Just FYI, pumping the gas pedal doesn't do anything on fuel injected engine....no accelerator pump like a carb had.
Your engine is getting too much fuel, I know duh, lol, lower MPG says that.
A cold engine needs more fuel(i.e. choke), so no issues on cold starts, colder weather/air also needs more fuel so again you wouldn't notice it as much with the denser air(cold air).
Warmed up the engine needs less fuel in the fuel/air mix.
So what controls fuel air mix at start up?
MAF is the main sensor for fuel/air mix
IAT(intake air temp) sensor plays a part, it is part of the MAF sensor on your year.
ECT(engine coolant temp) sensor also plays a part in fuel/air mix, fuel injected engines have the ECT "sensor" which has TWO wires, and a Temp SENDER, for the dashboard temp gauge, this one has ONE wire.
These look the same.
O2 sensors are not used on startup
Since MAF was cleaned already and IAT is probably OK because it pays a small part, I would check the ECT sensor first.
ECT sensor is easy to check with OHM meter but hard to get to, it will be under the throttle body on the front of the lower intake just above thermostat housing, remember you want the TWO wire one.
For the ECT to effect warm start but not cause CEL(check engine light) to come on it would have to go to high resistance(ohms) when sitting and then jump to lower resistance(ohms) once engine starts.
Computer sends ECT sensor 5volts which passes thru the ECT's resistance and goes back to computer, high resistance(cold) would pass lower voltage to computer so it "thinks" engine is cold and sets "choke mode" for cold starting, so extra fuel is sent to engine.
As ECT warms up the resistance gets lower so computer gets a higher voltage and it reduces the extra fuel.
Computer has a timer that starts with the cold engine if ECT voltage is not changing within 5 minutes CEL will come on.
If ECT wire is disconnected, 0 volts at computer, CEL will come on.
When warmed up ECT will show about 2,000 OHMs, when cold about 20,000 OHMs, these are not meant to be exact, computer learns ECT values, so "about" is the key word.
If ECT resistance is changing(so no CEL) but is not going low enough then computer continues to run engine rich which lowers MPG.
New ECT sensor should cost about $35, so up to you if you think changing vs testing is better, these rarely fail, not never fail, just rarely.
Another test to do is for sticking/leaking injector, it could be causing a rich condition which effects warm starts and MPG.
All fuel injected engine have a "clear flooded engine mode"
To get computer into this mode do this:
Turn key ON, do not crank engine yet
Press gas pedal to the floor and hold it there(no pumping, lol)
Now crank the engine
Engine should NOT fire or start, this "mode" turns off fuel injectors but leaves spark on.
If engine fires a bit or starts then there is fuel in the intake, and there shouldn't be
Warmed up engine is best for this test.
If you release gas pedal while cranking engine should startup normally
Last edited by RonD; 04-15-2014 at 10:00 AM.