Ok, well the trip to canton I calculated 518 miles on 30 gallons. That's not exactly accurate, but we put 15 gallons in twice and it went down toward e, but no light was on, and when we came back it was about the same place it started, so it doesn't seem right, but if i could write it out, it'd make since.

basically, we ended on the same place that we started, and it took 518 miles to get there.

that's about 17.25 mpg highway.

Now, the reason for this thread.

I've got about 100 miles on nearly half a tank, roughly 13 gallons = 7.7 mpg city.

Alright, what in the world can cause a 10 mpg difference?

My cat's not that great, but i don't see how it got any worse.

I have been driving more hills with this fill up than previously, so I was expecting a 4-5 mpg decrease. but 10?

Yikes.

Only other thing wrong with it is the temp gauge is not working. I wrote about that in another thread though. No way that affects 17 to 7 mph though.

Any advice?

 

You absolutely can NOT check your MPG going by the fuel gauge.

 

with a drop that drastic in mpg, the first place I would start is the catalytic converter.

Mine was gone after 68,000 miles. Totally F'ed... Try to see if shaking the cat causes the cell inside the cat to rattle. If it does, time to replace it. Otherwise, start checking other things like the temperature sensor. I beleive that that may be one of the things that the PCM relies upon in order to properly run in closed loop.

this is my reference from http://www.v8performance.com/tipsandtricks.htm

Open and Closed Loop Operation:

There are 7 different modes (conditions) that are used (besides Key-ON and Key-Off mode). Here is a table of what sensors are monitored and what determines the mode. One thing to remember, under OPEN LOOP Operation, the O2 Sensor(s) are ignored by the PCM.

Engine Start-Up Mode (OPEN LOOP)
Battery Voltage
Coolant Temp
Crank Position
Intake Air Temp (IAT)
MAP
TPS
Starter Motor Relay
Cam Position

Engine Warm-Up Mode (OPEN LOOP)
Battery Voltage
Coolant Temp
Crank Position
Intake Air Temp (IAT)
MAP
TPS
Cam Position
Park/Neutral Safety Switch (Automatics Only)


Idle mode (Once at Operating Temp: CLOSED LOOP)
A/C Select (if equipped)
A/C Request (if equipped)
Battery Voltage
Coolant Temp
Crank Position
Intake Air Temp (IAT)
MAP
TPS
Cam Position
O2 Sensors
Park/Neutral Safety Switch (Automatics Only)

Cruise Mode (CLOSED LOOP)
A/C Select (if equipped)
A/C Request (if equipped)
Battery Voltage
Coolant Temp
Crank Position
Intake Air Temp (IAT)
MAP
TPS
Cam Position
O2 Sensors
Park/Neutral Safety Switch (Automatics Only)

Acceleration Mode (OPEN LOOP)
A/C Select (if equipped)
A/C Request (if equipped)
Battery Voltage
Coolant Temp
Crank Position
Intake Air Temp (IAT)
MAP
TPS
Cam Position
Vehicle Speed Sensor
Park/Neutral Safety Switch (Automatics Only)

Deceleration Mode (OPEN LOOP)
A/C Select (if equipped)
A/C Request (if equipped)
Battery Voltage
Coolant Temp
Crank Position
Intake Air Temp (IAT)
MAP
TPS
Cam Position
Park/Neutral Safety Switch (Automatics Only)
Vehicle Speed Sensor

Wide Open Throttle - WOT (OPEN LOOP)
Battery Voltage
Coolant Temp
Crank Position
Intake Air Temp (IAT)
MAP
TPS
Cam Position

 

have u hooked a vacuum gauge up to ur intake manifold as a instant mpg gauge it helps out alot