Nissan Repair: 1986 nissan d21 fuel mileage, fuel pressure gauge, fuel pressure regulator

Question
QUESTION: Hello sir,

I am having a problem with the fuel mileage on my old Nissan truck. I've calculated the mileage on the highway and I'm getting 25 mpg which is great. My problem is when I start driving in the city, my mileage goes down to about 12 mpg. I have a new fuel and air filter, the plugs and wires are pretty new and the PCV valve is also new. I am just wondering what in the world could be the problem? Could it be the EGR valve?

Thanks for the help!

ANSWER: Eric,

If yours is a carburetor, sorry I forget, the power valve is probably blown and needs to be replaced.  Email me and let me know if yours is carburetor or fuel injection.

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QUESTION: The engine is a manual 2.4L 4 cylinder with two throttle body injectors.

Answer
Eric,

Check the fuel pressure regulator for a blown diaphram.  If the vacuum is removed the fuel pressure should increase.  You will need a fuel pressure gauge to do the test.  SO, if the vacuum to the fuel pressure regulator is removed and the fuel pressure stays the same would mean that the fuel pressure regulator is defective.