1984/5 toyota tarago 3y 2litre

Sounds like the catalytic converter is clogged. Disconnect or loosen it, then see if the problem still exists. If that corrected the problem, get another converter.

have to clean pipe line which is going from fuel tank to engine

I dont use this word much but im Assumeing that YOUR mechanic scanned the computer and checked the coolant sensor or other possible causes,Ive ran across this many times before so bare with me,Have your mechanic try a known good Airflow meter and dont forget to check the main airboot for rips,cracks or tears in the seams,gotta inspect all the areas where it is accordianed for flexibility,another one is moisture penetrating into the distributor cap,got more if ya need them hope this helped let me know have a good day Darrell

LPG 3Y engine toyota crown comfort automatic - fuel consupmtion is 13.30litre per 100km? How can this be reduced?

The mechanic is most likely correct the vehicles have double intake as well as double exhaust valves which allows the motor to retain most of its power even if one of the valves burns out. However if an intake valve burns or fails there usually is a popping sound or misfire upon acceleration in this case you most likely have a burnt or worn exhaust valve, you have about six months to a year before the truck starts really misbehaving. Just address the side of the engine or head that has the burnt valve don't let them talk you into doing both cylinder heads you may go 250,000 to 300,00 miles before having to do a full valve job on the truck. These motors are fairly tough so just do the one head not both when the time comes.

your choke is sticking

If this idles and starts fine, then fuel pump and distributor should be ok. If pump was bad, it would not likely start or run at all. If you have a distributor, then definately check your timing. That's what it sounds like is your timing is off. Do go over all your vaccum lines, that could have same result, but would probably idle fine but accelerate poorly if you had vaccum leak. If timing was off, then it would idle rough and have poor acceleration.
Do you have fuel injection or carbs? I like to use a can of "seafoam" Which removes moisture, depositsm, grime, dirt and anything elese in the fuel system that doesn't belong there.

remove the carb. and clean the float , and the float bowl . when gas setts to long it gums up that area and doesn't get the right amount of fuel into the bowl , so when you hit a hill its starving for fuel

You said you replaced the plugs - did you use the toyota platinum plugs? I found out the hard way that even the platinum plugs from bosch or any other kind of platinum plug just doesn't work right. Yes, they are more expensive from toyota, but they can make a difference in the way the engine runs.