Horrible gas mileage

[Twister]

So we have an 04 E350 super duty van with the SOHC 5.4 auto. Over the course of a couple months the gas mileage has tanked. Yes I know I shouldn't expect 16mpg, but I should be getting better than 7mpg, we have been getting 7-8mpg average. We do about 80% city and 20% highway. The CEL isn't on and I have recently replaced the coolant and serp belt.*Anyone have any suggestions?
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[adsm08]

Filters and plugs. That should still be a 2V engine, so no vct system to cause problems.

Postin' from teh Galaxy

 

[brokemechanic3000]

Try putting premium gas in and see what that does, Avoid ethanol blend fuels like the plague. Ethanol blend fuels really drop the fuel mileage I started putting only premium in and the mileage increase more than pays for the extra cost each fill up.

 

[Mark_88]

I agree with filters and plugs...especially filters...air filters...and check the tire pressure...nothing like loosing 20 psi when the weather turns warmer...

 

[Twister]

Filters and plugs. That should still be a 2V engine, so no vct system to cause problems.

Postin' from teh Galaxy

Oh lord, doing plugs on this thing is a PITA lol, but if it helps ill do it.

Try putting premium gas in and see what that does, Avoid ethanol blend fuels like the plague. Ethanol blend fuels really drop the fuel mileage I started putting only premium in and the mileage increase more than pays for the extra cost each fill up.

All it gets is non ethanol, I avoid the 10% ethanol pretty much at all cost.

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[RonD]

On Fuel Injected engines they use a temperature sensor as the Choke, this is called the ECT sensor(engine coolant temp), this is a 2 wire sensor not a 1 wire that would be used for the dash board gauge.

The ECT is a resistance sensor so easy to test
The PCM(computer) supplies the sensor with 5volts and then monitors the return voltage, the lower the return voltage is the colder the engine, so sensor has high resistance when cold and lower resistance when warm.
As long as this sensor is passing voltage there will be no CEL.

When the PCM is "told" by the ECT that the engine is cold the PCM will run the engine rich and with advanced timing, like a Choke did, as the voltage passing through the sensor increases the PCM starts to lean out the mixture.
If the ECT is not increasing voltage as it warms up then engine runs rich all the time, sending MPG into the basement.

You can test the ECT(usually located near the t-stat housing), with an OHM or volt meter.
With an OHM meter remove the ECT connector and test it's resistance, with engine cold this should be a high number.
Reconnect ECT start engine and drive it, when you get back home after engine has warmed up test ECT again, remove connector and test, resistance should be much lower.

Now to add more confusion some ECT sensors are reversed, they get more resistance as they get warm, so voltage goes down when engine warms up.
But for testing purposes what you are looking for is a difference when cold and hot, that's the main point.

A Voltage test must be done with key on engine off, set Volt meter to 20volts DC and use pins(sewing pins) to pierce the wires, test voltage cold and then hot, should be a 4v change, +/- .5v


The 5.4 also has a CHT sensor(cylinder head temp), I don't think this is used for part of the Choke, although it could be used to change air/fuel mixture.