This is about my wagon I dropped a sohc ZC into.

For the last half a year I've consistently gotten ~34mpg city driving.

This last tank it suddenly and unexpectantly plunged to 26mpg.

My driving patterns haven't changed, it seems to run as perfect as ever, and I cant detect any fuel leaks. The car has the same power it always has, no strange smells or exhaust, no check engine lights and my tires are properly inflated. I'm pretty stumped. What else should I check?

 

leaking fuel injector? have you done a tune-up recently? maybe it has weak spark.

 

Fuel filter could be a issue depending on its age but most time its an o2 going week or a catalyic convertor

 

The cat has been punched out, I checked my timing, the fuel filter was replaced new when I dropped the motor, injectors are dry, no gas smell while running.

Is there anything else at all I can check?

Oh yes, plugs were replaced new at motor drop as well.

Maybe I got a bad tank of watered down gas?

 

i also had seen a dramatic reduction in mileage, down to about 25/g. haven't really given it much thought.

 

valve timing but i would double check the fuel filter will let you no if nething is going on in that tank! your car is still obd0?? you may not be having any check engine light but you can always look for pending codes see if your ecu flashes at any time while the cars ignition switch is on

 

also bad gas mileage can be due to any kind of exhaust least even the ones you done here! most leakes can be found were piping meets each other! a crush donut a header gasket and also a bad muffler

 

Join the club. My MPG has unexpectedly been bad nearly this whole year and I can't figure out why. I get 27 MPG when I used to get 35+ at least. All this summer, my MPG dropped down to 25 due to my A/C use. It can eek out about 32 MPG on a pure highway tank, which isn't terrible but it should do 40 easy.

The 25 MPG in the summer was really aggravating. I mean I can get that in my Maxima.

Hopefully yours is just a one time fluke tank.

 

Its a thought but when this cold weather rolls in the IAT sensor reads a colder temp at start up and so does the coolant temp sensor...making it run rich until the o2 sensor warms up and takes over the fuel injection. The obd0 o2 sensor is a one wire sensor without a heating circuit...hence it takes longer (especially when its really cold outside) to heat up and take over. Once you start driving the o2 sensor should be warm enough that it takes over the fuel injection...so most of your mpg can be going down not much by driving but mostly at startup. If you jump in your car and just take off without the o2 sensor reaching its operating temperature then your car will most likely run rich while on a load lowering your mpg's. newer cars with 4 wire o2 sensor have a heater that helps the o2 sensor reach operating temp within a few seconds. Also an old o2 sensor most of the times will not throw a code but will transmit slow sluggish signals to the ecu. When it completely fails then most likely is when the CEL comes on. Try and replace your 02 sensor it just might help. Hope this helped you out

 

your also adding winter gas into the mix. i expect at least a 3 mpg drop. if you let your car warm up every start expect to see a loss of 5 or 6

 

Cool ill try a new o2 sensor, sounds like its worth a shot, thanks guys

 

I wouldn't waste money on a new 02 sensor. I would see what you get with another tank of gas. I was in the same situation once. Changed gas stations and got back in the high 30s.

 

Yeah that's what else was strange, I used the same station I always do.

I'll try another tank before buying the sensor, it may well have been a fluke.

 

^^ This

 

definitely this...