I've been observing a drop in my MPG lately with no effect on how she runs. Everything checks out ok but still the MPG has dropped significantly in the last week and a half. My driving style hasnt changed, nor have the driving conditions. Any ideas as to what can be causing this? Im stumped.

 

Maybe the O2 sensor?

 

o2 Sensor, Manifold Leak, Injector Leak, Cold Start Valve Leak, FPR Problem, Timing Incorrect, AFM, and on and on and on and on and on and on...

In other words... Good luck!

Rod.

 

Crucial . . .And it would run great despite any of these failing? Its not missing, no iratic idle, no smoke, no timing adjustment within the past year. I dont want to just start replacing stuff. Any indicators of failure for any of the above listed items?

 

Is your gas cap locked?

 

Locking the cap wouldnt have any adverse effect, would it? Cuz yes it is, but ive been locking it since ive owned the Z.

 

Locking it won't matter.... but making sure it's tight to prevent evaporation. I think it's an O2, AFM, or manifold leak problem like Rod Said.

How many MPG difference are we talking? I always pay attention to my mpg per tank... sometimes it differs a lil.

 

Well, I haven't done the math just yet, but for arguments sake lets say I was getting around 22-24 with it being about 15-18 now. Its that drastic. Again those figures are fictitious. I'll figure it out as i drive more.

 

Being that drastic, I would think you could tell a difference in the way it runs. Have you pulled the sparkplugs to see what they look like?

 

id say do the usual tune up stuff. that way you have a good baseline to start from, who knows it could be the cheap stuff.

 

Well I would say being that it runs the same I would start looking for that leak in the fuel system!! I would say, on its way back to the tank maybe. Then again depending on where you live the oil companies change fuel formulas for the summer...something to do with the clean air act, may not be as good of a fuel. But I would look for the cheapest thing first like a fuel leak and work my way up...is you air filter old? This problem may have happend over a period of time as to let you adjust to the problem without noticing a difference in how she runs or feels. Also the hotter it is outside the more fuel your car will use as the air is less packed with O2 as apposed to cold air. Are you running A/C? Have someone follow you and make sure it is not smoking, some smoke dissapates quickly as to not let you see it in the mirror.

 

Ive checked the simple things. All check out OK. There is no leak of any kind to my knowledge, doesnt smoke in the slightest and she runs great. Im going to start with the 02 sensor and FPR. Those are fairly cheap and could use a replacement anyway.
The change in MPG is recent. It started last week and I thought it might have been due to the weather but the past 4 or 5 days have been overcast and cool. I guess i have no option but to start replacing crap.

 

Didn't you just do a manifold gasket replacement... ?

Rod.

 

True, but i cant find any leaks, everything is tight and i cant pinpoint what the cause of the problem is

 

Check the sparkplugs, maybe those need to be changed.

 

I was implying that someone was siphoning.

I think the car is ignoring the o2 sensor signal.

 

The reason I mentioned the fact that you just replaced the manifold gasket, is that it seems more than a tad coincidental that your new problem arose so shortly after doing that job... i don't want to imply that you screwed anything up or that you made a mistake or anything, but the timing is a little more than ironic. I would re-trace and re-check everything that was removed around the rail and intake while you were doing the job. Maybe the wire to the o2 sensor got pinched or buggered or a vacuum line or something in that area got whacked... It just seems to make sense that your problem may have something to do with that job...


Rod.

 

As usual Rod is the man. My 02 was not properly connected after the manifold job. Now I just hope that it really was the actual cause of the raise in fuel consumption. Funny how it ran great though. . . Maybe I still need a new one?

 

The reason the car ran fine is that the spring in the airflow meter is what supplies the "fuel map".

This "map" is always richer than when running in closed loop.

Hence the car running okay and mileage suffering.

The car will default to this when the 02 sensor is not recieved for a certain amount of time.