excessive amount of gas

Probably need to change the plugs and wires, fuel filter, and run some fuel system cleaner through it. Tune it up if you can.

Hello, There are several causes for excessive fuel consumption . Dirty or clogged air filter, leaking gas hose or fuel lines, under inflated tires, wrong sized tires, excessive weight in the trunk, a heavy foot on the gas pedal are only a few.

Out the exhaust, unless a small amount of coolant
is going into the transmission

Have to use an exhaust gas analyzer at the exhaust
tail pipe or coolant fill cap or over flow container

You got a failed head gasket or worse, a crack that opens
up whenever it wants to

Wow. This could be most anything. Bad ignition coil, bad plugs and wires, bad coolant temp sensor, EGR, vacuum leak, bad timing chain....

If it has a caby then check why the float is not controlling the needle and seat that controls the amount of fuel into the carby. Check the needle and seat for something like a hair or the rubber on the needle is cracked or the seat is loose in the carby body. Concentrate on the carby as the over fuelling problem is in that area. Check that the choke is not coming on

check for a bad coolant temp sensor remove the plug and see if the connectors look green if so change both the sensor and connector

Hi, P0402 code means that OBD detected an excessive amount of EGR.
EGR stands for Exhaust Gas Recirculation. It is part of the vehicle emmissions system, and is used to reduce combustion temperature and pressure to control Oxides of Nitrogen. There are generally three parts to the EGR system: the EGR valve, an actuator solenoid, and a differential pressure sensor (DPF). These things work together to deliver the correct amount of recirculation based on engine temperature, load, etc.
With a P0402, it is common for people to replace the EGR valve, only to have the problem return. The most likely solution is to replace the DPFE sensor.

It could be the problem, but i would do some more checking before saying its the computer.

It sounds like you have a plugged exhaust to me.

have you checked your tires? a low tire will cause fuel consumption

Unless the Check Engine Light is ON, there is no vehicle-related issue to fuel consumption. The computer will deliver the exact amount of fuel needed to satisfy the demands placed on the engine, using input from a large variety of sensors, including the accelerator pedal position sensor. Fuel economy estimates in the USA are just that, estimates, derived from a chemical analysis of emissions, and not actual driving. And no vehicle ever hits those numbers.

There are numerous outside factors that have a negative impact on fuel economy:
Vehicle maintenance, driving conditions (stop & go driving KILLS fuel economy), accelerator pedal usage, overall vehicle speeds, loads carried, etc.

Remember, the more you use the gas pedal, the more you use the gas station!