Just wanted to update everyone on what has happened since I first posted about the Horrible Gas Mileage. It has taken over 6 years with no help from Mechanics or forums on what exactly was going on.
My nephew replaced timing Belt, water pump, and new thermostat. Found Out the Timing Belt was not put on correctly-Which was done at the Toyota Dealership back in 2000-Timing belt was not put on to correct timing. He also Adjusted timing with distributor cap. I was getting 8 Miles to gallon. Now with the timing belt set correctly I am getting more power and 11 mpg. We still have to replace the Emissions/Vaccuum Switch and Thermo Time Switch and a couple of Sensors. Which Should help. I read the Emissions/Vaccuum switch can cause bad gas mileage. We shall see. Hope fully in 2 or 3 weeks we will replace all of that and we will see.
Also After replacing the Timing Belt and adjusted timing - Truck is not Sluggish when it is warmer outside. It now has power.

Note - Prior To Replacing Timing Belt - Had the timing adjusted and it did not help with the gas mileage.

This was the original Post

I am getting 8 MPG on my 1991 Toyota Pickup V6 3vze Fuel Injected Engine Automatic Transmission 4 x 4. Truck has 199,892 miles on it. I had the truck tuned up by my mechanic in 2019. They replaced spark plugs, wires, distributor cap, rotor, and checked timing. After they tuned up the truck my gas mileage went from 10 mpg to 8 mpg. I took it back to them and told them my gas mileage went down after the tune up. They still did not fix it. I also replaced the O2 Sensor on it at that time.
My problem is after reading all the posts here a lot of people have the same problem but I do not see any responses as to what worked to solve it.

My nephew has worked on the truck and found the mechanics did an awful job on the tune up. He did a new tune up on it.He also found the fuel line to the fuel pressure regulator was kinked. We replaced the fuel pressure regulator and the fuel line hose. Funny the mechanic never spotted this. Result was a better running truck. Thought it would have helped with the Gas Mileage but still getting 8 MPG. Nephew checked the timing on it and it is at factory spec Compression test results 150 on all cylinders. So we are stumped as to why the truck is still getting horrible gas mileage. He checked for error codes and pulled a 12 and 24 error code. Another issue is that when the temp outside is hot-since living in Arizona. the truck runs sluggish versus when the temp is cooler outside it runs great.

 

Actually, when you posted the first time the first thing I brought up was timing.

Check tps setting, i rec using the timing light method
I'd only install the recommended ND or NGK sparkplugs. i prefer ones with a solid terminal. You don't need platinum or iridium sparkplugs unless the application calls for it
Any other brand of sparkplug i'd remove,; Bosch, Champion, Motorcraft,