So I have a 96 civic EX, D16Y8 manual trans car

I blew a head gasket so I put a new honda oem head gasket on and a spare head that I bought a while back and was told was good (to save on machining costs with the old head)

Before I blew the head gasket I was getting around 34mpg for an all highway tank and about 27 for an all city tank

Now I'm getting around 29-30 mpg for an all highway tank and around 24 for an all city tank

What gives? I kept the original intake manifold so all those sensors are what the car came with. The only thing I can think of would be the coolant temp sensors in the head. The cluster temp gauge stays in the usual range so I assume the sensor is good and correct

Is there a different ECU coolant temp sensor for OBD2A and OBD2B? Could that be a difference? Or they the same and mine could be bad?

 

verify timing.

 

I would bet the replacement head gasket and maybe the head decreased your compression ratio a tad. . I have had the stock head milled and with the replacement gasket noticed a power decrease. I would bet the stock gasket used when the original motor was assembled was thinner than the replacement gasket from Honda by a few thousands... Of course there could be minor intake air leaks or several other minor things that add up.........jmho...